Saturday, June 23, 2012

Tropical Storm Debby forms in the Gulf of Mexico

National Hurricane Center best track forecast as of 7PM CDT, Sat Jun 23, 2012

Hurricane Bonnie (1986) tracking map, provided by Unisys Corporation. Green = tropical depression, yellow = tropical storm, red = hurricane

Tropical Storm #1 (1941) tracking map, provided by Unisys Corporation. Green = tropical depression, yellow = tropical storm, red = hurricane

Tropical Storm Debby formed this afternoon in the Gulf of Mexico. This is an interesting storm, as the models have predicted a wide range of possible outcomes. In general, the GFS model has insisted on a more eastward track, while other models have arrived at a solution that brings Bonnie on a more westward track. Most models seem to indicate that Debby will drift to the north or northwest in the near future.

The National Hurricane Center has leaned toward the westward track, taking Debby on a slow progression south of Louisiana, toward the Texas Coast. However, it's important to understand that with such low model consensus, anything is really possible.

This blog looks at historical hurricanes and tropical storms that resemble active storms to try to gain context about storm surge potential. We pull our surge data from SURGEDAT, the world's largest archive of historical storm surge data. SURGEDAT now has peak surge data for more than 500 surge events around the world, including more than 200 for the U.S. Gulf Coast. We have also built entire inundation envelopes for most storms, giving us a total of more than 4,600 high water observations for the U.S. Gulf and Atlantic Coasts.

As the forecast for Debby's track and intensity is quite uncertain, I just looked up historic tropical storms that tracked from east to west, with slow forward motion, south of the Louisiana Coast. Such a track is possible Monday into Tuesday, according to the NHC forecast. I'm not looking into surge history for the Central Texas Coast yet, as much uncertainty exists and Texas impacts would come later. The best two historical comparisons I could find related to westward-moving, slow tropical storms south of Louisiana were...drumroll please....

Hurricane Bonnie in 1986 AND Tropical Storm #1 in 1941

Hurricane Bonnie 1986
Tracking and Max Winds: This storm slowly moved to the west as a tropical storm while S of the LA Coast, then became a hurricane while still S of LA. Then storm tracked to NW, and produced max winds of around 86 MPH before making landfall in SE Texas

Storm surge observations:
Peak Surge: 5.2 feet at Sabine Pass, TX
Additional Surge Obs:
3-4 feet at Bolivar Peninsula, TX
2.7 feet above Mean Sea Level (MSL) at Paris Road, LA
2.4 feet above MSL at Bayou Bienvenue, LA
2-3 feet at Calcasieu Pass
2.2 feet at Galveston Island
1.7 feet above MSL at Rigolets, LA
1.6 feet above MSL at Industrial Canal, LA
Source of obs:

Tropical Storm #1 in 1941
Tracking and Max Winds: This storm developed into a tropical storm and tracked slowly to the W, staying S of the LA coast. It eventually turned toward NW and made landfall in SE TX as a minimal tropical storm. Max winds only reached 46 MPH.

Storm surge observations:
Peak Surge: Approximately 3.25 feet at Sabine Pass, TX.

"…the storm center passed about 50 miles south of Burrwood, and moved inland west of Port Arthur on the 14th. Tides slightly over 3 feet were reported along the coast and very little damage occurred." 

Source of obs: (U.S. Army Corps Report, titled, History of Hurricane Occurrences along Coastal Louisiana, pg 30.)

A few thoughts:
Again, the track and intensity for Debby are still up in the air. However, based on the best track and intensity provided by the NHC, I've pulled surge data for two storms that tracked slowly from east to west, staying south of the LA coast, Bonnie (1986) and TS #1 (1941).  Both turned to the NW and made landfall in SE TX. Bonnie became a minimal hurricane, while TS #1 barely maintained TS status. Based upon the current forecast, it may seem reasonable that Debby tracks west with an intensity somewhere in between those two storms. The peak surges for both events were observed at Sabine Pass, TX; Bonnie's surge was 5.2 feet and TS #1's peak surge was 3.25 feet.

While these events may provide some context for Debby's surge, please keep in mind that every event is different and surge heights can be very localized. Debby is forecast to move very slowly in the near future before making the turn to the west, and this pattern should enable prolonged E winds to impact SE LA. Generally, prolonged E winds in this area pile up water on the east side of the MS River delta, from near Venice, north to the Lake Borgne/ Shell Beach area, as well as Lake Pontchartrain, the Rigolets, and even the Mississippi Sound. Surge levels this afternoon and evening already approached two feet above normal at Shell Beach, according to NOAA Tides and Currents.

Stay tuned to this blog over the coming days. I will provide updates every morning and evening, as well as possibly some actual footage (pics and maybe video) from southeast LA and MS.

-Hurricane Hal

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