Sunday, August 26, 2012

This week in Mid-South weather with a special focus on Isaac

Mid-South meteorologists have a very challenging week ahead thanks mainly to one very large unknown: the eventual forecast track of what will become Hurricane Isaac in the Gulf of Mexico.

Mid-South: Monday-Wednesday

For the first half of the week, the forecast is somewhat predictable. Humidity has returned on a southerly fetch of Gulf air, creating typical summertime conditions - hot and humid with small rain chances in the afternoons. We saw that today with a high of 94 and dewpoints well into the 60s. A smattering of afternoon showers brought rain to a few folks, but not like the steady rain that many saw Saturday morning as a warm front moved through. Monday and Tuesday will see a few more afternoon showers, but likely no more than a 20% chance. Temps will reach the mid 90s for highs with lows in the 70s. A weak low pressure trough will move through late Monday, pulling wind back to a more northeasterly direction and slightly reducing humidity values on Tuesday.  After a reasonably dry but hot day Wednesday, the tricky part of the forecast will be Thursday through Saturday and will be entirely based on where Isaac decides to go between now and then.

Tropical Storm (for now) Isaac

Over the past 24 hours, Isaac has moved through the Florida Straits and just past the Florida Keys as a strong tropical storm. It now embarks on a trek across the very potent Gulf of Mexico. Organization of the storm this afternoon should lead to strengthening by tomorrow and with plenty of very warm water and low wind shear values ahead of it, we expect to see Hurricane Isaac tomorrow. Unfortunately, computer models continue to struggle mightily with the eventual path of Isaac with landfall points ranging from the TX/LA border to the panhandle of Florida.  As a whole, model tracks have generally shifted west slowly, enough that the National Hurricane Center (NHC) now has landfall early Wednesday very near the MS/LA border, or just east of New Orleans.  Hurricane Warnings have been posted from Destin, FL to Morgan City, LA.  The Hurricane Center's intensity forecast calls for a strong category 2 storm at landfall, with maximum wind near 100 mph.

Sunday afternoon computer model forecasts for Isaac. These are NOT official forecasts, just possibilities.
NHC's official forecast track for Isaac as of 4pm CDT Sunday
Being that my meteorological expertise is not truly in tropical forecasting, I am not one to go against the experts and forecast something different, however I like the latest solution that NHC has come to with regards to the forecast track.  I tend to lean towards a stronger storm given the optimal conditions ahead of Isaac, however that is tempered some by it's behavior the past couple of days when I expected we'd see a little more strengthening.  Computer models are tending to favor a major hurricane by landfall (category 3+), which makes a lot of sense, but NHC may be on to something.

Gulf region impacts

Given the NHC forecast, impacts to the central Gulf Coast region will be widespread and destructive, particularly near and to the right (east) of where the center makes landfall.  In their scenario, this would especially target the Mississippi Gulf Coast.  Storm surge flooding of  up to 8-12 feet would be possible in this area, which would push Gulf water perhaps miles inland. In addition, torrential rainfall, some tornadoes, and prolonged 50-80 mph wind would do a great deal of damage.  The Alabama Gulf Coast and western panhandle of Florida, including Mobile Bay, Gulf Shores, Pensacola, and Panama City Beach would also see a surge of water, sustained gale force wind, heavy rain, and possibly tornadoes.  Flooding would be a serious threat in these areas, as well as over much of Mississippi and southern Alabama.

Tropical warnings as of 4pm CDT Sunday with areas already seeing tropical storm force wind in orange.
Hurricane Warnings are posted from Louisiana to Florida.
Probability of tropical storm force wind based on the current NHC track
In eastern Louisiana, including New Orleans, the impacts would definitely be felt, but may not necessarily be devastating  if they stay on the west side of the center of the storm. On that side, wind would blow offshore and the storm surge would be minimal. Gale force wind would still be likely and torrential rain could produce some flooding concerns.  HOWEVER, if the track shifts any further west, placing New Orleans under and east of the center of the storm, all bets are off and infrastructure rebuilt after Hurricane Katrina will get it's first true test. So, though we don't know exactly where landfall will be, I can all but guarantee that some places are going to feel the wrath of a mighty storm.

Mid-South Isaac impacts: late week

After landfall, areas inland will deal with the remnants of Isaac.  Inland effects are worse on the eastern side of the storm than the western side, with widespread heavy rain and thunderstorms, strong wind, and isolated tornadoes all possible to likely.  On the west side of the storm, especially within about 100 miles of the center, breezy to windy conditions and rainfall are likely, but the tornado threat is very low.  Areas outside of that radius would get a breeze, but probably not much rain.

The NHC forecast brings the center of Isaac's remnants basically up I-55 in Mississippi and into the Memphis area Friday afternoon. Computer models are all over the place with respect to the remnants however. The NHC forecast is not a guarantee that we'll see the effects of the storm, other than perhaps a noticeable breeze, as the path is highly uncertain.  The impacts outlined above apply here, so if the remnants move across eastern Arkansas, we'll be in a position to get much greater impacts than if it moves into northeast Mississippi or further east.  (When the leftovers from Katrina moved through northeast Mississippi, Memphis received wind gusts that peaked at 54 mph and about 3" of rain.)
Rainfall forecast through Friday evening, given the current NHC track for Isaac. Notice the tight precipitation gradient along the Mississippi River, where west TN is forecast to receive 4-8" of rain and east AR less than 1".
For now, the MWN Forecast holds off on predicting a specific outcome until there is more certainty. Though the rainfall forecast above looks ominous for west TN, this is highly subject to the track of Isaac and we're not ready to bite just yet!  Many have asked about next weekend's outlook.  At this point, we can hope that even if we get the remnants of Isaac, most of it will be cleared out by Saturday and Sunday. I simply make no guarantees other than we'll keep you posted if you follow along!

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