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New disturbance 94L a threat to the Lesser Antilles

By: Dr. Jeff Masters, 3:51 PM GMT on July 13, 2008

A tropical wave midway between Africa and the Lesser Antilles Islands has become organized enough this morning to be classified as a threat area (Invest 94L) by NHC. This morning's QuikSCAT pass missed the disturbance, but last night's pass revealed 94L's large, elongated circulation near 9N 35W. Visible satellite loops show this circulation getting better defined this morning, although there is not yet much organized heavy thunderstorm activity. The disturbance is embedded in a large area of tropical moisture, and 94L should be unaffected by dry air or Saharan dust over the next few days. Water temperatures are favorable for development--27.5°C. Wind shear is favorable for development--10 knots.

Figure 1. NHC's graphical tropical weather outlook for Sunday Jul 13 2008 at 8 am EDT. Area "1" is Invest 94L. Image credit: NHC.

The forecast for 94L
NHC is giving 94L a medium 20-50% chance of developing into a tropical depression by Tuesday morning. Wind shear is expected to remain low, less than 10 knots, for the next four days, and the water temperatures will be plenty warm enough to support development. Three of the four global models we trust predict 94L will develop into a tropical depression. The ECMWF develops it and puts 94L into the Lesser Antilles islands on Saturday. The UKMET model does not develop it, but also puts it into the Lesser Antilles islands on Saturday. The GFS develops it but foresees that Bertha will be close enough and strong enough to pull 94L north of the Lesser Antilles about, 6-8 days from now. The NOGAPS model is much slower, and does not foresee a threat to the Lesser Antilles this week. In summary, there is the potential for a tropical depression to form later this week from 94L, and residents of the Lesser Antilles should anticipate the possibility of tropical storm conditions affecting the islands by Saturday.

Tropical Storm Bertha stayed too long in one spot, and has churned up so much cold water from the depths that it has weakened to a tropical storm. Visible satellite loops still show a large and well-developed circulation, and the storm does have the potential to re-intensify if it can move away from the cold water it upwelled. However, the steering currents are very weak and are expected to remain so for several more days, making it unlikely Bertha can find any warm water. The outer spiral bands of Bertha are very close to Bermuda (see links below), and the storm is now visible on Bermuda radar. Bertha spent six days as a hurricane, making it the fourth longest-lived July hurricane on record (Emily of 2005 holds the record, at seven days). Bertha has been a named storm for 10 days, and will easily break the record for longest-lived July named storm (12 days).

Links to follow:
Bermuda radar
Current weather at Bermuda
Bermuda WebCam

High surf of 12-18 feet is expected to affect Bermuda through Monday, according to the Bermuda Weather Service. There is about a 64% chance the island will experience sustained winds of tropical storm force (39 mph), according to the latest tropical storm wind probability forecast by NHC. Given the very weak steering currents predicted to affect Bertha through Tuesday, the storm could easily approach closer to the island than the current official forecast.

South Carolina disturbance
A small low pressure system has developed off the coast of South Carolina at the tail end of an old cold front. This morning's QuikSCAT pass shows the circulation quite clearly, as do visible satellite loops. The low is too small to develop quickly, and is under about 20 knots of wind shear, which should also keep any development slow. The low is expected to move northeast, parallel to the coast.

My last blog entry until Saturday
This will be my last blog entry for five days (until Saturday). I'm headed to Lake Michigan for some camping and vacation. If you happen to be in Mackinaw City Tuesday night, be sure to catch the Straits Area Concert Band in Concert at 8pm at the band shell by Lake Huron. My dad and I will be in the trombone section, puffing our way through the band's usual assortment of Sousa marches, Gershwin medleys, and patriotic fare.

In my absence, our guest tropical blogger, Bryan Woods, will be doing my blog this week. Bryan has done a great job over the past three years blogging on the tropics over at thestormtrack.com. Here's Bryan's bio:

Bryan received his BS in Meteorology from the University of Massachusetts in Lowell, MA in 2005, and his M.Phil. in Geology & Geophysics from Yale University in New Haven, CT in 2007. Bryan is currently a Ph.D. candidate at Yale where he is also the graduate and professional student body president.

Bryan has spent two summers working on a National Science Foundation (NSF) sponsored field micrometeorological research project in Atlanta, GA, studying evapotranspiration rates in urban forest canopies. Currently, Bryan's research is focused on combining wavelet techniques and aircraft data from the NSF/NCAR Gulfstream V to diagnose energy and momentum fluxes from atmospheric gravity waves. Bryan has spent the past three hurricane seasons writing blogs on the tropics for thestormtrack.com.

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.