Isaac Landfall Means Increasing Tornado, Flood Threat

Isaac has now been upgraded to a Category 1 Hurricane with max winds of 75mph, still moving NW towards SE Louisiana. Here's the latest satellite image of Isaac showing some robust convection near the center, but also some dry air being entrained right into the center of the storm. Earlier, an eye was trying to form, but the battle with dry air really has won out and prevented Isaac from really strengthening further. This storm is beginning to slow down, so there is still time for intensification, but at this point the main concern will be extensive coastal flooding from 6-12 foot storm surge, as well as inland flooding due to relentless rainfall over the next 36 hours.

Here is the HWRF's depiction of early Wednesday morning at 1 AM, note, the center of the storm is still offshore:

As a secondary concern, with any landfalling tropical system, the threat for spin-off tornadoes exists, especially in the eastern quadrants of the storm in the spiraling bands. We've already had a tornado warning issued for parts of Hancock County and Orleans and St. Tammany Parish earlier today. Here is a current radar picture, showing a gusty squall just south of Hattiesburg moving W:

In terms of the immediate tornado threat, the SPC has issued 2 tornado watches for coastal Mississippi, Louisiana, Alabama, and the western Florida Panhandle, as well as another watch for parts of south Mississippi. Both watches will be allowed to expire at 7 PM, however, additional watches are very likely as Isaac continues to track ashore:

In terms of landfall, it appears Isaac is still on track to come ashore very late tonight and early tomorrow morning near Barataria Bay in SE Louisiana, moving up the bayou towards Lafitte and Port Sulphur. 

With the storm tracking west of New Orleans, we will be seeing strong, persistent onshore flow upstream of all the bayous and bays, and this will pose a great chance of flooding, not to mention the heavy rain. Coastal flooding has already been a problem along the MS coast as well as the FL panhandle. Even though Isaac is not a major hurricane, this system still poses a real threat, especially to low-lying coastal areas. Inland, communities still need to be prepared for extended power outages, really as far north as I-20, downed trees and power lines, and flash flooding in the coming days. 

Here's the latest rainfall estimate from the HPC:

Westward Shift Continues on Isaac

NEW: South Florida will face Isaac's lashing Sunday as the eastern side of the storm will bring the potential for tornadoes and flooding rains. The SPC has placed most of the South Florida in a slight risk for severe weather, including the likelihood of tornadoes in Isaac's outer bands:

After killing 3 in Haiti, Tropical Storm Isaac is now tracking back over open water, parallel to the north coast of Cuba and is heading towards the Florida Straits. By Sunday afternoon, this system will solidly be in the Gulf of Mexico, where rapid intensification seems very likely.

If you've been following Isaac closely, you know that the latest trends in the models have favored a farther track west, suggesting a central Gulf Coast landfall early next week (LA, MS, AL). Notice the 00z model runs shifted even farther west, targeting the SE Louisiana coast for a landfall:

Isaac's track really depends on the upper air pattern evolving over the US- the ridge building over the four corners region and the trough developing over the eastern half. The trough will have the most influence on Isaac; if the trough is deeper and farther south, this will likely 'grab' Isaac and lure the storm more northeast, which would mean a landfall farther east- likely the Florida panhandle as was previously thought. However, if the trough is flatter and stays farther north, Isaac will likely stay on its NW track and likely move into Louisiana.

The European model, which originally was an outlier with the farthest west landfall, is is now showing Isaac tracking closer to the FL/AL state line, near Pensacola Tuesday night:

-photo courtesy of Garrett Bedenbaugh

The image above is valid Tuesday at 7PM.

The GFS has shifted farther west with each run today, and now suggest Isaac will make landfall along the SE Louisiana coast Tuesday evening:

The Canadian solution is very close the the GFS in respect to location, but is 6 hours slower; below is valid at 1AM Wednesday morning, showing Isaac near the mouth of the MS River:

How strong will Isaac be? That answer may be a bit troubling. First, let me show you the HWRF 00z run. This model is suggesting Isaac will strengthen into a major category 4-5 hurricane before making landfall near Port Sulphur, LA, Tuesday afternoon:

This seems to be one of the faster models and is probably too fast- but the agreement with the Canadian and GFS on location cannot be ignored. This model has one of the most bullish intensity forecasts, but with the upper air pattern over the Gulf favoring development and warmer than normal water temperatures, this cannot be ignored!

Here again, is the *official* forecast from the National Hurricane Center, suggesting Isaac will make landfall near Fort Walton Beach, Florida, late Tuesday night into Wednesday. It is important not to focus solely on the center line as the effects of Isaac will be felt for hundreds of miles around the storm center, especially if this thing becomes a major hurricane:

Stay tuned for further updates!

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!

Tropics Watch: The Latest on Isaac

Tropical Storm Isaac continues tracking west across the northern Caribbean Wednesday afternoon. As of 5PM AST, max winds remained at 45 mph and the system's motion was west at 22 mph. Numerous tropical weather watches and warnings remain in effect:

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...
 
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* MARTINIQUE
* DOMINICA
* GUADELOUPE AND THE SURROUNDING ISLANDS...AND ST. MARTIN
* ST. KITTS...NEVIS...ANTIGUA...BARBUDA...MONTSERRAT...AND ANGUILLA
* SABA...ST. EUSTATIUS...AND ST. MAARTEN
* BRITISH VIRGIN ISLANDS
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. VIRGIN ISLANDS
 
A HURRICANE WATCH IS IN EFFECT FOR...
* PUERTO RICO...VIEQUES...CULEBRA...AND THE U.S. AND BRITISH VIRGIN
ISLANDS
* SOUTH COAST OF DOMINICAN REPUBLIC FROM ISLA SAONA WESTWARD TO THE
HAITI-DOMINICAN REPUBLIC SOUTHERN BORDER
 
A TROPICAL STORM WATCH IS IN EFFECT FOR...
* NORTH COAST OF DOMINICAN REPUBLIC FROM THE HAITI-DOMINICAN
REPUBLIC NORTHERN BORDER EASTWARD TO NORTH OF ISLA SAONA

Isaac's track still remains very uncertain as the global dynamic models and the tropical models all show differing solutions, especially in the period beyond the next 4-5 days. If the European solution is correct, this would have the biggest impact on our forecast, as this model brings Isaac into the Gulf and ashore along the central Gulf Coast as a major hurricane. At this time, the Euro is the outlier among all the models, but cannot be totally ignored. Here is what the European is forecasting for NEXT Thursday morning, Aug 30th:

Most opposite to the Euro's solution is the NAM, which keeps Isaac in the Atlantic through the next 84 hours, east of Florida. Here is the NAM's output, valid Saturday evening at 7 pm: 

The GFS and Canadian's solutions basically split the difference between the NAM and Euro, and target the Florida peninsula then the eastern seaboard.

Isaac's evolution of intensity is also in question due to the rugged terrain of the Caribbean Islands it will be crossing in the coming days. Isaac is expected to cross right over the Island of Hispañola and the eastern part of Cuba. The mountainous terrain of this area will really tear Isaac apart and the extent to which any re-strengthening will occur remains to be seen. 

Dry air to the north of the cyclone is also being entrained towards the center, preventing rapid strengthening. The storms structure is not nearly as compact as it has been, as it has become more elongated in nature and overall, appears to be disorganized:

Here is the official NHC track:

As far as the model spread, the majority have shifted slightly west with time, bringing Isaac up the west coast of the Florida peninsula:

It is still WAY too early to make a call as far as ultimate destination. Stay tuned for further updates...