Wednesday, September 18, 2013

Turning Soggy into the Weekend

By Jay Grymes & Steve Caparotta

We saw a few more showers running east-to-west across the viewing area this afternoon as compared to past days.  We expect even more clouds on Thursday afternoon and we’ll post rain chances in the 30% range for Thursday PM.
Rain chances take a big jump up for Friday and Saturday, with showers and t-storms “likely” for both days as a cool front advancing from the northwest takes advantage of an abundance of Gulf moisture.  We’ll back the rain chances down for Sunday, especially by the afternoon, but by then our attention may be on a potential tropical system in the Gulf.

As you can imagine, a wet forecast could mean the third Friday in a row where Mother Nature plays havoc with high school football and downtown’s “Live After Five.”  And after dodging rain and lightning near the scheduled kick-offs over the past couple of Saturdays in Tiger Stadium, we could be looking at rains extending through much of the Tigers’ SEC opener -- trouble for tailgaters and for fans-in-the-stands.
Admittedly, we can certainly use the rain.  In fact, you probably already know that most of the northern half of the state is under a burn ban and almost all of northern and western Louisiana is posted as “in drought” according to the weekly U.S. Drought Monitor.  None of the WAFB viewing area is currently included in the state’s drought areas but experts have our western communities rated as “abnormally dry” and on the verge of drought for this time of year.
The set-up through the coming days could make for a very wet spell -- the NWS Weather Prediction Center (WPC) is calling for 2” to more than 3” of rain for much of our viewing area between now and Saturday night.  That would not only take a big bite out of the local dry spell, but could lead to some local “nuisance” street flooding and standing water in the “usual suspect” spots over the weekend.  However, most of our local rivers and bayous are running at or near “base flow” (their lowest flow rates) so they have plenty of capacity for rain-generated runoff.  Therefore, we don’t anticipate river flooding at this time.
Then there’s the potential development in the SW Gulf.
Invest 95L -- the disturbance over the Yucatan and headed towards the Bay of Campeche -- appears on a slow path of becoming a tropical depression and could even become T.S. JERRY in the next couple of days.  As a depression, at least, it will be the 5th tropical cyclone to develop in the western/SW Gulf this season.  The four prior systems are: T.S. BARRY, T.S. FERNAND, ‘Cat 1’ INGRID and T.D. #8.  (You’ve got to believe that residents along the Mexico Gulf Coast are ‘storm weary.’)

As you may recall, INGRID’s track was reasonably-well forecasted and ‘she’ never prompted any serious concerns for the central Gulf Coast as her track into Mexico was never really in much doubt.  Unfortunately, the future of 95L is far from clear.
The latest forecasts (‘spaghetti plots’ and intensity forecasts) suggest that a slowly-developing system remains over the western Gulf into the weekend, maintaining a slow motion towards the NW.  But after about 2 to 3 days or so, the models diverge greatly in their solutions.

When models show good agreement through a 3-to-5 day window it suggests that they are all picking up on the same atmospheric steering -- both in terms of direction and speed.  Obviously, such good agreement in multiple models out 3 to 5 days provides confidence.  In this instance, however, we are seeing differences between the models as to what will become the main atmospheric “chauffeur” for the future of 95L into and after the weekend.  In addition, the models are not even in much agreement about potential intensification.
An upper-level trough over the eastern U.S. will send a cool front into the lower Mississippi Valley this weekend, with that front expected to stall along or near the northern Gulf Coast.  Our local rainy outlook for Friday, Saturday and possibly early Sunday will be a product of this front.
That same upper-trough’s impact on steering for the weekend’s Gulf system is the main question at this stage -- and you can find just about any possible outcome depending upon which model you choose.
A couple of models are suggesting that the trough essentially misses the tropical cyclone (whether a depression or tropical storm) -- fails to influence its trajectory -- leaving it to meander in the western Gulf.  That scenario likely favors another landfall for Mexico, this time possibly closer to the TX/MX border.  Other models are showing the trough grabbing the system and bringing it north or northeast, potentially interacting with the stalled front and threatening the U.S. Gulf Coast anywhere from Louisiana to Florida.
Then there is a more intriguing solution that suggest the trough “grabs” most of the system but leaves some of its energy behind, effectively stretching the tropical system across the Gulf and making a big mess of things.  While this could make for a string of very wet days into next week for several Gulf states, the potentially good news here would be that the system would have a very difficult time ramping-up into a destructive tropical wind-and-surge generating storm.
Those of you that have been “storm watching” for a while know that there are some computer models that experts tend to lean towards and others that they tend to discount.  But at this stage -- without a true cyclone yet -- even the more trustworthy models can bounce around in their projections from one computer run to the next.  The point: a ton of uncertainty remains. 
We’re going to have to just sit and wait a while on this one until the steering currents become a bit more apparent.

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