hurricanes

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.

In 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.

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.
In 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.”

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.”