Tropical weather analysis - June 25, 2012
Debby refuses to move. As of the latest NHC advisory, the following was posted on the storm:
Wind: 45 mph, with higher gusts
Location: 29.2°N 85.1°W
Movement: NE at 2 mph
Pressure: 992 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
The low-level center, while well-defined, is exposed well to the southwest of a large band of very deep convection due to about 20 knots of southwesterly shear associated with the upper low still over the western Gulf of Mexico, and a large anticyclone over the western Atlantic extending into the Caribbean Sea.
Figure 1. Latest infrared satellite image of Tropical Storm Debby. Image credit: NOAA
The cyclone's biggest legacy will continue to be torrential rains. Doppler radar estimates indicate a swath of 15+ inches of rain south of Tallahassee. Considering most of this has fallen over the last 24 hours, there is serious flooding ongoing across the Tallahassee metro area, especially in Wakulla County, where the heaviest rainfall has occurred. With Debby moving little, additional heavy rainfall is expected, and doppler estimates reveal that 2 to 3 inches per hour has been a common motif with Debby. Since the shear is pushing all the convection east, these dangerous rains may spread eastward, creating a more widespread flood threat. There has been mention by Dr. Greg Forbes, severe weather expert with The Weather Channel, that this is the most serious flood threat he's even seen in all his years of forecasting. While I seriously doubt Debby will surpass Tropical Storm Claudette's 24-hour rainfall record of 42 inches set at Alvin, Texas in July of 1979, over 20 inches of rain have fallen in western Wakulla County, and additional rainfall continues to train over this area. As senior hurricane specialist Stacy Stewart pointed out in the most recent National Hurricane Center forecast discussion for the storm, the cyclone center will have the tendency to follow the deep convection, in the absence of any large-scale steering mechanisms. The net result should be a flood threat that slowly spreads eastward with time, unless the shear unexpectedly relaxes, which is not anticipated. Truth be told, when this is all over, I don't think 35 to 40 inch rainfall totals in some locations should be considered unreasonable. I cannot place enough emphasis on how serious the ongoing flood threat across Florida is, the worst of which is currently occurring across the Tallahassee area. If you see high water, DO NOT attempt to drive through it. High water can be especially difficult to see at night.
Tropical storm force winds are also still impacting the state, although thus far I have been unable to find any official observations of sustained tropical storm force winds. However, locations with official National Weather Service anemometers are located within areas outside the strongest doppler velocities. Isolated tornadoes remain possible, but 0z upper air data over Florida indicates that any such threat will remain marginal. Indeed, the Storm Prediction Center hasn't reported any tornadoes across Florida in association with Debby today, despite the earlier tornado watch covering much of the peninsula. Although tropical storms don't normally produce significant storm surge, the slow-moving nature of the tropical cyclone will produce a prolonged fetch of onshore flow, and surge values of 1 to 5 feet near and to the east of the center will not be uncommon. The National Hurricane Center is predicting water level rises in excess of 3 to 5 feet near Apalachee Bay. This area is notoriously vulnerable to storm surge, so this is a reasonable prognosis.
Given the shear, continued dry air, and inevitable oceanic upwelling along the projected path, Debby is not expected to intensify while over the Gulf of Mexico. In fact, slow weakening seems most likely. When the system enters the western Atlantic, global models suggest the shear could decrease, and the cyclone will have the opportunity to restrengthen. This is consistent with the global models, which show Debby intensifying as it accelerates across the western Atlantic waters between Bermuda and the United States.
5-day intensity forecast
INITIAL 0300Z 06/26 40 KT 45 MPH
12 hour 1200Z 06/26 40 KT 45 MPH
24 hour 0000Z 06/27 40 KT 45 MPH
36 hour 1200Z 06/27 40 KT 45 MPH
48 hour 0000Z 06/28 35 KT 40 MPH...JUST OFF COAST
72 hour 0000Z 06/29 35 KT 40 MPH...INLAND
96 hour 0000Z 06/30 40 KT 45 MPH...OVER WATER
120 hour 0000Z 07/01 45 KT 50 MPH
The track forecast remains challenging, but a little easier than recent days. Water vapor and upper air data over the eastern United States indicates the absence of any synoptic scale steering mechanisms. This means Debby will move little for at least the next 48 hours, with only a slow east-northeast to northeast motion toward the Big Bend region of Florida expected. A weakness between the central United States ridge and the Atlantic subtropical ridge is evident along and east of 80W. This has a been a persistent feature with this storm, and is why Debby is expected to, eventually, cross Florida. The GFS and ECMWF remain in excellent agreement on this, and have converged tighter on the forward speed since the recent run of the 0z GFS. Although Debby is most likely to follow the path of least resistance out into the Atlantic, earlier runs of the GFS hinted that the large scale blocking pattern over the central US, which has been equally persistent relative to the trough over the east, would halt the forward progress of the cyclone while over the western Atlantic. I doubt Debby swings back into the Gulf of Mexico amidst that pattern, but I think now is a good time to alert everyone on the east coast to begin monitoring the storm. Out of deference to this possibility, I am not forecasting rapid acceleration.
Interestingly, the GFDL and HWRF bring Debby west from its current location, but this is probably contingent on a weaker system moving westward in the low-level flow. Since Debby has convection, this was discounted.
5-day forecast track
Figure 2. My 5-day forecast track for Debby.
Watches and warnings
THE TROPICAL STORM WARNING HAS BEEN DISCONTINUED WEST OF MEXICO
SUMMARY OF WATCHES AND WARNINGS IN EFFECT...
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE FLORIDA GULF COAST FROM MEXICO BEACH TO ENGLEWOOD
FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE