Wednesday, June 30, 2010

Peak U.S. surge levels maximizing between 3 and 3.5 feet

The green lines in the graphs below indicate that surge levels are maximizing between 3 and 3.5 feet at both South Padre Island and Port Isabel, TX, as Hurricane Alex makes its closest approach to the S TX coast. Alex is centered about 110 miles S of Brownsville, TX, as it bears down on the Mexican coast. Expect surge levels to remain near these levels before falling later this evening.







Surge levels rising at S Padre Island

The green line on this graph indicates that the water level at S Padre Island Coast Guard Station is between 2 and 3 feet above normal. This line measures the difference between predicted water levels (blue) and actual water levels (red), which essentially defines the storm surge.

Alex's eye is now visible on Brownsville's radar. Also, the wind at South Padre Island, TX at 1300 CDT shifted from the ENE at 35 MPH, an indication that Alex is approaching the coast. Surge levels should continue rising this afternoon.

Tropical storm force winds approach the TX coast



Tropical storm-force winds (over 39 MPH sustained) are approaching the TX coast. This image from the NHC depicts the extent of tropical storm-force and hurricane-force winds as of 1000 CDT.

The winds at S Padre Island Heliport have been from the NNE at between 29 and 36 MPH during the past four hours. Look for winds to increase and come from the NE, then E as the storm approaches. Surge levels should increase as the wind direction changes to the NE, then E.

NOAA tides and currents data reveals that water levels are already 2 feet above normal at the S Padre Island Coast Guard Station, as of 0700 CDT.

NHC best track shifts farther south, peak U.S. surge estimate reduced to 3 to 5 feet




Wednesday, June 30, 2010
0600CDT

Update: The NHC best track has shifted farther south since yesterday evening. The best track now places landfall about 100 nautical miles S of Brownsville, TX. Therefore, the estimate of peak U.S. surge has been lowered to 3 to 5 feet.

Peak U.S. surge estimate:
Location: South Padre Island, TX
Surge Height: 3 to 5 feet

Discussion:

Alex will likely make landfall about 25 miles S of the landfall location of Hurricane Emily (2005). Emily produced a 5-foot surge at S Padre Island, TX. Although Emily was stronger than Alex will be at landfall, Alex's circulation is very large, which tends to enhance surge heights. To give some idea of Alex's massive size, check out the radar picture in this blog post. Rain bands associated with Alex extend as far out as the western Florida Panhandle. Rain bands are pushing into Baton Rouge this morning, more than 600 miles from the center of circulation!

The NHC upgraded Alex to a hurricane last night around 2130 CDT and conditions are favorable for further strengthening. Alex is currently a Cat 1 and may make landfall as a Cat 2.

Onshore winds should persist in S TX today, reaching tropical storm force for perhaps 12-18 hours near S Padre Island.

This peak U.S. surge estimate is based on comparison of NHC best track and intensity forecasts to similar historical surge events. As mentioned in previous posts, the SCIPP program identified three similar surge events, which are useful in this comparison: Hurricane Emily (2005), Hurricane Gilbert (1988), and an Unnamed storm (1909).

Tuesday, June 29, 2010

Alex's broad circulation



Check out Alex's broad circulation, which covers most of the western Gulf of Mexico. Extensive circulation in tropical systems tends to enhance surge levels. Image Source: Weather Underground

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.

Discussion:

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?

Alex still a tropical storm



The latest position and storm track from the National Hurricane Center reveals that Alex is still a tropical storm as of 1000 CDT. However, the NHC expects Alex to strengthen into a hurricane today, and likely make landfall in NE MEX as a Category-1 hurricane.

Peak U.S. surge estimates:
Location: South Padre Island, TX
Height: 4-6 feet

See previous post from 0800 CDT for historical comparisons of Alex to Hurricanes Emily (2005), Gilbert (1988) and the Unnamed 1909 hurricane.

Tuesday, June 29, 0800 CDT




Alex’s peak surge estimate based on historical surge data #4
June 29, 2010
0800 CDT

Alex’s peak storm surge estimate based on historical surge data and the NHC forecast track and intensity:

Peak surge height estimate: 4-6 feet
Peak surge location estimate: South Padre Island, TX

Update: The NHC track forecast has shifted slightly north since yesterday evening. According to the present track, Alex should make landfall perhaps 30-50 nautical miles S of Brownsville, TX. However, Alex is still somewhat disorganized and still not a hurricane. It seems most likely that it would make landfall as a CAT 1 hurricane south of the TX border. I've not changed the surge estimate, as the storm should track slightly closer to the coast than previously predicted, but it will likely be a bit weaker than previously predicted.

Research through the SCIPP program identified three historical hurricanes that came off the Yucatan Peninsula, made landfall in northeast Mexico, and produced surges greater than or equal to 4-feet along the S TX coast. These events are:

Hurricane Emily
2005
Max surge: 5 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed 110 kts
Landfall position 75 nautical miles S of Brownsville, TX

Hurricane Gilbert
1988
Max surge: 6 feet at South Padre Island, TX
Cat 4 at landfall
Landfall wind speed 115 kts
Landfall position about 130 nautical miles S of Brownsville, TX

Unnamed Storm
1909
Max surge: 6 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed: 105 kts
Landfall position about 120 nautical miles S of Brownsville, TX

All three of these hurricanes produced surges between 5-6 feet at S Padre Island. Images of all three tracks are included in this blog.

The NHC track places Alex’s landfall closer to the TX border than any of these events. Alex is forecast to be weaker than these previous events, however, because Alex’s circulation is so large, the surge may be comparable to the other three events.

The NHC has forecast a 3-5 foot surge near and to the north of the point of landfall. However, this forecast may be a bit low, as typical category-1 hurricanes generate 4-6 foot surges, and Alex has a large area of circulation, which should produce an even larger surge. Based on historical comparisons, it would seem reasonable that the peak surges on the Mexican coast are 5-7 feet, and peak surges near South Padre Island are 4-6 feet.

Also, Alex has a broad circulation area, which means the surge values decrease more slowly than normal from where the eyewall makes landfall. Because the coast of extreme S TX faces east-northeast, onshore winds may be observed near S Padre Island for more than 18 hours, given the current NHC forecast track. It would make sense that this should enable at least 4 feet of surge to pile up, but, again, this is just an estimate based on historical comparisons.

Thanks!
Hurricane Hal

Disclaimer: This product is not a forecast. It is a surge estimate based on comparisons to historical storm surge events. This product is experimental and should not be used for decisions included, but not limited to, evacuations, storm preparations, insurance claims, or any decisions that may result in loss of life, injury, or personal, corporate or public economic losses. Louisiana State University, the University of Oklahoma, the SCIPP program, as well as Hal Needham and associated faculty, staff and students involved in developing this product, are not liable for any decisions that lead to injury, death, or any personal, corporate or public losses, including economic losses associated with this storm surge event.

Monday, June 28, 2010, 1915 CDT




Alex’s peak surge estimate based on historical surge data #3
June 28, 2010
1915 CDT

Alex’s peak storm surge estimate based on historical surge data and the NHC forecast track and intensity:

Peak surge height estimate: 4-6 feet
Peak surge location estimate: South Padre Island, TX

Update: The NHC point of forecast landfall is about where it was this morning, after shifting north, then shifting back south again. However, the intensity forecast has slightly diminished. Alex’s circulation still appears to be quite large. The only difference in this update is a reduced peak surge height estimate, because of the decrease in intensity.

Research through the SCIPP program identified three historical hurricanes that came off the Yucatan Peninsula, made landfall in northeast Mexico, and produced surges greater than or equal to 4-feet along the S TX coast. These events are:

Hurricane Emily
2005
Max surge: 5 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed 110 kts
Landfall position 75 nautical miles S of Brownsville, TX

Hurricane Gilbert
1988
Max surge: 6 feet at South Padre Island, TX
Cat 4 at landfall
Landfall wind speed 115 kts
Landfall position about 130 nautical miles S of Brownsville, TX

Unnamed Storm
1909
Max surge: 6 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed: 105 kts
Landfall position about 120 nautical miles S of Brownsville, TX

All three of these hurricanes produced surges between 5-6 feet at S Padre Island. Images of all three tracks are attached to this e-mail.

The NHC track places Alex’s landfall closer to the TX border than any of these events. Alex is forecast to be weaker than these previous events, however, because Alex’s circulation is so large, the surge may be comparable to the other three events.

Thanks!
Hurricane Hal

Disclaimer: This product is not a forecast. It is a surge estimate based on comparisons to historical storm surge events. This product is experimental and should not be used for decisions included, but not limited to, evacuations, storm preparations, insurance claims, or any decisions that may result in loss of life, injury, or personal, corporate or public economic losses. Louisiana State University, the University of Oklahoma, the SCIPP program, as well as Hal Needham and associated faculty, staff and students involved in developing this product, are not liable for any decisions that lead to injury, death, or any personal, corporate or public losses, including economic losses associated with this storm surge event.

Monday, June 28, 2010 0800 CDT




Alex’s peak surge estimate based on historical surge data #2
Monday, June 28, 2010
0800 CDT

Alex’s peak storm surge estimate based on historical surge data and the NHC forecast track and intensity:

Peak surge height estimate: 5-7 feet
Peak surge location estimate: South Padre Island, TX, early Thursday morning

Update: The NHC track shifted to the north, placing the estimated landfall about 65 nautical miles S of Brownsville, TX. Forecast intensity also increased. In addition, multiple sources indicate that Alex’s circulation is quite large.

Research through the SCIPP program identified three historical hurricanes that came off the Yucatan Peninsula, made landfall in northeast Mexico, and produced surges greater than or equal to 4-feet along the S TX coast. These events are:

Hurricane Emily
2005
Max surge: 5 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed 110 kts
Landfall position 75 nautical miles S of Brownsville, TX

Hurricane Gilbert
1988
Max surge: 6 feet at South Padre Island, TX
Cat 4 at landfall
Landfall wind speed 115 kts
Landfall position about 130 nautical miles S of Brownsville, TX

Unnamed Storm
1909
Max surge: 6 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed: 105 kts
Landfall position about 120 nautical miles S of Brownsville, TX

All three of these hurricanes produced surges between 5-6 feet at S Padre Island. Images of all three tracks are included in this blog.

The new NHC track places Alex’s landfall closer to the TX border than any of these events. Alex is forecast to be weaker than these previous events, however, because Alex’s circulation is so large, it is likely that Alex would produce a surge at least as high as these previous events.

The models still diverge considerably after 48 hours, so a drastic change in the storm track could change all of this.

Thanks!
Hurricane Hal

Disclaimer: This product is not a forecast. It is a surge estimate based on comparisons to historical storm surge events. This product is experimental and should not be used for decisions included, but not limited to, evacuations, storm preparations, insurance claims, or any decisions that may result in loss of life, injury, or personal, corporate or public economic losses. Louisiana State University, the University of Oklahoma, the SCIPP program, as well as Hal Needham and associated faculty, staff and students involved in developing this product, are not liable for any decisions that lead to injury, death, or any personal, corporate or public losses, including economic losses associated with this storm surge event.

June 27 1900 CDT




Sunday, June 27, 2010
1900 CDT

In regards to Alex's potential S TX surge, I found three historical events that came off of the Yucatan Peninsula, made landfall in NE Mex, and produced 4-foot or greater surges in S TX. I've included images of those tracks in this blog. The storms are:

Hurricane Emily
2005
Max surge: 5 feet at South Padre Island, TX
Cat 3 at landfall
Landfall wind speed 110 kts
Landfall position 75 nautical miles S of Brownsville, TX

Hurricane Gilbert
1988
Max surge: 6 feet at South Padre Island, TX
Cat 4 at landfall
Landfall wind speed 115 kts
Landfall position about 130 nautical miles S of Brownsville, TX

Unnamed Storm
1909
Max surge: 6 feet at South Padre Island, TX
Cat 3 at landfall
Landfall position about 120 nautical miles S of Brownsville, TX

All three of these hurricanes produced surges between 5-6 feet at S Padre Island

Comparison to Alex:
The average NHC forecast right now puts Alex's landfall at least 130 nautical miles S of Brownsville, which means it would make landfall at least as far south as all of these storms, if not farther. Also, all of these historical events interacted less with the Yucatan Peninsula, so they lost less strength, enabling all of them to make landfall as at least Cat 3 storms. Alex spent more time over the Yucatan Peninsula and will likely make landfall as a Cat 1 or 2 hurricane if the forecasts hold.

Therefore, a quick historical comparison would lead one to believe the peak surge heights in extreme S TX should be lower than these three historical events, perhaps around 3-4 feet. However, wave heights tend to be relatively high along the S TX coast, so we could see some decent waves and minor/ moderate beach erosion.

However, keep in mind that the models are divergent at this time and the surge could drastically change with a changing storm track. Just wanted to provide a quick historical comparison. Hopefully, we can get some similar discussions/ maps out to stakeholders before hurricanes strike this season.

Thanks!
Hurricane Hal

Disclaimer: This product is not a forecast. It is a surge estimate based on comparisons to historical storm surge events. This product is experimental and should not be used for decisions included, but not limited to, evacuations, storm preparations, insurance claims, or any decisions that may result in loss of life, injury, or personal, corporate or public economic losses. Louisiana State University, the University of Oklahoma, the SCIPP program, as well as Hal Needham and associated faculty, staff and students involved in developing this product, are not liable for any decisions that lead to injury, death, or any personal, corporate or public losses, including economic losses associated with this storm surge event.