Clark
(Meteorologist)
Tue Sep 14 2004 09:47 PM
Ivan

And now Ivan...well, what can one say about this storm? I know for one that it has pretty much cracked the Dvorak intensity scale -- not once during the regularly scheduled updates has it had a T-number higher than 6.5 (127kt/935mb), and only on a couple of occasions did it reach T7.0 (140kt/921mb) on intermediate advisories. Just goes to show you how beneficial recon can be, despite the high costs involved (into the millions of dollars).

Ivan has continued to move generally in a NNW fashion today. Most of the motion continues to be northerly, though every so often the eye jogs to the west, often times in wobbles at it tries to reform and consolidate. The storm is roughly due south of Pensacola right now along 87W.

Right now, Ivan is starting to look a bit more asymmetric as some drier air begins to infiltrate the SW side of the storm (thanks to sullynole for the heads-up on this one). The dry air is a remnant of the trough now up by Ohio that dove down to the northern Gulf last night; it has taken time to rotate around the circulation of the storm, but it is slowly but surely infiltrating the circulation. Outer bands are beginning to impact the Florida peninsula and big bend, while the outflow from the storm reaches far up the Atlantic seaboard.

Te shortwave trough over the SE US last night lifted faster than expected, but was enough to begin to turn the storm. However, it is not enough to lift it to the shore or turn it eastward, contrary to my previous thinking. The ridge located over Texas and Oklahoma, while small in nature, is beginning to build northeastward. If it builds faster than Ivan moves, it could result in a storm that stalls out much sooner than expected -- perhaps even over the Gulf. I don't anticipate that, but it would be a disaster scenario for the New Orleans-Panama City area if so. Instead, a slower forward speed to landfall, which is still roughly 42-48 hours away I believe, is more likely.

With the change in prognostic reasoning, a change in the landfall region is necessary. The area most under the gun from a direct landfall is from Mobile eastward to Destin, in fair agreement with (if a bit east of) the NHC path. It must be noted, however, that the wind field with this storm is so large -- along the north side, the tropical storm wind field extends 235 miles to the west and 265 miles to the east -- that areas well away from the exact landfall point will receive high winds and have the potential for damage. It is feasible that much of the hurricane warning area will receive hurricane force winds, at least in gusts, with this storm. If it can be surmised, the tropical storm wind field is a bit bigger with this storm than with Frances, and we all saw what happened as it moved inland.

Assuming a landfall point near Pensacola for a moment, tropical storm force winds would affect the region from Baton Rouge, LA to just east of Perry, FL, while hurricane force winds would affect the region from Biloxi, MS to Panama City, FL. In addition, rains along the path of this storm, especially to the east of a line from Mobile, AL to Chattanooga, TN, are going to be locally heavy. Beyond landfall, rains in the central Appalachians -- already devestated by Frances -- are going to just add to the problems for residents of North Carolina and the Virginias.

Intensity -- see no reason to substantially deviate from the NHC intensity forecast except to knock it down a tad. While the pressure is remaining steady around 930mb for the time being, the eyewall is in a constant state of flux, there is greater shear ahead (if only a little bit more before it hits land), dry air is about to infiltrate the storm (if it hasn't done so already, considering the open W-SE eyewall), and the height of the 700mb surface continues to rise, now over 2500m. A reduction of about 10kt in the NHC forecasted intensities is what I feel the most likely forecast, but the course of least regret is to prepare for a high end category 4 hurricane and hope for the best. Better safe than sorry.

I hope everyone in the projected landfall swath of the NHC has made all necessary preparations, including those well inland of the system. Tropical storm force winds will begin to impact the coast tomorrow and it will be downhill from there. Best of luck to everyone and, unless things change, I think I'm going to focus on Jeanne for now...things seem to have come into better agreement tonight, the models (trending eastward) and the observations (trending westward) included.

And time to eat a bit of crow on this one...though for folks around these parts (Tallahassee), one only needs to remember what Kate did despite relatively low winds (max wind < 70mph in Tallahassee) to see what this storm may still cause well inland and well away from the center of the storm.



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