WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

Alex still not a hurricane

By: Dr. Jeff Masters, 2:19 PM GMT on June 29, 2010

Tropical Storm Alex continues to slowly grow more organized as it heads towards the Texas/Mexico border. Satellite loops continue to show a slow increase in Alex's heavy thunderstorms and low level spiral bands. An impressive large thunderstorm with very cold tops has erupted near Alex's center in the past hour, and this may be a "hot tower" that presages the formation of an eye. The latest Hurricane Hunter center fix at 6:29am CDT found a central pressure of 982 mb, which is a typical Category 1 hurricane pressure. However, as of 9am CDT, the Hurricane Hunters has still not found hurricane-force winds at the surface in Alex. The clockwise flow around an upper-level high pressure system a few hundred miles west of Alex is bringing about 10 - 15 knots of wind shear to the storm, which is slowing intensification. Heavy thunderstorm activity is still limited on the storm's northwest side, thanks to the shear and some dry continental air flowing off the coast of North America. Sea Surface Temperatures are very warm, 29°C.

Hurricane local statements with projections for how Alex will affect the coast are now being issued by the National Weather Service in Brownsville and Corpus Christi. Since Alex is a large storm, it will have a storm surge that will affect most of the South Texas coast. NHC is giving a 30% - 40% chance of a storm surge of at least 4 feet affecting the Brownsville area, and 10% - 20% chance the surge will exceed 5 feet. In theory, a Category 2 hurricane moving WNW at 5 mph can bring a storm surge of up to 8 - 9 feet to the Southern Texas coast (Figure 1.) Of course, flooding damage from the expected 10 - 20 inches of rain from Alex will also be a major concern, as will wind damage. The combined wind, surge, and flooding damage from 2008's Hurricane Dolly, which hit near Brownsville, were about $1.05 billion. Dolly was a Category 2 hurricane offshore that weakened to a Category 1 hurricane with 85 mph winds when it made landfall. I expect Alex will be similar in its impacts to Dolly, though Alex's storm surge damage is likely to be greater.

Figure 1. Maximum Water Depth (storm tide minus the elevation of the land it is passing over) computed using the primary computer model used by the National Hurricane Center (NHC) to forecast storm surge--the Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. The accuracy of the SLOSH model is advertised as plus or minus 20%. The "Maximum Water Depth" image shows the water depth at each grid cell of the SLOSH domain. Thus, if you are inland at an elevation of five feet above mean sea level, and the combined storm surge and tide (the "storm tide") is ten feet at your location, the water depth image will show five feet of inundation. This Maximum of the "Maximum Envelope of Waters" (MOM) image was generated for high tide, and thus shows the worst-case inundation scenarios for a mid-strength Category 2 hurricane moving WNW at 5 mph. For more information on storm surge, consult our detailed storm surge pages.

Track forecast for Alex
The latest 0 UTC and 6 UTC (1am CDT) runs of our most reliable computer models did a pretty poor job, in general, of capturing the north-northwest motion of Alex early this morning, and are thus probably too far south in their landfall predictions. The official NHC forecast is thus taking Alex farther north than the models are. The most northerly landfall location, near Port Mansfield, is now being predicted by the HWRF model. The most southerly landfall prediction comes from the ECMWF model, which predicts landfall more than 100 miles south of Brownsville.

To get the probability of receiving tropical storm force winds or hurricane force winds for your location, I recommend the NHC wind probability forecasts. The 4am CDT (9 UTC) wind probability product predicted that Brownsville, Texas had the highest odds of getting a direct hit from Alex:

Brownsville, TX: 84% chance of tropical storm conditions (winds 39+ mph), 23% chance of hurricane force winds (74+ mph). This is the cumulative probability through Saturday morning. The wind probability forecasts also include separate probabilities for each 12-hour period between now and three days from now, and each 24 hours for the period 4 - 5 days from now.

Corpus Christi, TX: 44% tropical storm, 4% hurricane.

La Pesco, MX: 42% tropical storm, 3% hurricane.

Freeport, TX: 22% tropical storm, 0% hurricane.

Tampico, MX: 20% tropical storm, 0% hurricane.

Galveston, TX: 16% tropical storm, 0% hurricane.

Uncertainty in the NHC Cone of Uncertainty
A research project funded by NOAA known as the Joint Hurricane Testbed has produced a remarkable number of tools now in operational use at the National Hurricane Center to improve hurricane forecasts and warnings. One of these projects, called "Prediction of Consensus TC Track Forecast Error and Correctors to Improve Consensus TC Track Forecasts", was an effort by Dr. Jim Goerss at the Navy Research Lab to improve the accuracy of the NHC "cone of uncertainty" (AKA the "Cone of Death") showing where a storm is expected to track 2/3 of the time. The radius of the circles that make up the cone are based on error statistics of the official NHC forecast over the past five years. We can expect in certain situations, such as when the models are in substantial disagreement, a consensus forecast made using these models will have much greater than average errors. Since the NHC typically bases their forecast on a consensus forecast made using a combination of reliable hurricane forecasting models, it is instructive to view the "GPCE" (Goerss Prediction Consensus Error) circles to see if the uncertainty cone should be smaller or larger than usual. The consensus forecast I'll look at is called "TVCN", and is constructed by averaging the track forecasts made by most of (or all) of these models: GFS, ECMWF, NOGAPS, GFDL, HWRF, GFDN, and UKMET. In the case of this morning's 12 UTC (7am CDT) June 28 run of these models, here is what the radius of the "cone of uncertainty" should be, in nautical miles:

12 hours: 36 nm
24 hours: 59 nm
36 hours: 82 nm
48 hours: 119 nm

And here is the radius of NHC's "cone of uncertainty" for their official forecast, based on the average errors for the past five years:

12 hours: 36 nm
24 hours: 62 nm
36 hours: 85 nm
48 hours: 108 nm

So, the GPCE error estimates are showing that the latest forecasts for Alex out to 48 hours are are pretty close to average.

Figure 2. Tropical Cyclone Heat Potential (TCHP) for Monday, June 28, 2010 at 7pm CDT, with the official NHC forecast track for Alex overlaid. Alex will be passing directly over the core of a warm Loop Current eddy that broke off from the Loop Current back in July 2009. Values of TCHP in excess of 90 kJ/cm^2 commonly cause rapid intensification of hurricanes. Alex will be passing over waters with less TCHP than that. Image credit: NOAA RAMMB/Colorado State University.

Intensity forecast for Alex
Alex is approaching a region of ocean with with a warm, clockwise rotating Loop Current eddy that broke off from the Loop Current in July 2009 and moved west-southwest over the past 11 months. This eddy has moderately high total ocean heat content (Figure 2.) Wind shear is currently a moderate 10 - 15 knots, and is projected by the SHIPS model to decrease to the low range, below 10 knots, this afternoon and Wednesday. The combination of low wind shear and moderately high ocean heat content should allow Alex to intensify into a Category 2 hurricane. NHC is giving Alex a 79% chance of being a hurricane on Wednesday afternoon, and a 9% chance it will be a major hurricane at that time. Water vapor satellite images show the amount of dry air over the western Gulf of Mexico has decreased over the past day, though dry air may turn out to be a detriment to Alex on Wednesday as the storm approaches land. Another factor limiting Alex's intensification may be that the atmosphere is more stable than usual right now--temperatures at 200 mb are a rather warm -50°C, and are expected to warm an additional 1 - 2 degrees by Wednesday. I don't expect Alex to stall out again, so slow motion leading to upwelling of cold water will probably not be a problem for Alex. The main issue limiting intensification will be the fact that Alex is so large, and it takes more time for a large storm to organize. Thus, I think Alex has only a 20% chance of intensifying into a major hurricane before landfall.

Elsewhere in the tropics
The last two runs of the NOGAPS model has been predicting the formation of a tropical disturbance off the coast of Nicaragua on Friday or Saturday that will move northward towards Jamaica and Cuba. The GFS model, and the two models that use it for starting conditions, the GFDL and HWRF, are indicating the possibility that a weak extratropical storm may form along coastal Alabama late this week. It is unlikely that such a storm would be over water long enough to transition to a tropical storm.

Wind and ocean current forecast for the BP oil disaster
It currently appears that Alex's winds will not directly affect the oil slick location. However, because Alex is such a deep low pressure region, strong southeast to south winds of 10 - 20 knots will blow over the oil slick region today through Thursday, according to the latest marine forecast from NOAA. The resulting currents should act to push oil to the west and northwest onto portions of the Louisiana, Mississippi, and Alabama coasts, according to the latest trajectory forecasts from NOAA and the State of Louisiana. Oil will also move westward along the central Louisiana coast towards the Texas border. Alex is expected to bring a storm surge of 2 - 3 feet along the coast in the oil spill region, which will push oil deep into the marshlands in some locations. The long range forecast for the oil slick region is uncertain, due to the possibility a weak area of low pressure might develop late this week along the remains of a cold front draped across the region.

Resources for the BP oil disaster
Map of oil spill location from the NOAA Satellite Services Division
My post, What a hurricane would do the Deepwater Horizon oil spill
My post on the Southwest Florida "Forbidden Zone" where surface oil will rarely go
My post on what oil might do to a hurricane
NOAA's interactive mapping tool to overlay wind and ocean current forecasts, oil locations, etc.
Gulf Oil Blog from the UGA Department of Marine Sciences
Oil Spill Academic Task Force
University of South Florida Ocean Circulation Group oil spill forecasts
ROFFS Deepwater Horizon page
Synthetic Aperture Radar (SAR) imagery from the University of Miami

"Hurricane Haven" airing again this afternoon
Tune into another airing of my live Internet radio show, "Hurricane Haven", at 4pm EDT today. Listeners will be able to call in and ask questions. The call in number is 415-983-2634, or you can post a question in the comments area on my blog during the show. You can also email the questions to me today before the show: jmasters@wunderground.com. Be sure to include "Hurricane Haven question" in the subject line. Some topics I'll cover today on the show:

1) Alex
2) A look ahead at what may happen the rest of hurricane season

Today's show will be about 45 minutes, and you can tune in at http://www.wunderground.com/wxradio/wubroadcast.h tml. The show will be recorded and stored as a podcast.

Next post
I'll have an update between 2 - 3 pm CDT today, when the latest set of models runs will be available.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.