Thursday evening update on Tropical Storm Isaac

For the past day or so, we've been pushing out bits of information about Tropical Storm Isaac on our social feeds, however there is enough interest even in the Mid-South that it's time to cull the latest information and put it together in one place.  So here it is. We encourage you to follow us on social media for the latest info as it happens - links can be found at the end of this blog.

So far, Isaac wins the award for the largest mess of a tropical system I can recall.  While nearly 1000 miles wide, it has so far remained highly disorganized, thanks in large part to a great deal of dry air that has been caught up in the northern half of the storm.  Multiple times today, the National Hurricane Center and Hurricane Hunters flying into the storm have noted "multiple centers" to the storm - individual areas of low pressure within the cloud mass and convection (thunderstorms). If this storm can get organized (which we expect it to tomorrow), it could become a large one to reckon with.

Obviously in forecasting, including for systems like these, meteorologists rely strongly on computer models to provide a clue as to what path a storm will take, how strong it will be, and how fast it will move. Then, intuition kicks in and adjustments are made to the forecast. I'll admit I am NOT a tropical forecaster and have not been specifically trained in hurricane meteorology. However, I can parse a great deal of incoming information and I follow many bright minds in the business for clues on a storm's structure and tendencies.  While I don't claim to be able to forecast the track or intensity myself, I DO know a fair amount about what causes these storms to tick, what factors tend to strengthen and weaken them, as well as the potential impacts given a known set of parameters.

First I introduce the current conditions. Below is a recent infrared satellite image as the storm moves generally west, to the south of Puerto Rico and the Dominican Republic.  Maximum sustained wind is near 45 mph.  The strongest convection (storms) is in the reds and dark grays, mainly southwest of the center.  I expect we may see that center re-located somewhat if convection continues in that area as it seems the most likely place for the primary "center" of the storm to appear.

8:15pm infrared satellite image of Tropical Storm Isaac

Next, we look at the official track from the National Hurricane Center. It indicates a west-northwest motion across Haiti tomorrow, a northwest path over the mountainous terrain of Cuba on Saturday, then emerging over the southeast Gulf of Mexico Sunday afternoon.  After a hit on the Florida Keys (with south Florida on the stronger east side of the storm), Isaac continues northwest across the Gulf towards the north-central Gulf Coast early next week.

UPDATED: 10pm CDT official forecast track of Tropical Storm Isaac from NHC

Isaac could briefly attain hurricane status before reaching Cuba, but will lose strength over the mountains this weekend if this path verifies.  Once emerging over the Gulf, conditions are highly favorable for re-intensification, perhaps rapidly depending on how disorganized it becomes over Cuba.  The less time it spends over land, the more likely it is to be stronger after emerging over water and begins strengthening again.

Now we look at a series of computer model forecasts, commonly called "spaghetti plots" - you can probably see why they are called that.  These potential tracks, as a group, have been shifting west with each new model run (which occur every 6 hours). First, the afternoon model runs, which show potential landfalls, as a group, between the Big Bend of FL and the MS Gulf Coast.

Computer model output from several models on Thursday afternoon showing potential tracks for Isaac

Next we check out the latest evening models, which clearly show a more westward shift with potential landfalls from southeast Louisiana to near Pensacola.

Computer model output for Thursday evening. Note a general westward "shift" in the potential tracks (as a whole).

Finally, we look at another suite of forecast tracks, all from the same model (the well-known GFS) but with various possibilities this model considers.  Again, the tracks cluster along the north-central Gulf Coast with landfalls ranging from Biloxi to Apalachicola (disregarding a couple of outliers).

GFS ensemble members forecast tracks for Isaac

Lastly, let's look at one of the reasons why intensification is likely over the Gulf - water temperatures. Many parts of the Gulf are nearly bathwater temperature, which is like feeding an athlete Wheaties, then injecting steroids. The orange-red colors are temperatures of 86-90+ degrees.  Any storm that passes over those waters will get a shot of high-octane fuel.

So, what does all this mean?  It means you should stay tuned.  It means that the northern Gulf of Mexico needs to be on high alert for the possibility that we could see the first major (category 3 or stronger) U.S. landfall in nearly 7 years (the last was Wilma in the infamous year of 2005, which also produced Katrina).  It also means that the entire east coast of Florida could be side-swiped, including Tampa - the site of the Republican National Convention next week. It also means that meteorologists will be closely monitoring the tracks and that it's conceivable the city of New Orleans could be tested for the first time since Katrina.  Landfall, wherever it occurs, will likely be in the Tuesday-Wednesday time frame with the storm moving out of the Gulf Coast region about 24-36 hours after landfall.

How about impacts on the Mid-South?  It's really too early to tell. However, one thing we are sure of is this - the farther west the track goes in the Gulf, the more likely it is we could feel some impact.  A landfall in Louisiana would give us a higher likelihood of rain than landfall in the Florida panhandle.  I don't know anyone who wouldn't welcome some rain in the Mid-South after the drought of summer 2012!

Stay with us for the latest via the MWN Tropical page on MemphisWeather.net as well as our Twitter and Facebook feeds linked below.

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