Time to Tone Down Hurricane Season Prognostications?

August 3, 2010 · 3 comments

With a third of the Atlantic hurricane season over and just three storms named (albeit accompanied by one tropical depression), should hurricane season prognosticators consider backing down from their early season forecasts of a wild season? And we’re not just talking about one or two Punxatawny Phils here — this year realized eight separate forecasts of named storms and hurricanes for the six-month season, which began June 1. Predictions of the number of named storms ranged from 17 to a lofty 23 — far above the average of 11 named storms realized over the last 60 years.

2010 hurricane season forecasts

The real meat of hurricane season is from mid August through mid October, when about 90% of a season’s storms form. Based on the May and June forecasts, that would equate to about 15-21 tropical storms and hurricanes — still a substantially busy season. But the chatter has begun on the blogs (2nd topic on this page) and in the online and mainstream news that this year will not be like 2005. By this point in that season the Atlantic had already seen eight named storms, including two major hurricanes. The 2005 season went on to realize 27 named storms, including Category 5 Hurricanes Katrina, Rita, and Wilma, and one unnamed storm added to the tally in the post-season.

So what drove the early season forecasts? And why might they need to be lowered? As in 2005, sea surface temperatures (SSTs) across the Atlantic basin have been well above average since spring. In fact, record warm SSTs have dominated the main tropical cyclone development region—from 10°N to 20°N between the coast of Africa and Central America (20°W – 80°W)—for five consecutive months (see the 2nd topic  entry on this page). Combine that with lower-than-normal surface pressure basin wide and the fact that El Niño was not only ending but appeared poised to transition to La Niña conditions (which it did) in the tropical Pacific, both of which are factors that can lead to more than the usual number of storms, and forecasters had almost no choice but to set their sights rather high. Conditions appeared very favorable for a quick start to a long and busy season, not unlike 2005.

Problem is, that hasn’t happened. The tropical cyclones that have developed this year have struggled. Despite all the favorable features, it appears dry air and more importantly strong wind shear across the Caribbean and Gulf of Mexico in June and July have kept storms in check. Typically, the atmosphere over the Atlantic Basin moistens significantly starting in August as the westward-moving Saharan dust outbreaks wane. And seasonal wind shear also becomes more conducive for storm development by August. Still, the next four months would need to see the pace of tropical storm and hurricane formation come fast and furious to realize the forecasts. It could happen: in 1995, 16 tropical storms and hurricanes, including five major hurricanes, formed one after another after another from the last days of July through the end of October, leaving just 10 days in the three-month period free of any storms. But that kind of hurricane history isn’t likely to repeat itself. Even 2005 had more storm-free days in the same portion of the season.

So what will forecasters do? Time will tell as two of the leading forecast teams—NOAA and the Colorado State University Tropical Meteorology Project, led by Phil Klotzbach and William Gray—update their forecasts this week. (Check these links for their updated forecasts: CSU (Aug. 4) and NOAA (Aug. 5).

Update …

Largely Unchanged Forecasts Point to Busy Months Ahead

With their August updates, hurricane season forecasters have left their predictions generally intact. The CSU forecast (pdf file) remains the same with 18 named storms total, 10 hurricanes, and 5 major hurricanes. After increasing its forecast in July by one named storm that was likely to be a hurricane, Tropical Storm Risk (TSR), a private British forecasting company, is back to its early June forecast numbers, with its August update (pdf file) again calling for about 18 named storms, 10 hurricanes, and 4-5 major hurricanes projected. NOAA shaved down the upper range of its seasonal forecast numbers while keeping the lower end of the range intact for named storms and hurricanes, and narrowing the range of major hurricane expected. Its updated forecast is for 14-20 named storms, 8-12 hurricanes, and 4-6 major hurricanes, versus its June forecast of 14-23 storms, 8-14 hurricanes and 3-7 major hurricanes. Florida State University lowered its prediction by 2, from 17 to 15 named storms and from 10 to 8 hurricanes.