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 , 4:59 AM GMT on September 02, 2011
Tropical Depression Thirteen
As has been predicted by the major models this week, the broad area of disturbed weather that began in the western Caribbean Sea and meandered into the Gulf of Mexico has developed into Tropical Depression Thirteen. The most recent satellite image of the depression from 11:00 p.m. CDT (Figure 1) shows deep convection mainly confined to east of center, but westerly shear of 20 knots is expected to relax over the 24 hours. While this relaxation, in combination with sea surface temperatures well above the threshold for intensification, should lead to some strengthening, the sheer size of the depression would require significant time for the system to become a hurricane.
Figure 1: Infrared image of Tropical Depression Thirteen. The depression is centered south of Louisiana and is expected to move very slowly toward the coast over the next few days.
Track of Tropical Depression Thirteen
The depression's center position is still quite uncertain, as is the overall track. We should get a better idea of what is going on inside of the depression, along with pinpointing its center, after a couple of hurricane hunter aircraft complete their investigation over the next 12-24 hours.
The initial track, although uncertain, strengthens the depression into a tropical storm (which would be named Lee) on Friday while curving to the northwest. After Friday, it is possible that the system would gradually turn to the northeast, making landfall along the coast of Louisiana Sunday evening. It is important to note, however, that the track is uncertain at this time and could vary over the next few days. The depression is currently south of a high pressure ridge that is not giving much in the way of steering currents. This is also the reason for its very slow movement.
Figure 2: 5-day forecast map of Tropical Depression Thirteen.
Effects of Tropical Depression Thirteen
Tropical Storm Warnings have already been posted for the central Gulf Coast due to its proximity to the area. At this time, hurricane force winds are not expected for the area, but because the depression/tropical storm will be slow-moving, it may be a large precipitation-producer throughout the Southeast. You can already see rain from the northernmost part of the depression on radar out of New Orleans. Southern Louisiana through southern Alabama is set to receive 10-15 inches of rain, while higher amounts near 20 inches are possible in the hardest hit areas. This could lead to a precipitation pattern much like Hurricane Danny in 1997, except peak areas would receive less rain. Unfortunately for Texas, the major models are not currently producing any rain in the drought-stricken state, aside from areas bordering Louisiana. But, Tropical Depression Thirteen is likely to aid drought conditions in other parts of the Southeast, including Louisiana. (Figure 5).
Figure 3: Tropical Storm Warnings have been posted for the central Gulf Coast.
Figure 4. Rainfall amounts from Hurricane Danny July 17-26 1997. Note the astonishing 37.755 inches recorded at Dauphin Island, AL.
Figure 5. Drought Monitor image from August 30 showing exceptional drought conditions through much of Texas and parts of Louisiana.
We will be monitoring this storm as it moves slowly through the northern Gulf of Mexico over the next few days. You can also follow us on Twitter (@wunderground) and on Facebook for the latest developments.
Dr. Masters will be back in the morning with another blog highlighting the latest with Tropical Depression Thirteen.
Thanks for reading,
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