Derechos are less common than Midwestern tornadoes, but occur almost every year in that region as well as other parts of the United States. Yet they remain exotic and mystifying. Even to meteorologists.

The reason appears to lie in the complex way the long-lived windstorms form. Derechos are often incorrectly referred to as inland hurricanes; their damaging winds can reach hurricane force, but they are straight-line in nature, rather than circulating around a common center. When people suffer, though, they can be forgiven for using the wrong windstorm term. By definition derechos have to meet specific criteria, such as causing damage continuously or intermittently in a lengthy line of at least 400 miles that’s at least 60 miles wide. But just like hurricanes, they come in a variety of intensities.

2020_derechoLast week’s derecho, roaring 750 miles from eastern Nebraska across Iowa, Illinois and Indiana, was particularly ferocious with winds in multiple swaths across Iowa gusting to over 100 mph. The National Weather Service found damage to an apartment complex in Cedar Rapids, Iowa, consistent with wind gusts of an astonishing 130-140 mph. But even these extreme winds and the severe damage they wrought don’t tell the whole story of the variation in these windy storms with the seemingly odd name.

Iowa_wind_gustsThe AMS Glossary of Meteorology states: “The term derecho derives from a Spanish word that can be interpreted as “straight ahead” or “direct” and was chosen to discriminate between wind damage caused by tornadoes, which have rotating flow, from straight-line winds.” It defines derechos as widespread convectively induced straight-line windstorms.

Specifically, the term is defined as any family of particularly damaging downburst clusters produced by a mesoscale convective system. Such systems have sustained bow echoes with book-end vortices and/or rear-inflow jets and can generate considerable damage from straight-line winds.

Two very different derechos: Base reflectivity composite radar data for (a) 2056 UTC 4 Apr 2011 and (b) 2234 UTC 29 Jun 2012, with surface observations plotted using conventional station plot format. Convective system in (a) is composed of a series of loosely connected, largely linear bands with low-amplitude LEWPs, whereas that in (b) consists of a single, intense arc of storms with a well-defined bow echo. Animated radar and satellite imagery for these events may be viewed at and
Two Very Different Derechos: Base reflectivity composite radar data for (a) 2056 UTC 4 Apr 2011 and (b) 2234 UTC 29 Jun 2012, with surface observations plotted using conventional station plot format. Convective system in (a) is composed of a series of loosely connected, largely linear bands with low-amplitude LEWPs, whereas that in (b) consists of a single, intense arc of storms with a well-defined bow echo. Animated radar and satellite imagery for these events may be viewed at and


This updated definition was the work of a team of meteorologists about five years ago who compared different straight-line windstorms—all meeting the damage criteria of derechos at the time but clearly having different mechanisms driving them. Their published paper in the Bulletin of the American Meteorological Society was “A Proposed Revision to the Definition of ‘Derecho’.” With it, the authors made the case to improve the definition of derecho to make it more physically based and more closely mirror other definable meteorological terms such as tornadoes, extratropical cyclones, and cirrus clouds.

While the paper had a positive effect among the meteorological community, a web search still turns up different definitions of derecho among meteorological websites, including within the NWS.

We asked the lead author of the BAMS paper, Stephen Corfidi (NOAA NWS, CIMMS, University of Oklahoma), for his thoughts on last week’s Midwestern derecho, to put it in perspective and also to help explain the reasons behind the difficulty with meteorologists not only defining but using the meteorological term differently.

BAMS: Was last week’s Midwest fast-moving line of severe thunderstorms with damaging winds a derecho? If so, did it fit the Glossary definition of a derecho?

Stephen Corfidi: Last Tuesday’s Midwest wind storm met the AMS Glossary definition of a derecho. Most significantly, it satisfied the criterion for the presence of “one or more sustained bow echoes with mesoscale vortices and/or rear-inflow jets.” A large mesoscale vortex, located on the north end of the larger-scale bow-shaped convective system from central Iowa to Lake Michigan, appears to have been associated with some of the strongest and longest-duration surface winds that accompanied the event. While the swath of organized damaging winds was not particularly long (on the order of 500 miles (800 km)) in comparison to some past events, the length well exceeded the somewhat flexible 400-mile (~ 650 km) cut-off used to distinguish derecho storms.

BAMS: Among derechos, how does this one rank?

SC: The Iowa-Illinois storm is certainly one of the most memorable of 2020—a year that already has seen other noteworthy events (e.g., those in Colorado, the Dakotas, and Pennsylvania in June). What made the Iowa-Illinois event especially noteworthy was the sizable number of reliably-observed significant surface gusts. There were numerous reports in excess of 80 mph (35 meters/second), and incidental evidence that some locations experienced speeds greater than 100 mph (45 m/s). The strong winds—likely enhanced by the presence of the mesoscale vortex—in some places persisted for more than 30 minutes. The vortex to some extent resembled the “warm-core” (hurricane-like) circulation that formed within the Kansas-Missouri-Illinois derecho of 8 May 2009.

BAMS: What made this one stand out?

SC: The duration of the storm’s high winds over eastern Iowa and western Illinois—again, likely due in part to the presence of the large mesoscale vortex—was certainly outstanding. This is quite a statement considering that the event occurred in a region known for its propensity for derechos (Recall that Gustave Hinrich’s nineteenth century studies of straight-line wind events were based in Iowa).

BAMS: Why are derechos so perplexing and difficult for meteorologists to acceptably define?

SC: There are several reasons why derechos are not only difficult to define, but also challenging to forecast. Unlike supercell thunderstorms that, in most cases, are well-delimited in time and space, broad swaths of damaging convective winds can arise in many different ways. Some of the processes involved in derecho development remain poorly understood and / or are only partly resolved by current observational platforms.

More significantly, unlike weather phenomena like supercells, derechos may or may not be accompanied by meteorological structures that readily are apparent to human observers. For example, most meteorologists recognize a supercell when one appears in the sky. But specific cloud formations are not associated with derechos, and the characteristic meteorological structures that do accompany many derechos (e.g., embedded vortices and rear-inflow jets) are too large or subtle to be recognized by human observers without the aid of remote-sensing devices such as radar and satellite.

These same aspects make derechos difficult to forecast. One can say that derechos arise when a unique combination of known and unknown necessary ingredients is present over a sufficiently broad area to support rapid, repetitive, downshear thunderstorm development. That said, applying this concept in practice presents a formidable challenge. In some cases, the known ingredients are present, but the extent of the favorable environment is too limited for derecho status to be realized. Conversely, expansive environments sometimes appear that are supportive of widespread destructive winds, but not those strongly dependent on the smaller-scale processes associated with rear-inflow jets and mesoscale vortices.

As with any weather forecast, when “missed” derecho events occur, the root causes can be traced to the existence of too many unknowns—both observational and theoretical. In the case of derechos, both unknowns, at present, remain sizable.

BAMS: What has been the impact of your 2016 BAMS paper and proposed new definition?

SC: It does appear that the updated definition proposed in the paper has been adopted to at least some extent in the severe weather community; the updated definition in the AMS’ Glossary likely abetted the effort. There is, of course, no “official” arbiter of general meteorological terms.  This explains, in part, the range of derecho “definitions” that appear in some sources. The lack of consistency in the use of the term “derecho” was, in fact, a motivating factor in drafting the paper. We’d like to think that the paper focused needed attention on the value of increased precision in meteorological terminology, but that is not something that is easy to measure. It is probably safe to say that the definition of “derecho” will continue to change as the underlying processes responsible for the most intense storms become better understood.

BAMS: What would you like readers of your 2016 paper to learn about and from the re-defining of derecho?

SC: I think perhaps the most important take-aways are that (1) the existing definition was outdated because of the significant changes that have occurred in observational data, record-keeping, and understanding since the mid-1980s, and (2) that if we are to better understand and forecast high-wind-producing convective systems, we need to first better classify those systems that are observed.

BAMS: You and your colleagues at NOAA-NWS-NCEP put together a comprehensive and impressive website About Derechos. How did you become interested in them?

SC: Convective systems, in general, are interesting because there are so many “moving parts” involved in their development, sustenance, and motion. It is especially challenging—but also rewarding—to try to assess the strength and likely longevity of those processes in real-time so as to prepare useful forecasts. Derechos are high-impact events; they are one area where society would benefit greatly from increased meteorological knowledge.

BAMS: What was the biggest challenge you encountered in your work to update the derecho definition?

SC: Having thoughts on introducing any new idea is one thing, but putting those concepts into words is another. Unlike most other papers in our “business,” in this one I felt we had to be somewhat persuasive. I was not at first comfortable with taking such an approach. But the contrasts presented by the two derecho events introduced in the first part of the paper furthered my conviction that persuasion was needed to both defend and encourage discussion on the definition topic.

BAMS: What got you initially interested in meteorology?

SC: My interest in clouds and storms goes back to my earliest days as a young kid growing up in a house kept super-clean by my mom. Because I was forbidden to play in the yard when the grass was wet (I’d track dirt into the house, of course!), I soon came to appreciate “Mr. Sun.” In particular, I noticed that I’d often not see the sun the next day if certain cloud formations had appeared in the sky on the previous one. A bit later, in first grade, my dad introduced me to weather books in the local library. Some had cloud pictures indicating which formations were “bad,” and which were associated with good weather. I was hooked! Thunderstorm days were always a favorite because the clouds on such days seemed to change the most rapidly—and provided the most surprises. I was very interested in things like rocks and plants, too—but weather was always #1.


Through July, 2020 has been on almost the same track as 2016: the two years had the hottest first seven months in NOAA’s 141 year dataset of global surface temperatures (land and ocean combined). Since 2016 turned out to be the hottest year on record, it might seem as if this fast start puts 2020 on track to set a new record, too, or to be a near miss. NOAA says it’s already “very likely” 2020 will be among the five hottest years on record.

But in the strange reality of ever rising global temperatures, it’s not so much the first half of the year that puts 2020 on the verge of a record. It’s the underlying trend of warming: 2020 was already on the verge on New Year’s Day.

As the new State of the Climate in 2019 released last week points out, the six hottest years in the last century and a half were…exactly the last six years, 2014-2019. Due to global warming, practically every year’s surface temperature is going to be a hot one. Just by showing up at the starting line, every year is a serious threat to set a new standard.

A paper recently published in the Bulletin of the American Meteorological Society puts this relentless streak of rising temperature expectations in terms of probabilities. There’s a greater than a 99% chance that most of the next 10 years through 2028 will be ranked among the top 10 warmest.

The study, by Anthony Arguez (NOAA/NCEI) et al. also finds an 82% chance that all years in the next decade will rank in the top 15 warmest years as global warming continues. Its authors suggest that record warm years are already “baked into the cake” of Earth’s global climate and that it would take “an abrupt climate shift for even a few years within the next decade to register outside the top 10 warmest years.”

To determine these odds, the researchers analyzed the monthly version of NOAA’s Merged Land Ocean Global Surface Temperature Analysis Dataset (NOAAGlobalTemp) to project annual global temperature rankings in the future. The ever-shifting expectations for global temperatures render the usual way of keeping tabs on the data—by comparing to 30-year normals—inadequate. So Arguez et al. formulated a new way to compare each year to surrounding years:

We introduce a “temperature score”  to help NOAA communicate the coolness or warmth of a given year relative to the long-term trend. We believe this is the first such projected ranking and temperature score currently produced operationally. Our objective is to use this tool to improve the communication of climate change impacts to the general public.

Top 10The temperature score from 1 (a very cold year) to 10 (very warm) is useful to distinguish between warmer and colder years relative to the long-term trend. As examples, the authors note that 2008 and 2011 were considerably cooler than surrounding years and below the overall trend, whereas 1998 and 2016 were not only the warmest years on record but were also notably warmer than surrounding years.

The study only includes average annual global temperatures through 2018. But as reported in the annual State of the Climate, 2019 ranks as the second or third warmest year on record (depending on your favored dataset), adding another year to the recent string of those warmer than any years back to the mid 1800s. The report notes that each successive decade since 1980 has been warmer than the previous. Arguez’ research suggests that not only will this continue but it will worsen dramatically.

This is a testament to the exceptional warmth experienced over the last few decades, punctuated by the last [5] years [2015–19], which have separated themselves from “the pack.”

We asked Arguez a few questions (more found in the latest print/digital issue of BAMS) about this work as well as about his background and what sparked his interest in meteorology.

Anthony_ArguezBAMS: What would you like readers to learn from your study of record global temperatures?

Anthony Arguez: I would like the general public to know that there is not a great deal of suspense that most years—if not all—over the next decade will likely register as top 10 years. In fact, the data suggest we should expect this, as it would likely take a pretty abrupt change to get us off this trajectory.

BAMS: How did you become interested in finding new ways to analyze the global temperatures and their trend?

AA: I feel like I’ve been staring at the annual global temperature time series continually over the past 15 years or so because it is just so interesting in many ways. I find it challenging and rewarding to develop methods to translate volumes of data into answers to specific questions posed by the general public. I’ve drawn inspiration from Nate Silver, whose penchant for expounding on and communicating the “signal” hidden in the “noise” informs the way I would like to see myself and fellow climate scientists communicate to the general public more effectively.

BAMS: What surprised you most in doing this work?

AA: I was very surprised that the ranking errors we found were so small! Before calculating the results, I had a gut feeling that these errors would be modest, but the mean absolute ranking error of ~2 spots a full 10 years out was well below anything I could have imagined. I clearly under-appreciated the predictability inherent in the observed upward trend when it comes to annual global temperature rankings.

BAMS: What was the biggest challenge?

AA: I think the biggest challenge we faced was that we were not aware of any similar operational products in existence (neither for projected rankings or global annual temperature scores), or of any papers that had characterized ranking errors in a similar fashion, so we were in uncharted territory to some extent.

BAMS: This isn’t the biggest climate challenge, or surprise, you’ve ever faced…

AA: I became interested in meteorology as a teenager in 1992 when Hurricane Andrew totaled my parents’ home in Miami, Florida.



Today’s publication of State of the Climate in 2019 marks the 30th annual release in this series of supplements to the Bulletin of the American Meteorological Society. The report is not just a service for immediate use as the latest status report on climate. It’s a resource that people worldwide will use throughout the year, and indeed as a reference through the coming years. The report, now online only, is meant to stand a test of time as a bedrock of other reports and decisions.

SOCcover2Despite the rapid pace of writing, editing, and reviewing, this is obviously not the first (or last) assessment of 2019’s climate. So we still often get asked—why now? Why release in August 2020 a comprehensive, validated check-up on the health of our planet’s climate in 2019 August, instead of in January, when it’s still hot news?

Co-editor Jessica Blunden of NOAA addressed the question a number of years ago, with a helpful look behind the scenes of State of the Climate. You can appreciate, first of all, the amazing job she and coeditor Deke Arndt (also of NOAA) do to pull this all together so fast—they’re coordinating the work of more than 500 authors and chapter editors from 61 different countries. So naturally, at first glance, churning out this report in only a matter of months is a managerial triumph–a testament to international cooperation:

The production of this document really does “take a village”; without the dedication and hard work of every single one of the people who contribute to this process, the quality and scope of the report would not be possible. Each year the number of authors tends to increase as we add new information to the report.

SOCauthormap2In just the past decade alone Blunden and Arndt have added 150 authors and 13 additional countries. Why so many authors?

The authors are asked to contribute based on their expertise in a specific field. For our Regional Climates chapter, which is comprised of annual summaries for countries around the world, the authors are often affiliated with a specific country’s official meteorological/hydrological agency and provide analysis based on data from that agency. it’s not just any process of coordination. State of the Climate is an elaborate scheme to make a scientifically worthwhile document:

The development of the report is quite rigorous, with writing, two major peer-review processes, technical editing, layout, and approval. After the calendar year has ended, authors are given about six weeks to develop their content and provide an initial draft that is reviewed by the chapter editors.

Then the chapter editor has the draft reviewed by two or three scientists with expert knowledge in that field. Generally, we allow one to two weeks for this review to be completed and another one to two weeks for the authors to make revisions, as needed, and for the chapter editors to prepare the new version for a formal, external review.

The external review process involves anonymous peer reviews, and BAMS allows three weeks for these reviews to be completed. The authors and chapter editors then have two weeks to make revisions based on these comments and submit the final draft for approval.

Then there’s editing and layout and so on . . . as Blunden summarizes:

This document takes the time to provide the most accurate information available on the state of the climate system.

But the time isn’t actually about writing and reviewing; it’s the comprehensiveness of 429 pages and a bazillion references (no, we didn’t count them). A report that started as a 30-pager gets bigger and more precise with each iteration, because the value increases:

The longer a data record is and the larger the area it covers, the more useful it is for putting a particular climate indicator into context, for example comparing one year to another, or detecting trends over time. Today we are fortunate to have technologies and capabilities that were not available to us decades ago, such as satellite observations, but to use all those observations for climate research means combining observations from multiple sources into a single, seamless climate data record, which is neither fast nor easy.

With both satellite and direct observations, it is important to reconcile data discrepancies and inaccuracies so that the climate records are correct, complete, and comparable, and this painstaking process can take years. For our report, a high-quality dataset is ready for inclusion only after its development processes and methodologies have been scrutinized through peer review with published results. That way readers of the State of the Climate reports can depend on detailed journal articles if they want to understand the details of a data record.

The process of creating a climate quality data set and then having it evaluated by other scientists through peer review is so challenging, no more than a few are added to the State of the Climate report each year.

So the State of Climate is a testament to a complex process, with complex, interrelated data sources that cry out for the reconciliation and comparison that makes the report unique. And of course, all about a climate that is nothing if not the paragon of complexity.

As Deke Arndt explains about Earth’s climate (in a webinar to watch before using State of the Climate): “If the Earth didn’t spin, and we didn’t have day and night, it would be very simple.”

That sums up the reason the State of the Climate is not simple . . . or small, or fast. It is all connected.





Even tall tales have their facts, but in historical fiction the myriad factual details often far outshine the story itself. In the ever popular books of Laura Ingalls Wilder, the telling details turn out to be the truly epic—and real—weather of the past. Barbara Mayes Boustead (University of Nebraska—Lincoln) and her coauthors show us in a recent BAMS article that Wilder’s, The Long Winter, isn’t just good history wrapped into a great novel–it’s also valuable climate data.

The cold, snowy season of 1880-81 featured in The Long Winter was strikingly difficult across much of the Plains and Midwest. A number of accounts have referred to it as the “Hard Winter” or “Starvation Winter.” Wilder’s story, set in De Smet, Dakota Territory (present-day South Dakota; 60 km west of Brookings), is fiction, but it contains many verifiable facts about the weather.

Clearing snowBoustead and co-authors Martha D. Shulski and Steven D. Hilberg set out to determine which parts of Laura’s stories are based in fact, and in the process, filled in the gap left by the absence of analysis or documentation in scientific literature about the Hard Winter of 1880-81. In the process, Boustead et al. show that the Hard Winter places recent severe winters, such as 2013-14, into context.

The winter began early, with a blizzard in eastern South Dakota and surrounding areas in mid October. Following a respite thereafter, wintry conditions returned by mid-November, followed by a number of snow and potential blizzard events in December. After a cold but relatively snow-free period, storm frequency increased from early January through February, producing snow almost daily in eastern South Dakota. In March, most days remained below freezing, though snowfall frequency decreased. Cold conditions continued into the first half of April. The BAMS article goes into detail describing why the winter of 1880-81 was so severe.

pAWSSIBAMS asked a few questions of Boustead to gain insight into her research. A sampling of answers are below:

BAMS: What would you like readers to learn from your article?

Barbara Mayes Boustead: Literature and other creative work can provide windows into past weather events and climates – including everything from documentary evidence to the impacts of those events on individuals and communities. We can connect those works to other historical weather data sources, from observations to reanalysis data, to reconstruct what occurred during these noted events, and why. By researching weather and climate related to a popular-interest subject like Laura Ingalls Wilder and the Little House stories, I have been able to reach audiences that otherwise might not have been so engaged, sparking interest in weather and climate by presenting it through Laura’s perspective.

BAMS: How did you become interested in investigating the weather of Wilder’s book?

Boustead_PhotoBarbara Mayes Boustead: The Long Winter research began over a decade ago as I reread the book as a “comfort read” on the tail end of a winter, reminding myself that even the longest winters do eventually end. I’ve been reading Laura Ingalls Wilder’s books since I was in elementary school, and I had always wondered if the winter was really as Wilder had described it. And then I got to thinking – I am a meteorologist, and I have the tools to look it up! The deeper I dug, the more that my questions led to more questions. I especially got excited as I found data that verified much of the weather that Wilder had described. And I knew I had found a resonant topic when I presented the work at a conference called LauraPalooza in 2010 (it’s real and it’s serious!) and was overwhelmed with questions and discussion following my presentation.

BAMS: What got you initially interested in weather and, more importantly, these novels?

Barbara Mayes Boustead: It seems that many meteorologists started with either a memorable event or a fear of a weather phenomenon. I was in the latter group, afraid of thunderstorms in my preschool years. My mother and sister took me to the library so that I could read books about weather, hoping that understanding would help me conquer fear. I had plowed through all of the books in the library in about a year, and I was hooked! As for my interest in Laura Ingalls Wilder, I can again thank my mom and books. She purchased Little House on the Prairie for me at a garage sale when I was in first grade and ready for chapter books. I turned my nose up at it, but she encouraged me to give it a chance. I did, and of course, Mom knows best – I was hooked and plowed through the rest of the book series, too.

BAMS: What surprised you the most in doing this research?

Barbara Mayes Boustead: Laura Ingalls Wilder was an excellent weather observer. Having researched the winter of 1880-81 extensively, as well as the rest of the identifiable weather and climate phenomena throughout the Little House books, I found that while many elements of the books were fictionalized, she recounted weather and climate events with great accuracy. Almost every weather or climate detail in her books really did occur and usually occurred just as she described it. She occasionally moved some timelines around, but the events themselves were spot-on.

BAMS: What was the biggest challenge you encountered in the research?

Barbara Mayes Boustead: There were times during my research that I would have gone to great lengths to obtain true snowfall measurements from one of the observing sites near the area of interest, or to fill in the spatial gaps. Snowfall data just don’t exist for the central U.S. in the early 1880s.

BAMS: What’s next?

Barbara Mayes Boustead: Research into the weather and climate of Laura Ingalls Wilder’s books and life continues as I work to document other weather and climate events from her other books and stories. Given the popular interest in Laura Ingalls Wilder, some of the research is and will be written for broader audiences, providing a window into the world of science (meteorology and climatology) for non-specialists by standing on the shoulders of Laura Ingalls Wilder’s storytelling and characters. What began as a side project has transitioned into decades worth of research and storytelling! Her books include everything from tornadoes and hail storms to blizzards, droughts to floods, extreme cold to extreme heat. There is fodder for research for years to come!


Thursday NOAA updated its forecast to an “extremely active” Atlantic hurricane season. That has some news outlets linking the  19-25 predicted named storms to Earth’s future—even warmer—global climate. The future looks like it will indeed bring high levels of overall “activity” due to the intense, damaging hurricanes of a warming world (regardless of whether the frequency of storms overall changes). And, of course, settling into a new “norm” isn’t going to happen while warming is ongoing. But the huge number of storms forming? That’s a lot of what the public takes away from the forecast, and that profusion of named storms is not projected to be characteristic of seasons to come.

As we blogged here in May, recent research published in the Bulletin of the American Meteorological Society finds there’s no evidence to support an increasing trend in tropical cyclone frequency.

NOAA-2020-outlook-update2In that assessment of the current literature, Tom Knutson (NOAA) and other top tropical experts reviewed a number of peer-reviewed studies and determined that a majority found the numbers of named storms actually decrease in climate projections as we move deeper into this century. But there was no consensus among the authors to either support or refute those studies since their research also showed that “there is no clear observational evidence for a detectable human influence on historical global TC frequency.”

Their assessment did find that we can expect stronger and wetter hurricanes in our warming world and, notably, a possible uptick in the number of intense (Category 4 and 5) hurricanes. It’s these storms that have Knutson and his colleagues most concerned since a majority of hurricane damage is done by the big ones. Their increase is alarming even if the number of storms goes down.

Notable with this week’s forecast update is a prediction close to record territory. “We’ve never forecast up to 25 named storms” before—more than twice a season’s typical 12—noted Jerry Bell, lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center. He went on to say there will be “more, stronger, and longer-lived storms than average” in the Atlantic Basin, which includes the Caribbean Sea and Gulf 0f Mexico. In an average season there are six hurricanes,  and three of those grow into major hurricanes.


Tropical Storm Isaias is soaking the Mid-Atlantic states with what is expected to be three times as much rain as is typical for the area. Today’s heaviest tropical showers could trigger potentially deadly flash floods.

The projection is the finding of a new Intuitive Metric for Deadly Tropical Cyclone Rains, which we blogged about on The Front Page in June. The extreme rainfall multiplier (ERM) used the quantitative precipitation forecast (QPF) from the Storm Prediction Center last night to generate an ERM forecast for Isaias.


“Since Isaias is a fast-moving storm (currently moving NNE at 23 mph), the heaviest rain is forecast to fall with[in] a 24-hour period today (Aug 4)”, wrote the study’s lead author, Christopher Bosma, a Ph.D. student at the University of Wisconsin-Madison, in a-pre-dawn e-mail. “Peak rainfall totals are projected to be just over 6 inches (approx. 150 mm), mostly in a narrow region just south of the DC Metro [area].”

In contrast, the region’s heaviest single-day, 2-year rainfall event is a bit more than 50 mm. Bosma uses that comparison in generating an ERM around 2.86 (152 mm / 53 mm). Rainfall may exceed the projections, but that gives a rough idea of how the storm compares to others in residents’ recent memory.

According to the study, which was published in the Bulletin of the American Meteorological Society in May, the average value of an ERM in U.S. landfalling hurricanes and tropical storms is 2.0. ERMs can hindcast the severity of precipitation for such storms, like 2017’s Hurricane Harvey. Harvey deluged Texas with as much as 60 inches of rain and reached an ERM of 6.4—the highest calculated.

Those having lived in the D.C. area in the early 2000s might recall a tropical storm that Bosma says is comparable to Isaias: Isabel. After landfall in eastern North Carolina as a Cat. 2 hurricane the morning of September 18, 2003, it barreled north-northwest through the Mid-Atlantic delivering flooding rains and damaging winds that night.

“Isabel was also a fast mover at landfall, and was responsible for similar one-day rain totals of just over 6 inches, based on CPC-Unified gauge-based gridded data,” Bosma wrote.” The peak ERM for Isabel was 2.8. One thing to note from Isabel is that localized rainfall totals were higher in some spots, particularly in the mountains of Virginia, highlighting the threat of localized flash flooding that might also be present today with Isaias.”


Indeed, flash flood warnings were issued all across the interior Mid-Atlantic this morning. This was despite drought conditions in parts of the area.

Bosma and colleagues Daniel Wright (UW-Madison), J. Marshall Shepherd (University of Georgia), et al., created the ERM metric to focus on the deadly hazard of extreme tropical cyclone rainfall. Getting word out about the threat using only the wind-based Saffir-Simpson Scale “was a problem brought to light with Hurricanes Harvey and Florence,” Shepherd says.

Wright also in an e-mail last night stated that for Isaias in and around Washington, D.C., it’s “a fairly large amount of rain, though certainly not unprecedented for the region.”



All the research ships and aircraft of atmospheric science may never be able to gather in one place for testing. But small, portable unmanned aircraft systems (UAS) are another matter. An international vanguard of scientists developing these atmospheric observing capabilities is finding that it is really helpful to get together to pool their insights—and devices—to accelerate each other’s progress. Together, their technology is taking off.

In the May 2020 BAMS, Gijs de Boer (CIRES and NOAA) and colleagues overview one of these coordinate-and-compare campaigns: when 10 teams from around the world brought 34 UAS to Colorado’s San Luis Valley for a week of tests, laying groundwork for new collaborations and future field programs. The July 2018 flight-fest conducted 1,300 research flights totaling more than 250 flight hours focused on observing the intricacies of the lower atmosphere.

Dubbed the LAPSE-RATE campaign—Lower Atmospheric Profiling Studies at Elevation–A Remotely-Piloted Aircraft Team Experiment—it was one of the fruits of a new community of scientists, the International Society for Atmospheric Research Using Remotely-Piloted Aircraft (ISARRA).

UAV_launch_ready2At a “Community Day,” the scientists shared their aircraft and interests with the public as well. Working together all in one place has huge benefits. The teams get to see how they compare with each other, work out the kinks with their UAS, and move faster toward their research goals. It’s one reason they are getting so good so fast.

Below, de Boer answers some questions about the campaign and how he got started with UAS.

BAMS: What are some of the shared problems revealed by working together—as in LAPSE-RATE—with other UAS teams?
Gijs de Boer: There are common problems at a variety of levels.  For example, accurate wind sensing has proven challenging, and we’ve definitely worked together to improve wind estimation. Additionally, different modes of operation, understanding which sensors are good and which are not, and sensor placement are all examples of how the community has worked together to lift up the quality of measurements from all platforms.

BAMSWhat are the most surprising lessons from LAPSE-RATE?
GdB: I think that the continued rapid progression of the technology and the innovation in UAS-based atmospheric research is impressive.  Some of the tools deployed during LAPSE-RATE in 2018 have already been significantly improved upon.

BAMS: What are some examples of this more recent UAS improvement?
GdB: Everything continues to get smaller and lighter.  Aircraft have become even more reliable, and instrumentation has continued to be scrutinized to improve data quality.  Battery technology has also continued to improve, allowing for longer flight times and more complex missions.

Yet, we have so much more to do with respect to integrating our measurements into mainstream atmospheric research.

BAMS: What are some challenges to doing more to integrate UAS into research?
GdB: Primarily, our UAV research community is working to demonstrate the reliability and accuracy of our measurements and platforms.  This is critical to having them accepted in the community.  There are also some other challenges associated with airspace access and development of infrastructure to interface these observations in both mainstream research and operations.

BAMS: It seems like there’s been success in this mainstreamed usage of UAS.
GdB: Campaigns like LAPSE-RATE have paved the way for UAS to be more thoroughly included in larger field campaigns.  A nice example is the recent ATOMIC (Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign) and EUREC4A (Elucidating the role of clouds-circulation coupling in climate) field campaigns, where three different UAS teams were involved and UAS were operated alongside manned research aircraft and in support of a much larger effort.

BAMS: How did you become interested in unmanned aviation?
GdB: In 2011, I worked with a small group on a review article about our knowledge of mixed-phase clouds in Arctic environments.  We took a good look at critical observational deficiencies, and I began to realize that many of the gaps involved a lack of in situ information, quantities that I thought could be measured by small platforms. This sent me down the road of investigating whether UAS could offer the necessary insight.


The Saildrone vehicle returning to San Francisco on 11 Jun 2018. The wind anemometer is visible at the top of the wing and solar panels are on both the wing and the vehicle hull. Image credit: Saildrone/Gentemann.
The Saildrone vehicle returning to San Francisco on June 11, 2018. The wind anemometer is visible at the top of the wing and solar panels are on both the wing and the vehicle hull. Image credit: Saildrone/Gentemann.


You’ve heard of drones in the air, but how about on the ocean’s surface? Enter Saildrone: A new wind and solar powered ocean-observing platform that carries a sophisticated suite of scientific sensors to observe air–sea fluxes. Looking like a large windsurfer without the surfer, the sailing drone glides autonomously at 2–8 kts. along the surface of uninhabited oceans on missions as long as 12 months, sampling key variables in the marine environment.

In a recent paper published in the Bulletin of the American Meteorological Society, author Chelle Gentemann and her colleagues explain that from April 11 to June 11, 2018, Saildrone cruised on a 60-day round trip from San Francisco down the coast to Mexico’s Guadelupe Island to establish the accuracy of its new measurements. These were made to validate air–sea fluxes, sea surface temperatures, and wind vectors derived by satellites. The automated surface vehicle also studied upwelling dynamics, river plumes, and the air–sea interactions of both frontal and diurnal warming regions on this deployment—meaning Saildrone’s versatile array of instruments got a workout not only above surface but just below it as well, in the water along the hull.

BAMS asked a few questions of the authors to gain insight into their research as well as their backgrounds. A sampling of answers are below:

Chelle Gentemann Chelle Gentemann

BAMSWhat would you like readers to learn from your article?

Chelle Gentemann, Farallon Institute: New measurement approaches are always being developed, allowing for new approaches to science. Understanding a dataset’s characteristics and uncertainties is important to have confidence in derived results.

BAMSHow did you become interested in working with Saildrone?

Gentemann: The ocean is a challenging environment to work in: it can be beautiful but dangerous, and gathering ship observations can require long absences from your family.  I learned about Saildrones in 2016 and wanted to see how an autonomous vehicle might be able to gather data at the air–sea interface and adapt sampling to changing conditions.  There are some questions that are hard to get at from existing remote sensing and in situ datasets; I thought that if these vehicles are able to collect high-quality data, they could be useful for science.

BAMSHow have you followed up on this experiment? 

Gentemann: We sent two more [Saildrones] to the Arctic last Summer (2019) and are planning for two more in 2021.  There are few in situ observations in the Arctic Ocean because of the seasonal ice cover, so sending Saildrones up there for the summer has allowed us to sample temperature and salinity fronts during a record heat wave.

Sebastien de Halleux, Saildrone, Inc.: I believe we are on the cusp of a new golden age in oceanography, as a wave of new enabling technologies is making planetary-scale in situ observations technically and economically feasible. The fact that Saildrones are zero-emission is a big bonus as we try to reduce our carbon footprint. I am excited to engage further with the science community to explore new ways of using this technology and developing tools to further the value of the data collected for the benefit of humanity.

BAMSWhat got you initially interested in oceanography?

de Halleux: Having had the opportunity to sail across the Pacific several times, I developed a strong interest in learning more about the 70% of the planet covered by water—only to realize that the challenge of collecting data is formidable over such a vast domain. Being exposed to  the amazing power of satellites to produce large-scale remote sensing datasets was only tempered by the realization of their challenges with fine features, land proximity, and of course the need to connect them to subsurface phenomena. This is how we began to explore the intersection of science, robotics, and big data with the goal to help enable new insights. Yet we are only at the beginning of an amazing journey.

BAMS: What surprises/surprised you the most about Saildrone’s capabilities?

Peter Minnett, Univ. of Miami, Florida: The ability to reprogram the vehicles in real time to focus on sampling and resampling interesting surface features. The quality of the measurements is impressive.

Saildrones are currently deployed around the world. In June 2019 , there were three circumnavigating Antarctica, six in the U.S. Arctic, seven surveying fish stock off the U.S. West Coast and two in Norway, four surveying the tropical Pacific, and one conducting a multibeam bathymetry survey in the Gulf of Mexico. In 2020, Saildrone, Inc. has deployed fleets in Europe, the Arctic, the tropical Pacific, along the West Coast, the Gulf of Mexico, the Atlantic, the Caribbean, and Antarctica. NOAA and NASA-funded Saildrone data are distributed openly and publicly.


The Thanksgiving holiday weekend has long been heralded as the start of the Western United States winter ski season. But new research using regional climate models sees Thanksgiving skiing going cold turkey.

As climate change ramps up into the mid twenty-first century, we can expect shorter ski seasons from the Southwest to the northern Rockies. This includes projections for less snow as well as poorer conditions for artificial snowmaking in the mountain states of the interior West. These are the findings from new research presented by Christian Lackner (Univ. of Wyoming and Johannes Gutenberg-Univ. of Mainz) this week at the American Meteorological Society’s 19th Conference on Mountain Meteorology. Despite being entirely on-line, the meeting achieved record attendance.

Large decreases in the percentage of years with at least snow days during the Thanksgiving period, Nov. 22 - Dec. 1.
Large decreases in the percentage of years with at least 8 snow days at Rocky Mountain ski resorts during the Thanksgiving period, Nov. 22 – Dec. 1.


Lackner’s presentation, co-authored with Bart Geerts and Yonggang Wang, showed that the downturn in the ski season is projected to impact lower-elevation ski areas such as those in Arizona and New Mexico the most. Ski seasons by 2050 will start about two weeks later and end two-to-three weeks earlier than in the baseline period of 1981-2010. For many resorts that means the season length is seen to fall below the 100-day threshold long viewed as the make-it-or-break point for staying viable in the ski industry.

Higher-elevation ski resorts in Colorado, Utah, and western Wyoming, as well as higher latitude ski areas in Montana and Idaho, will fair better, although they’ll see their seasons shrink by 10-20 days. That will drop them below 120 days—the high-elevation, high-latitude resorts’ economic threshold—by 2050.

Lackner et al.’s study looked at climate change impacts at 71 ski resorts in Arizona, Colorado, Idaho, Montana, New Mexico, and Wyoming from November 15-April 15, the key cold-season months.

The good news is the Christmas holiday week still looks good for shooshing down Western slopes, despite the climate projections.

Large decreases in the percentage of years with at least snow days during the Thanksgiving period, Nov. 22 – Dec. 1
Almost no change in the percentage of years with at least 8 snow days at Rocky Mountain ski resorts during the Christmas period, Dec. 23 – Jan. 1.


Observations and models–that’s often an uneasy relationship. It’s not always easy to find the common ground needed to turn observations into model input and then models themselves into physically realistic output consistent with those observations.

DOE’s Atmospheric Radiation Measurement (ARM) program is trying to pull observing and modeling—ranging over vast time and space scales—tighter together, into effective bundles of science. Naturally, they’re using an initiative called “LASSO.”

Focused on shallow convection (often small, low-level scattered clouds), LASSO, or, the “Large-Eddy Simulation (LES) ARM Symbiotic Simulation and Observation” project centers on the capabilities of DOE’s Southern Plains observatory in Oklahoma. LASSO is designed to “add value to observations” through a carefully crafted modeling framework that evaluates how well the model “captures reality,” write William I. Gustafson and his colleagues in a paper recently published in the Bulletin of the American Meteorological Society (BAMS).

Renderings of cloud water isosurfaces (10−6 kg−1), every 2 h, show the diurnal evolution of a cloud field from a simulation forced by the VARANAL large-scale forcing on 30 Aug 2017. Cloud shadows can be seen in the surface downwelling shortwave radiation (colors; W m−2).
Renderings of cloud water isosurfaces (10−6 kg−1), every 2 h, show the diurnal evolution of a cloud field from a simulation forced by the VARANAL large-scale forcing on 30 Aug 2017. Cloud shadows can be seen in the surface downwelling shortwave radiation (colors; W m−2).


LASSO bundles data such as observations, LES input and output, and “quick-look” plots of the observations into a library of cases for study by modelers, theoreticians, and observationalists. LASSO includes diagnostics and skill scores of the extensive observations in the bundles and makes them freely available with simplified access for speedy use.

The goal of the data packaging approach is to enable scientists to more easily bridge the gap from smaller scale measurements and processes to the larger scale at which most modeling parameterizations operate.

We asked Gustafson to explain:

BAMS: What would you like readers to learn from this article?

Gustafson: In the atmospheric sciences we work with so many scales that we often get siloed into thinking in very scale-specific ways based on our sub-specialty and the type of research we do. This can happen whether we are modelers trying to wrap our brains around comparing parcel model simulations with global climate models, or as observationalists trying to rationalize differences between point-based surface measurements and big, pixel-based satellite measurements. The LASSO project is one attempt to get past limitations sometimes imposed by certain scales. For example, the DOE ARM program has such a wealth of measurements, and at the same time, DOE is developing a new and improved climate model. LASSO is one way to help marry the two together to add value for researchers working with both sets of data.

How did you become interested in the topic of this article?

My training is as a modeler, and over the years, a lot of my research has looked at issues of scale and how atmospheric models can better deal with unresolved detail—the so-called subgrid information. We know that subgrid information can be critical for properly simulating things like clouds and radiation. Yet, we cannot run global models with sufficient resolution to track this information. So, we need tools like large-eddy simulation to help us make better physics parameterizations for the coarser models used for weather prediction and climate change projections. Marrying the LES more tightly with observations seemed like a great way to help the atmospheric community move forward and make progress improving the models.

What got you initially interested in meteorology or the related field you are in?

I find weather fascinating and awe inspiring, and science has always been one of my interests alongside computers. Coming out of my undergrad years with a physics degree, I knew I wanted to pursue something related to computing, but I did not want to do it for a company for the sole purpose of making money for somebody. Atmospheric modeling seemed like a great way to apply my computer interests in an impactful way that would also be a lot of fun. Not many people get to play on giant supercomputers for a living trying to figure out what makes clouds do what they do. I have never looked back and much of my job I see as a grown-up playground where I get to build with computer bits instead of the sand I used to play with as a kid.

What surprises/surprised you the most about the work you document in this article?

This is not an article with filled with “ahah” moments. It is the result of years of effort put into developing a new data product that combines input from a large number of people with many different specialties. So, I would not say that I came across surprises.

However, I have come to really appreciate the help from so many people to make LASSO happen. We have people helping to collect input from dozens of instruments that have had to be maintained, data that has to be quality controlled, computers that are maintained, the actual modeling and packaging of the observations with the model output, the database and website development to make the product findable by users, the backhouse archive support, and communications specialists that have all been
critical to make LASSO happen.

What was the biggest challenge you encountered while doing this work?

Working with a long-term dataset has been one of our big challenges. We have been trying to put together a standardized data bundle that would make it easy for researchers to compare simulations from different cases spanning years. However, instrumentation changes from year to year, which means we continually have to adapt. Sometimes this presents itself as a new opportunity because of a new capability, such as a new photogrammetric cloud fraction product we are starting to work with. Other
times, existing instruments malfunction or are replaced with instruments that do not have the same capabilities, such as a switch from a two-channel to a three-channel microwave radiometer. The latter, in theory, could offer improved results, but in reality, led to years of calibration issues.

What’s next? How will you follow up?

The LASSO activity has been well received and we are excited to be expanding to new weather regimes. During 2020 we have been developing a new LASSO scenario that focuses on deep convection in Argentina. This is really exciting because storms in this area are some of the tallest in the world. It will also be a lot of fun working with LES of deep convection with all its associated cloud motions and detail. We plan to have this new scenario ready for release in 2021.

William Gustafson visited the DOE ARM's Southern Great Plains Central Facility in 2016 near the beginning of the LASSO activity. Seeing the locations of the instruments within their natural environment has really helped put the them into context and use them in conjunction with the LASSO LES modeling.
William Gustafson visited the DOE ARM’s Southern Great Plains Central Facility in 2016 near the beginning of the LASSO activity. Seeing the locations of the instruments within their natural environment has really helped put them into context and use them in conjunction with the LASSO LES modeling.