Saturday, July 24, 2010

Bonnie disorganized; surge levels should be low



The latest satellite imagery indicates that Bonnie is still disorganized. The system is not producing substantial onshore winds yet along the Central Gulf Coast, and the latest NHC forecast questions if Bonnie will even redevelop into a tropical storm.

For these reasons, expect surge levels to be very low, less than 3 feet, in areas with onshore winds later today and tonight. As most communities in the impacted area are located at about 3 feet above sea level, this means that storm surge flooding of communities, roads, etc. is unlikely.

For some reason, the NHC still forecasts the possibility of a 4 to 6 foot surge (1 to 3 feet above ground level). Here is the surge information from the latest advisory (7AM CDT), issued about 10 minutes ago:

STORM SURGE...STORM SURGE WILL RAISE WATER LEVELS BY AS MUCH AS 1 TO
3 FEET ABOVE GROUND LEVEL ALONG THE IMMEDIATE COAST NEAR AND TO THE
RIGHT OF WHERE THE CENTER MAKES LANDFALL ON THE NORTHERN GULF
COAST.

This appears to be an overestimation of surge, based on historical comparison. Generally, historical records show that systems must be at least moderate tropical storms ( around 50 to 65 MPH winds), moving forward less than 15 MPH, to produce 4 to 6 foot surges. Bonnie will be a weak tropical storm, at best, and is moving too fast to pile up this much surge.

Tropical storms Brenda and Bertha, featured in the first Bonnie blog post, both generated surges within this range, but both produced winds over 60 MPH near the LA Coast, and were moving forward more slowly.

It is encouraging, however, to see the NHC using surge estimates above ground level, as this system should help the public get a better idea of surge levels in coastal communities in future surge events.

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