Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 8:26 PM GMT on August 08, 2006
NOAA's August 8 hurricane forecast was issued today, and calls for a less active season than their May 22 outlook did. However, they still predict a 75% chance of a more active than usual hurricane season, with 12-15 named storms, 7-9 hurricanes, and 3-4 intense hurricanes. An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. NOAA's May 22 forecast called for 13-16 named storms, 8-10 hurricanes, and 4-6 major hurricanes. The key statistic that should cheer us up is the forecast reduction in the number of intense hurricanes, by 1-2.
NOAA follows the lead of the other major forecasting groups, which have all reduced their forecast number of named storms and hurricanes by 1 or 2 since May. Here's a comparison of what the four groups currently are forecasting:
NOAA Aug 8 forecast: 12-15 named storms, 7-9 hurricanes, 3-4 intense hurricanes.
Dr. Bill Gray Aug 3 forecast: 15 named storms, 7 hurricanes, 3 intense hurricanes.
Cuba Institute of Meteorology Aug 1 forecast: 15 named storms, 9 hurricanes. Last named storm ends in mid-November.
Tropical Storm Risk, Inc. Aug 4 forecast: 16 named storms, 8 hurricanes, 3-4 intense hurricanes.
Why the reduction in storms?
NOAA cites three main reasons for reducing their forecast numbers:
1) Sea surface temperatures anomalies (departures from normal) cooled during June and July. This happened due to stronger trade winds over the Atlantic. In addition, surface pressures have been higher than average. Hurricane formation is enhanced when lower surface pressures are present.
2) La Nina died quicker than expected. This has resulted in higher wind shear over the Atlantic.
3) The persistent upper-level ridge (and associated westward extension of the Bermuda High) over the eastern U.S., which contributed to the extremely active 2003-2005 hurricane seasons, is not present this year.
NOAA does not make seasonal forecasts of where hurricanes might make landfall, but notes that similar above-normal seasons have historically averaged 2-3 landfalling hurricanes in the continental United States and 2-3 hurricanes in the region around the Caribbean Sea.
Tropical wave approaching the Lesser Antilles
A tropical wave moving westward at 15-20 mph near 13N 49W, about 700 miles east of the Lesser Antilles Islands, is looking more impressive this afternoon. A blow-up of intense thunderstorms has developed over the circulation center, which is a key sign that a tropical depression may be trying to form. It remains to be seen if the storm can hang onto this thunderstorm activity; 10-20 knots of wind shear are still interfering. This shear has been oscillating in strength during the last two days, periodically blowing away all the heavy thunderstorms the storm has managed to build. I wouldn't be surprised to see a sudden increase in shear rip away most of this current burst of deep convection. However, I think it more likely that the storm will hang onto this burst and become a tropical depression by Wednesday night.
The wind shear is greatest to the system's north, so the further south it can stay, the more likely it is to develop. If the storm does make it into the Caribbean, its chances are much better than if it turns more west-northwest and takes aim at Puerto Rico. The wind shear to the north is expected to retreat a bit to the north over the next two days, and should stay in the 10-20 knot range over the wave, which is low enough to allow a tropical depression to form. Dry air will continue to be a problem for the wave, and will likely keep development slow.
Last night's GFDL model predicted that the wave would develop by Thursday into a weak tropical storm, and move through the central Lesser Antilles Islands and into the Caribbean Sea. This morning's run of the GFDL model is no longer showing any development, and none of the other computer models develop the wave.
The wave should move through the Lesser Antilles Islands on Thursday morning. At this point, if I had travel plans in the Caribbean, I wouldn't change them, since any development of this system is likely to be slow. However, I would check the situation frequently, as surprises are common. The Hurricane Hunters are on call to investigate the wave on Wednesday at 2pm EDT.
Figure 1. Preliminary model tracks for the mid-Atlantic tropical wave.
Elsewhere in the tropics
A tropical wave moving through the Bahamas and South Florida is under 20 knots of wind shear and is not expected to develop.
I'll be back Wednesday morning with an update.
Comments will take a few seconds to appear.