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Outlook for the remainder of hurricane season

By: Dr. Jeff Masters, 1:57 PM GMT on September 11, 2009

Hurricane Fred continues its slow decline, as wind shear of 25 knots tilts and stretches the storm, allowing dry air to penetrate into the hurricane core from the southwest side. With sea surface temperatures down near the threshold needed to support a tropical cyclone and wind shear expected to increase to 30 knots Saturday, Fred should continue to weaken the next few days. The storm's very slow motion will hasten this decay, since the storm will stir up cold waters from the depths that it will not be able to move away from. Fred will likely die by Tuesday.

Figure 1. Hurricane Fred (upper left) and a new tropical disturbance (right), newly emerged from the coast of Africa. At the time, Fred was a Category 1 hurricane with 85 mph winds.

Elsewhere in the tropics
A large tropical wave emerged from the coast of Africa last night and has a well-organized surface circulation, as seen on this morning's QuikSCAT pass. The wave as yet does not have much in the way of heavy thunderstorm activity associated with it, and wind shear is relatively high, about 20 knots. By Sunday, the shear will drop below 20 knots, and the wave may begin to develop. The GFS model predicts this will be a tropical depression by early next week.

A low pressure system is over eastern Texas and the adjoining waters along the Texas Gulf of Mexico coast. This low is under high shear, about 25 knots, and is not tropical. Shear is expected to remain high, 20 - 30 knots, over the next five days. The low should remain non-tropical during this time, but will bring much-needed heavy rains to drought-stricken south Texas. Flooding problems may also occur, and flash flood watches have been posted for six counties in extreme South Texas.

Early next week, we should be alert for tropical storm development along an old frontal zone stretching from the Florida Keys across the Florida Peninsula to the waters between the Bahamas and North Carolina. The NOGAPS model predicts development could occur on the Gulf side of Florida, and the GFS model forecasts that development is more likely on the east side of Florida.

Figure 2. Departure of Sea Surface Temperature (SST) from average for September 10, 2009, shows that SSTs were 0.5 - 1.0°C above average over most of the Main Development Region (MDR) for hurricanes in the Atlantic, from the coast of Africa to Central America, between 10°N and 20°N. Image credit: NOAA/NESDIS.

Outlook for the remainder of hurricane season
September 10 marks the halfway point of the Atlantic hurricane season, and the season has thus far been average to a bit below average. What, then, can we expect for the remainder of the season? First, let's take a look at Sea Surface Temperatures (Figure 2). SSTs are 0.5 - 1.0°C above average over most of the Main Development Region (MDR) for hurricanes in the Atlantic, from the coast of Africa to Central America, between 10°N and 20°N. This means that the ocean will be capable of supporting the development of major hurricanes well into the middle of October. Approximately 85% of all major hurricanes form in the Main Development Region.

Wind shear and El Niño
The current weak El Niño conditions in the tropical Eastern Pacific show no signs of weakening, and these El Niño conditions are likely contributing to the enhanced levels of wind shear observed over the Caribbean over the past month (Figure 3). However, wind shear has been near average over the remainder of the Main Development Region, which allowed two major hurricanes (Bill and Fred) to form in the region between Africa and the Lesser Antilles Islands. The latest 16-day forecast from the GFS model projects near-average wind shear levels across the Main Development Region for the remainder of September. The latest long range wind shear forecast from NOAA's CFS model predicts high levels of wind shear for the entire tropical Atlantic for the remainder of hurricane season--the typical pattern for an El Niño year. This model's September wind shear forecast doesn't look very accurate, so I mistrust the model's call for high wind shear for the remainder of hurricane season. It does not appear that this year's El Niño conditions are resulting in as much wind shear as a typical El Niño event generates over the tropical Atlantic. This may be because this year's El Niño conditions have some characteristics of a modiki El Niño. These "modiki" El Niños tend to have warming concentrated in the Central Pacific, instead of the Eastern Pacific. This year, the warming is greatest in the Eastern Pacific (see Figure 2), but there is also quite a bit of warming in the Central Pacific.

Figure 3. Departure of wind shear from average for the 31-day period August 9 - September 8, 2009. Wind shear has been above average over the Caribbean and Gulf of Mexico, and much below average over the Eastern Pacific--a typical pattern observed during El Niño years. Image credit: NOAA Climate Prediction Center.

Steering Currents
We've been lucky this year that the steering currents have aligned to keep our two major hurricanes, Bill and Fred, out to sea. This year's hurricane season has featured a very favorable steering current pattern, with an abnormal number of intense troughs of low pressure over the U.S. East Coast. These troughs have brought westerly winds far to the south, which have acted to recurve to the north all the tropical storms that have formed from African tropical waves. The latest 16-day forecast from the GFS model shows no change to this steering current pattern. It will remain difficult for tropical storms spawned from African tropical waves to make the long crossing of the Atlantic and impact the Caribbean, Bahamas, or Florida. The land areas at greatest risk of receiving strikes from hurricanes that develop from African tropical waves will continue to be Bermuda, North Carolina, New England, and the Canadian Maritime provinces.

Only one hurricane has made landfall this season--Category 1 Hurricane Bill, which did minimal damage to Newfoundland, Canada. Although it is an El Niño year, and the steering current pattern will continue to be favorable for keeping most of our storms out to sea, I expect we will get a hurricane strike somewhere in the Atlantic this season that will require a disaster response.

Twenty years ago on this date
On September 11, 1989, Tropical Depression Twelve continued to grow more organized, building a large region of heavy thunderstorms near its center. Two hooking spiral bands formed, prompting the National Hurricane Center to upgrade the depression to a tropical storm in their 11 am advisory. The new storm's name: Hugo. Tropical Storm Hugo continued to trek westward across the open Atlantic at 20 mph, still four days from the Lesser Antilles Islands.

That day at NOAA's Miami-based Office of Aircraft Operations--the hurricane hunting division of NOAA--we joked about the fearsome new storm with the same name as the director of NOAA's Atlantic Oceanographic and Marine Laboratory (AOML), Hugo Bezdek. AOML housed the offices of NOAA's Hurricane Research Division, whose scientists would decide whether or not our hurricane hunting group would intercept the new storm once it got close enough to the Lesser Antilles Islands. Even if Hugo was a dud, we figured we'd be flying the storm for sure, since it shared the same first name as the big boss of the hurricane research scientists.

After work that evening, I celebrated my 29th birthday by biking through the sun-dappled shaded streets of Coconut Grove. As I stopped to watch a perfect tropical fuchsia-red sunset, my thoughts roamed out over the eastern horizon. What kind of birthday present had the weather gods delivered me today? I was first on the list of flight meteorologists that would deploy to meet Hugo, should we fly the storm.

Figure 4. AVHRR visible satellite image of Tropical Storm Hugo taken on September 11, 1989. Image credit: Google Earth rendition of the NOAA HURSAT data base.

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.