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 , 2:12 PM GMT on July 29, 2007
A tropical disturbance (98L) a few hundred miles north of the central Bahamas has become only a little better organized since yesterday. Heavy thunderstorm activity has increased, and the beginnings of some upper-level outflow to the northeast is apparent on visible satellite loops. A QuikSCAT satellite pass from 6:38am this morning revealed that 98L is attempting to form a closed circulation at the surface, and had top winds of at least 25 mph.
Water vapor satellite loops show an upper-level low pressure system to 98L's northeast, and this upper low is bringing about 15 knots of wind shear over the disturbance. The GFS and GFDL models predict that the upper low will move north-northeast in tandem with the disturbance, keeping low enough shear over it that a tropical depression could form. The other reliable models do not develop 98L. At present, it appears that Bermuda is the only place that needs to concern itself with 98L. SSTs are warm enough to support tropical storm formation until 98L reaches a point 500 miles or so north of Bermuda. The Air Force Hurricane Hunter aircraft scheduled to investigate 98L this afternoon was canceled, and no new flights are planned.
Figure 1. Preliminary model tracks for 98L.
Early August hurricane outlook
In the first half of August, Atlantic tropical cyclone activity starts to pick up. Since the current active hurricane period began in 1995, eight of 12 years have had one or more named storms form during the first half of August, including the last seven years in row. The fact that we've had a quiet July does not mean we can expect a slower than average hurricane season. To illustrate, consider 2004--the first storm, Alex, did not get named until August 1, yet that season had 15 named storms, 9 hurricanes, and 6 intense hurricanes. Five named storms formed in first half of August of 2004. One item of comfort, though, is the fact that 2007 is definitely not a repeat of 2005--we were already up to "G" in the alphabet at this point in 2005.
Figure 2. Historical Atlantic hurricane and tropical storm activity, 1851-present. Figure modified from NOAA's original.
As seen in Figure 2, hurricane activity does take a bit of an upward jump around August 1, but the real action doesn't start until August 18. It should not surprise us, then if we go a few more weeks without a named storm. I am still expecting an above-average hurricane season with 12-14 named storms (10-12 more, since we've already had two), with at least one major hurricane hitting the U.S. However, the decline in SSTs relative to normal over the past two months means we should have a less active season than originally thought. I'm guessing that the Dr. Bill Gray/Phil Klotzbach August forecast, due to be released Friday August 3, will have two fewer named storms compared to their May 31 forecast.
Figure 3. Tracks of all tropical storms and hurricanes since 1851 that formed August 1-15.
Sea Surface Temperatures
Early August storms can occur anywhere, and strike anywhere (Figure 3), since the oceans have finally heated up to the point where the entire tropical Atlantic can support hurricane formation. Sea Surface Temperature (SSTs) remained near average over the tropical Atlantic between Africa and the Lesser Antilles in July (Figure 4), thanks to plenty of African dust keeping sunlight from heating up the ocean. A stronger than average Bermuda High has also helped cool the ocean more than normal, thanks to the faster trade winds it brought over the ocean in June and July. However, SSTs are 0.5-1.0 °C above average over much of the Caribbean and Gulf of Mexico, so there is still good reason to expect an above-average number of tropical storms and intense hurricanes this hurricane season. The total amount of heat energy in the upper layer of the ocean (the Tropical Cyclone Heat Potential, TCHP) remains high in the Western Caribbean--near the record levels observed in 2005. However, TCHP is much lower over the rest of the tropical Atlantic, including the Gulf of Mexico.
Figure 4. Sea Surface Temperature (SST) departure from average for July 26, 2007. Image credit: NOAA.
Wind shear has been near normal, averaged over the tropical Atlantic during July. Wind shear is predicted to be near or below normal for the first half of August.
Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa. By early August, dust activity typically diminishes, but it is not well-understood how to forecast these dust outbreaks. We have seen a decline in the amount of dust over the past week, as one might expect from climatology. Since the long-range GFS forecast does not show any major changes to the weather pattern over Africa the next two weeks, I am expecting African dust activity to remain near normal through mid-August.
The hurricane steering pattern for the next two weeks should be near normal, with no areas at above-average risk for a hurricane strike. I discussed this in detail in Friday's blog. Steering current patterns are not predictable more than about two weeks in advance, and there is no way to tell if this steering current pattern will remain in place past mid-August.
Recent history suggests a 75% chance of at least one named storm occurring in the first half of August. I'll go with climatology and forecast a 75% chance this year, as well, since SSTs, wind shear, and African dust/dry air should all be near normal. With the steering current patterns expected to be near normal, no areas can be singled out as being at higher risk than average. We'll have to keep a careful eye out late this week, when a cold front is expected to sweep off the East Coast of the U.S. Several of the models are indicating the possibility that a tropical depression could form at the tail end of the cold front by Thursday or Friday. This would most likely happen off the Carolina coast, but could also occur in the Northeastern Gulf of Mexico, near the Florida Panhandle. I think it is still too early to get a tropical storm forming between Africa and the Lesser Antilles Islands this week, but next week we could see something.
Last blog for a week
This will be my final blog until Monday, August 5, as I am taking a week's vacation to do some camping and paddling along Lake Huron's gorgeous Georgian Bay. I've arranged for an able substitute blogger to make daily posts here this week, but I will be able to do some blogging beginning Friday if something nasty pops up.
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