Tuesday, June 29, 2010

Alex's track shifts south, peak U.S. surge estimate holds at 4-6 feet

Tuesday, June 29, 2010
1730 CDT

Update: The NHC shifted Alex's track a bit to the south. However, the best estimate of Alex's peak U.S. surge height, based on historical comparison, remains unchanged:

Estimate of Alex's peak U.S. surge height:
Location: South Padre Island, TX
Height: 4-6 feet

Note: This estimate pertains to Alex's peak surge height. Surge heights will become lower to the north of South Padre Island, TX.


This estimate is based on comparing NHC track and intensity data to historical storm surge records. As mentioned in previous blog posts for Alex, the SCIPP program identified three historical surges in which the hurricane tracks were similar to Alex. Emily (2005), Gilbert(1988) and an Unnamed storm (1909) all crossed the Yucatan Peninsula, made landfall in NE Mex, and generated surges between 5-6 feet at South Padre Island, TX. See previous posts for images of these hurricane tracks.

Alex will likely make landfall in a similar location to Emily. Emily was more powerful, a Cat 3 storm at landfall, packing winds of 125-130 MPH. However, Alex has a broader wind field, which tends to enhance surge levels.

A quick comparison between these storms reveals that Emily's 50-knot wind field in the northeast quadrant extended for 100 nautical miles, while Alex's wind field in same direction is only forecast to extend 70 nautical miles, meaning Emily had a broader wind field at this speed (the area covered by winds exceeding about 56 MPH). However, Alex's 34-knot (38 MPH) wind field in NE quadrant is actually forecast to be larger than Emily's, as Alex is forecast at 150 nautical miles and Emily was only 140 nautical miles (see NHC Emily archive from 0300Z Jul 20 2005). This means that Alex will likely produce tropical-storm force winds over a larger area than Emily.

This also means that if they make landfall at same point and these forecasts hold, some points along Texas coast will feel stronger winds from Alex than Emily, even though Emily was a Cat 3 and Alex will likely only be a Cat 1. This extensive area of tropical-storm force winds will likely enable Alex to produce similar surge levels along S TX coast as Emily, even though the max winds in Emily's eyewall were much stronger. A quick view of radar from S TX shows the broad area of circulation that is already bringing outer bands into the coast from the northeast.

Another element that will enhance S TX surge is the "aspect," or direction which the coast faces. As the coast faces east-northeast, and Alex is moving towards the NW, and will stay south of TX, onshore winds approaching or exceeding tropical-storm force will impact S TX for perhaps more than 18 hours. This will help force surge levels up. The winds at S Padre Island have been from the N at between 22-31 MPH during the past three hours and should shift towards the NE as Alex approaches.

For these reasons, we can expect Alex to likely produce peak surge levels of 4-6 feet in extreme S TX. The NHC has forecast a lower peak surge for the past several advisories, 3-5 foot surge near the point of landfall. One would likely interpret this to mean perhaps 2-4 foot surge levels in S TX. We'll see what happens.

Follow the water levels at NOAA Tides and Currents as Alex approaches. I'd provide the link, but the Website is down at the moment. :) Bad timing, huh?

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