Significant Storm System to Affect the Region by Tuesday Night

Early Morning Weather Briefing for Emergency Managers November 30, 2009---

In this update:

· Significant Storm System to affect the region by Tuesday night

· Multifaceted hazardous weather threat expected including severe weather

An area of low pressure will develop over the Western Gulf of Mexico on Tuesday afternoon as shown in the image above.  This area of low pressure will initially move along a stalled frontal boundary across the Northern Gulf.  As this low moves to the northeast, the front will begin to lift back north bringing a more moist and unstable airmass back into the region.

In addition to the incoming moist and unstable airmass, this low pressure area is forecast to strengthen rapidly as it interacts with a strong upper level system moving across the Lower Mississippi River Valley.  This will result in strong southerly winds ahead of the low pressure system by Wednesday morning.  These strong onshore winds will create very high surf and the potential for coastal flooding, particularly in the flood prone areas of Apalachee Bay.

Finally, this storm system will also bring the potential for very heavy rainfall.  Thus, urbanized flooding during the event and river flooding in the days to follow is certainly possible with  this system.

Severe Weather Threat:

On Wednesday, the Storm Prediction Center has placed our region in a Slight Risk for severe weather.  Though there is certainly potential for some of this risk area, especially near the coast, to be increased to a moderate risk in the days to come.  This is because there is a considerable amount of wind shear associated with this system.  However, as is usually the case, the amount of instability is uncertain.  As Wednesday approaches we will likely have a better understanding of how unstable we will get across the region.  If more instability is realized, the potential exists for a significant severe weather event to accompany this system with destructive straight line winds and isolated tornadoes, especially with a squall line along the cold front on Monday afternoon.

Coastal Flooding:

Currently, areas from Cape San Blas to the Suwannee River Entrance are under a coastal flood watch.  This means that we are seeing indications that inundation of low lying areas along the coastline is possible.  If confidence in coastal flooding continues to increase, this watch may be upgraded to a warning later tonight or on Tuesday.

Strong southerly winds ahead of this system on Wednesday morning will begin to push water into Apalachee Bay.  These strong onshore winds will likely coincide with the time of high tide on Wednesday afternoon.  As you can see from the tide chart below, there is very little difference between the high and low tides on Wednesday afternoon, thus the potential exists for a prolonged period of inundation for low lying areas along the coast.

It should be noted that some of our storm surge models are showing storm surge values in excess of 4 feet in Wakulla, Jefferson, Taylor, and Dixie Counties, which would create overall
storm tides near 7 to 8 feet, or values very similar to Hurricane Gustav in 2008.

Coastal Flood Summary:

For more information on Storm Tide visit http://www.nws.noaa.gov/mdl/etsurge/index.htm?coast=gulf&load=gfmap.htm&map=0-48

In addition to the coastal flood impacts, dangerous high surf which could cause beach erosion from Saint George Island westward to Destin is possible. The surf could reach heights of 7 to 10 feet, particularly from Panama City to Cape San Blas.

Inland Flooding:

The rainfall with this system is predicted to be heaviest across the western half of the region where storm total amounts of up to 4 to 6 inches is possible.  Somewhat lesser totals of 2 to 4 inches are expected across the eastern half of the region.  Please see the map below for more details:

Notice that the heaviest totals are located from Albany to Apalachicola and areas to the west.  If the low pressure area tracks further to the east, this heavy rain could impact areas further to the east than presently expected.

Timing:

At present, we are expecting the threat for severe weather and heavy rain to develop late Tuesday evening across the Florida Panhandle and then begin to spread further inland into Southeast Alabama on Wednesday morning.  Then, by mid morning Wednesday, a squall line is predicted to develop and spread eastward rapidly across the region.  This squall line could be particularly severe if enough instability is in place.  The main threats early in the event will be for isolated tornadoes.  Then, with the squall line, destructive straight line winds and isolated tornadoes will be possible.

Watches Currently in effect:

**Coastal Flood Watch in effect for Gulf, Franklin, Wakulla, Jefferson, Taylor and Dixie from Wednesday morning through Thursday morning

**Gale Watch in effect for the coastal waters from late Tuesday night through Thursday morning.

Summary:

**A significant storm system with the potential for severe weather, coastal flooding, inland flooding, and beach erosion will approach the region Tuesday night and into Wednesday.

**Coastal flooding, beach erosion and heavy rainfall are more certain impacts.

**Severe weather is certainly possible but there remains some uncertainty to how significant the impact will be.

The National Weather Service in Tallahassee will continue to monitor this storm’s progress over the next couple of days.  If you have any questions, please give our office a call at 850-942-8833 or on our toll free line at 800-598-4562 and ask to speak to a meteorologist.  We are available 24 hours a day, 7 days a week.  You can also reach us on our Southern Linc phone at 1*77*184.

This information originally published on November 30, 2009.

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Written by :

mkwestmark

 

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