re: Hurricane Florence - Catastrophic Flooding Potential

Weather Channel is in morning that not enough people were killed by Gordon, so wants Florence to hit the east coast head on. Advertisement money is good.

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Although 9 days out but the last 2 runs of Euro is trending east coast for Florence.

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We might get all kinds of Fujiwhara effects...

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That’s what happened with Harvey last year in 48 hours and nobody evacuated Houston because it was supposed to be a weak storm

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Harvey was never supposed to, and didn't hit Houston as a wind event. Harvey was a disaster here because it sat around for 4 days and dumped a ton of rain, which was predicted.

They’ve been so wrong on the forecasts for Gordon. It’s still sitting near Jackson and most of the rain is all but gone. Hell the heavy rain in West Alabama has drifted away from the center toward East Alabama.

Weirdest thing I’ve ever seen

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Without wind shear, Gordon could have been a category 1 hurricane. The National Weather Service in Slidell was following the NHC lead.

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LIX did fine. No complaints here. Flood watches, TS warnings reasonable until it was clear areas were definitely out of the woods. Good with setting potential wind and rain expectations in the forecasts.

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Without wind shear, Gordon could have been a category 1 hurricane. The National Weather Service in Slidell was following the NHC lead.

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No doubt, had a healthy warm eddy across the central gulf. Gordon was never wanting for moisture, just couldn't wrap it. You could make the case if those ULLs had lined up right it could have been a bit stronger than a minimal hurricane. Which is why those eastern parishes certainly had reason to be very cautious.

The rest just watch and have a plan to close out if things change.

That thing is gonna be a bitch

Rained almost all day in Birmingham.

Nothing was forecasted for Birmingham...I really don't GAF but Damn did they miss this one.

The Beach Club on Fort Morgan recorded 78 MPH winds on the 10th floor. I do not believe that was an "official" recording station and also because it was not on ground level this recording could not be used to place Gordon as a hurricane.

Florence defied the odds and the forecasters and looked great early today as a major hurricane.



This evening the storm is clearly getting sheared as the SW eyewall has thinned and downshear convection has increased.




CIMSS analysis shows strong shear over the system this evening and that is to be expected with a huge PV streamer (upper trough) sagging across the Atlantic.



PV streamer sagging across the Atlantic enhancing shear and transporting dry air into the tropics.



Why does any of this matter? Florence has been giving the models fits and has continually been much stronger than anticipated. One of the results is that the models have had a short term west bias. Euro track errors below.



In fact, Florence is currently near the outside edge the 12z Euro EPS spread.



Up to this point, the models have expected a weaker system that had a more westward component to the track but have been trying to correct back east as the system has been much stronger than anticipated.

Between now and Saturday night the big question will be how much does the system weaken and does that bring the system back towards the western side of guidance? Saturday/Sunday will be the key days to watch to see if the system can escape out to sea before getting blocked.

By Saturday night, a trough in the northern stream is moving out into the Atlantic splitting the two ridges, this looks like a clear cut escape route for the system. If Florence is far enough north (current trends seem to indicate that might happen) and/or the trough is deep enough then the curve back out to sea happens.



On the 18z GFS, the trough lifts out and the system gets trapped under a ridge that forces it back to the west. Also, notice 92L down in the MDR.



Things get really messy from this point moving forward but between now and Sunday the things to watch are how much does the system weaken, does it stay on the right side of guidance or shift back west, does it hit the escape route?

All of this will also play a role in the future strength and track of 92L. Going back to the current setup tonight we can see the PV streamer that is increasing shear across Florence.



Going back to a Gordon post:

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If you think of the TUTT in a PV framework then you can see how convective driven outflow can thin or fracture the PV streamer (TUTT). Then the PV streamer fracturing can result in an ULL that eventually provides an outflow channel. However, models struggle with this process and it is common for them to fumble ULL motion and strength. It's a tricky setup that could result in shear staying strong and keeping things in check or just the right outflow setup resulting in rapid strengthening.

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Many of the same processes are in play again but Florence is a mature system vs. Gordon that was trying to develop. The upper level convective outflow from Florence will start working to break down the PV streamer with an ULL eventually breaking off.



Eventually an ULL to the west of the system aids in the poleward outflow channel as a massive anticyclone builds over the top.



The anticyclone that builds over the top will also eventually increase shear over 92L. A weaker 92L tracks farther west but the spacing between the systems and the ultimate track of Florence will determine if 92L strengthens and takes an early out track or if it stays weaker and tracks farther west. Also, Florence could help "pump" the mid-level ridge over the top of 92L down the road.



At this point, we are well into fantasy land range but this is one example of how 92L could take a more western track. The strength of 92L and future track of Florence would determine if there is any chance of 92L getting into the Gulf.