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Isaac Days Away: A Detailed Storm Briefing & Possible Good News

By: Oscar Fann WTVY-TV meteorologist
By: Oscar Fann WTVY-TV meteorologist

Early Sunday morning….

There should be good news forthcoming for the WTVY-TV area with regards to Isaac’s path…but first a little background.

As I compose this between the hours of 2 to 5:30 am cdt Sunday morning, over the last 24 to 36 hours there have been several significant changes from the computer weather guidance models concerning the path and intensity of Isaac ‘when’ it moves into the Gulf.

(A quick word on weather model ‘flip-flopping’ on Isaac’s path prediction…the computer guidance is only as good as the data that is input before the computers do their work. If the weather data is changing more so than usual (which evidently it has been doing), then the results will incorporate those changes – thus, different paths…but I’m not sure I’ve seen so many changes in such a short time).

First, the window appears to be quickly closing for Isaac to move north through the Florida Straits into southern Florida, then up along the Florida EAST coast – thus, bypassing the Gulf. The trough (area of extended low pressure) to the northwest of Florida as expected reformed northeast of Florida early on Saturday and appears too far removed and too weak to ‘pick up’ Isaac and move it across the Florida peninsula into the Atlantic.

However, Isaac has short term but not overly significant issues. Another smaller trough is persisting in the northwest Caribbean. I pointed out this trough (full of dry air) on the Friday night weather on WTVY-TV – at that time it had made it to Havana. It then merged with another area of low pressure and is currently southwest of Issac. For much of Saturday (and continuing into Sunday morning), Isaac and this other area of low pressure have not been getting along.

Early this Sunday morning, looking at satellite imagery, I see Isaac just off the northeast coast of Cuba with the other low (trough) in the northwest Caribbean. This Caribbean trough is responsible for thunderstorms between the Yucatan Peninsula and Jamaica. In addition, the Caribbean trough with its dry air (along with high terrain across eastern Cuba) is disrupting flow into Isaac. In fact, Isaac’s main area of thunderstorms is split into 2 parts – one to the north of the center and the other separated to the south and southeast of Isaac’s center.

This weakening will prove temporary, but the disrupted flow is causing short term problems in Isaac’s intensity.

Indeed, Isaac has weakened in the last 24 hours and its sustained winds have dropped from 70 mph to 60 mph.
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WHAT TO LOOK FOR TODAY…

The National Hurricane Center (NHC) believes Isaac will be steered almost westward for awhile on Sunday due to the tug from the Caribbean trough…then Isaac turns northwest as it crosses the Florida Keys. A longer westward motion (say closer to the lower Keys, or Key West) before entering the Gulf would be better news for the WTVY-TV area (more on that later).

Fewer hurricane models continue to show, FOR NOW, a chance of a turn northward then northeastward toward Apalachicola or even east of there.

What I have seen from the hurricane models over the last 36 hours is the renewal of the trend for a more westward track through the Gulf of Mexico.

I have often mentioned the two primary weather models are the GFS and the ECMWF models. It just so happens they have been the best performing models this tropical season…the ECMWF has been the better of the two, but the GFS did better with Tropical Storm Debby back in late June that caused the flooding northeast of Apalachicola, especially in Wakulla county.

Without getting too bogged down in revisiting these models’ tendencies over the last week, I will keep the discussion to the last 2 days, which is much more important.

Late Friday, the GFS model started targeting Isaac landfall for southeast Louisiana (there are several different GFS models – all of which make up what’s called the GFS ensembles, which show variations of ‘what if’ scenarios).

Boy, did it ever target southeast Louisiana – and it hasn’t let up since.

Meanwhile, the ECMWF briefly gave life to the possibility of Isaac moving more north through the eastern Gulf and even turning northeast of Apalachicola and cutting across northeastern Florida and southeastern Georgia. This was a little surprising and a big change from its previous run (which was toward the western Florida panhandle).

I think rightfully in this recent ECMWF change the NHC decided to wait for more consistency from the ECMWF before giving it credibility.

Meanwhile, the GFS in the Saturday computer runs (the two main computer runs) showed remarkable consistency in stating its case for the southeastern Louisiana hit (the GFS’ conclusions showed 2 main targets: a hit near the mouth of the Mississippi River and then a path perilous close to New Orleans, OR a landfall about 50 miles south of New Orleans, then a move more west along the Louisiana coast).

Through the day Saturday the NHC briefly alluded to increasing model guidance moving Isaac’s path more westward, BUT only shifted its OFFICIAL landfall path slightly westward (from just west of Panama City to near Ft. Walton Beach).

Finally, what you might consider the coup de grace, the late day Saturday ECMWF came in and not surprisingly, ‘gave up’ (actually, better data was incorporated into the computer run) on the northeastern Gulf path – and now had the landfall target close to the Alabama / Missippippi line.

At the very least, the NHC should now begin a definitive shift of Isaac’s path farther to the west. Unfortunately, this also means a chance of a much stronger hurricane for the north central Gulf coast.
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4AM CDT SUNDAY AUG 26 ISAAC UPDATE

The 4am complete update has arrived from the National Hurricane Center (NHC)…and here’s the news (you can find the full NHC update under an accompanying article on the Home and Weather pages)…

Issac slightly stronger (65 mph, still a tropical storm), but here’s the important change…

The NHC has indeed shifted Isaac’s path significantly to the west.

This shift is actually farther westward than I anticipated – given the conservative nature of the NHC. Nevertheless, it is justified, as I already pointed out, the trend has been there from 12 to 36 hours.

An eventual shifting even farther westward is possible later Sunday or Monday (unless, of course, ridge and trough positions change, along with the steering winds for Isaac).

The current forecast landfall is near Dauphin Island and Mobile Bay around 2am cdt Wednesday Aug 29…still as a Category 2 (sustained winds near 105 mph).

Also, Isaac may move a little faster through the Gulf than previously thought. That could be bad and good news. A faster moving Isaac could create more tornado potential as its winds reach inland areas. However, the faster movement may also work to hold excessive rain totals down.

The longer soon-to-be Hurricane Isaac remains over the Gulf, the greater chance the intensity increases. (The NHC would be the first to tell you intensity forecasts for hurricanes leave much to be desired, especially in rapid intensification modes). Isaac will pass over very warm central Gulf waters, but Isaac should miss the warmest water that lies in the south central Gulf.
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IF THIS NEW FORECAST PATH HOLDS, this is what we can expect for the WTVY-TV area:

Hurricane Watches (already in effect) for the nearby coast..

Hurricane Warnings still likely from Apalachicola westward..

Tornado Watches for all of the Florida panhandle..southern and southeastern Alabama and probably southwestern Georgia.

There will be other watches (ESPECIALLY FLOODING POTENTIAL – 3 to 7 plus inches of rain still likely), warnings and advisories to come..

In short…CONTINUE PREPARATIONS in case the path changes yet again…STOCKUP ON SUPPLIES..PUT GAS IN YOUR VEHICLE..GET CASH from the bank / ATM..

REGARDLESS….BE PREPARED FOR POWER OUTAGES..DOWNED TREES AND POWERLINES..

The recent NHC update on Isaac is good news, but the danger potential remains.

All of us at WTVY-TV will be here as Isaac draws near.

ALL THE BEST and BE SAFE!!

Oscar Fann WTVY-TV meteorologist


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