Friday, July 23, 2010

Is Bonnie like Brenda?





Is Bonnie like Brenda? Brenda was a tropical storm that made landfall in eastern Louisiana in 1955, producing a 6 foot surge at Shell Beach, LA. Both storms may have similar landfall points. As Bonnie is still a depression, if it does redevelop, it may not happen until the system is just off the southeast coast of LA, in the similar region in which Brenda developed.

Brenda, however, moved slower, enabling the storm to build more surge. Also, Brenda was stronger than Bonnie should be. As Brenda approached eastern LA, winds exceeded 60 MPH. If Bonnie does redevelop into a tropical storm, max winds are likely to stay less than 45 MPH. So Brenna's 6 foot surge is likely higher than what we'd observe in Bonnie.

The latest surge info from the last NHC advisory on Bonnie, released at 10PM CDT tonight, predicts surge levels of as high as 2 to 4 feet above ground level...(adivsory is below)

STORM SURGE...STORM SURGE WILL RAISE WATER LEVELS BY AS MUCH AS 2 TO
4 FEET ABOVE GROUND LEVEL ALONG THE IMMEDIATE COAST NEAR AND TO THE
RIGHT OF WHERE THE CENTER MAKES LANDFALL ON THE NORTHERN GULF
COAST.

As most communities in extreme eastern Louisiana are around 3 feet above sea level, 2 to 4 feet above ground level means a 5 to 7 foot surge. This surge level would be similar to Brenda. However, Brenda was moving slower and produced stronger winds. Therefore, it is likely that NHC is overestimating surge levels.

Also, Brenda (and some similar storms) taught us that the peak surge can actually occur on west side of storm track, especially for storms that track in a NW direction and make landfall in E LA, or near the LA/ MS border. So don't be surprised to see peak surges to the west of storm track, perhaps in areas such as Shell Beach or Yschloskey. This is because the strongest winds in these events is from the east, which is not onshore in MS, but parallels the shore. This keeps water from piling up as much. In E LA, however, even in areas west of the storm path, strong winds from the E or NE actually pile water up from the Mississippi Sound.

Based on this historical comparison, we might expect Bonnie to produce a surge between to 2 to 4 feet if it does redevelop into a tropical storm. However, If Bonnie continues to weaken, just look for some choppy water and slight water rises along the central Gulf Coast.

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