Models Shift Back West: TS Isaac Update

Tropical Storm Isaac continues to be of concern to residents along the central Gulf Coast states from Mississippi to Florida as this system will likely make landfall early next week as a hurricane. A lot of uncertainty remains in both Isaac's future intensity and track which this post will address in depth.

First things first: Isaac remains a fairly disorganized Tropical Storm in the Caribbean, hugging the south coast of Hispañola (Haiti/Dominican Republic). The 10PM official observation has max sustained winds at 70 mph, moving NW at 14 mph. Of note, Isaac's forward speed has slowed considerably in recent days and now has begun to make that northwestward turn.

Possibly the biggest uncertainty lies in how Isaac will interact with these mountainous islands over the weekend. It is likely the system will 'survive' but how much damage will be done due to shear and interrupted low-level inflow. Until Isaac emerges into the far SE Gulf near the Florida Straits, confidence can only be so high. However, the forecast models are giving a general idea of Isaac's evolution and with this storm still being about 5 days out, that's pretty good. 

The main story continues to be how the models have been shifting west and east with each run. Luckily, the shift has not been overly dramatic, but the general targets have been near Mobile Bay east to near Apalachicola, FL. 00Z runs of the tropical models look a lot like last night, with a more westward path, closer to the Pensacola, FL area:

Not only has there been an issue of placement of Isaac early next week, there also is a disagreement among the dynamic models in terms of timing.

The Canadian (above) shows Isaac near Cedar Key, FL, Monday evening at 7pm.

The GFS shows Isaac near Gulf Shores, AL, Tuesday morning at 7AM.

The European is in good agreement with the GFS on Isaac's position near Gulf Shores, AL, however the Euro is timing landfall almost 24 hours later than the GFS.

The HWRF and GFDL seem to take Isaac farther west than any other model, and don't indicate a due northward turn like the global models. This could be because the global models may have a better handle on the upper air/steering patterns for Isaac.

Both models show Isaac approaching land Tuesday evening. 

In terms of intensity, the official forecast from the NHC which reflects the forecast models well, keeps Isaac as a minimal hurricane during its progression through the Gulf of Mexico. Here's the 00Z intensity guidance:

However, when the warm sea surface temperatures of the Gulf are considered, these predictions may seem a bit conservative. This map is in Celsius but the brown colors in the Gulf indicate temperatures in excess of 85F degrees:

If Isaac maintains the slow forward speed like it has now, intensification greater than what the models are showing now seems very likely. However, it Isaac starts traveling closer to 20mph which is fast for a tropical system, rapid intensification will be hard to come by.

Here's the latest *official* forecast from the National Hurricane Center:

SUMMARY: Official forecasts from the Hurricane Center will have to be monitored closely throughout the weekend as further changes in the track/intensity are likely. People who live and/or have interests along the central Gulf Coast, especially along the Alabama and Florida coast,  need to go ahead and think about early preparations and be ready to act come Sunday or Monday. 

Stay tuned for more updates here!