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NOAA hurricane forecast still calling for very active season

By: Dr. Jeff Masters, 3:10 PM GMT on August 09, 2007

NOAA released their August Atlantic Hurricane Season Outlook Update today. The forecast calls for a likely range of 13-16 named storms, 7-9 hurricanes, and 3-5 major hurricanes. The forecast is almost unchanged from their May 22 forecast of 13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes (a normal season has 10-11 named storms, 6 hurricanes, and 2 intense hurricanes). The forecast team cites the lack of El Nino, sea surface temperatures in the western tropical Atlantic and Caribbean Sea well above average (+0.56 C), and the elements shown in Figure 1 below, as justification for their continued forecast of much above normal hurricane activity this year. They also note:

Historically, similar conditions have typically produced 2-4 hurricane strikes in the continental United States and 2-3 hurricanes in the region around the Caribbean Sea. However, it is currently not possible to confidently predict at these extended ranges the number or intensity of landfalling hurricanes, or whether a given locality will be impacted by a hurricane this season.


Figure 1. Graphic of the meteorological justifications for NOAA's forecast of a much-above normal Atlantic hurricane season.

NOAA (13-16 storms), the Colorado State Dr. Gray/Phil Klotzbach team (15 storms), and TSR (15 storms) are all calling for an awful lot of tropical storm activity in a relatively short period of time. I think it is likely that the total number of named storms this year will be at the lower end of NOAA's range--13. This is close to what the new UKMET office forecast was calling for in June--12 named storms. Given the current SST patterns and behavior of the steering currents, at least one major hurricane affecting the Caribbean and one major hurricane hitting the U.S. is a good bet this season.

Tropical update
The Tropical Atlantic is relatively quiet. The Intertropical Convergence Zone (ITCZ), the region of strong thunderstorms usually present between Africa and the Lesser Antilles Islands, has been almost absent the past three days. Thunderstorm activity in association with a surface trough of low pressure is over the Dominican Republic and extending northwards over the southeastern Bahama Islands and southwards over the central Caribbean. There is 20-30 knots of wind shear over this region, and wind shear is expected to remain too high to allow any development over the next few days. An area of thunderstorms off the North Carolina/Virginia coast is associated with the tail end of a cold front that pushed off the East Coast yesterday. Sea surface temperatures are warm enough (80-84 F) and wind shear is low enough (15-20 knots) to allow some slow development over the next few days. However, conditions are marginal enough that I'd be surprised to see a tropical depression form here.



What the computer models forecast
Most of the computer models are forecasting very low levels of wind shear for the western Caribbean and southern Gulf of Mexico developing early next week. Two computer models, the NOGAPS and ECMWF, predict a tropical depression could form in this region as early as Tuesday. The GFS and UKMET models do not, but do show something developing off the coast of Africa next week. We are starting to approach the peak part of hurricane season, and I expect that our next tropical depression will form in one of these two areas by the end of next week.

Pacific storms
Tropical Storm Flossie formed yesterday in the Eastern Pacific, and could be a threat to Hawaii by Wednesday. In the Western Pacific, Tropical Depression Pabuk hit China this morning, bringing up to 10 inches of rain. Pabuk killed 11 in the Philippines earlier this week in rain-triggered landslides. However, Pabuk ended a 3-month long drought in the Philippines that had priests throughout the country urging their parishioners to pray for rain. Rains from Pabuk also added to flooding problems in Vietnam, where at least 43 people have died in severe flooding this week.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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