Friday, July 23, 2010

Bonnie downgraded to depression; possible weak surge along the LA, MS, or AL coasts this weekend

The NHC downgraded Bonnie to a tropical depression at the 4:00PM CDT advisory. The latest satellite loop, ending at 5:45PM CDT, indicates very poor organization. Circulation is not visible in this loop, and it appears as though Bonnie is dissipating.

Nonetheless, the official NHC forecast projects that Bonnie may once again form into a tropical storm. The track has moved a bit east, placing potential landfall near the LA/ MS border late Saturday night or early Sunday morning.

Based on historical comparisons, it appears unlikely that Bonnie will produce a surge 4 feet or greater along the U.S. Gulf Coast. If Bonnie does redevelop and make landfall in near the LA/ MS border as a tropical storm, it may be useful to compare Bonnie to Tropical Storm Brenda, from August, 1955. Brenda formed into the tropical storm south of the MS River Delta, then moved NW, making landfall near the LA/ MS border, producing a 6 foot surge at Shell Beach, LA. However, Brenda's wind speeds at the closest approach to Shell Beach were over 60 MPH, a wind speed that seems unlikely for Bonnie to obtain at this point. As Bonnie is forecast to become a minimal tropical storm, at best, one might conclude that Bonnie's surge would likely be lower, perhaps 1-3 feet.

An important observation from Brenda is that the point of greatest storm surge was actually on the west side of the storm track, instead of the usual east side. A similar situation occurred in November 2009, when Ida produced a 6.53 foot surge at Bay Gardene, LA, to the west of the storm track. This sometimes happens in eastern LA, as strong northeasterly winds that usually push water offshore along most of the Gulf Coast, actually can pile water up in places east of New Orleans or along the eastern levee of the MS River Delta. So if Bonnie does redevelop, don't count out several feet of surge in eastern LA, even if specific areas are on the "weak" (left) side of the storm. Areas experiencing onshore winds in MS or AL could also observe small surges.

The current NHC forecast map, as well as TS Brenda's 1955 track are included in this post.

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