re: Hurricane Florence - Catastrophic Flooding Potential

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rds dc

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Is this "Swiss HD 4x4" model for tropical cyclones worth a shite?

If its swiss it has to be good right?

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Nope. More left for Katrina than Georges or Ivan.

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Not really good examples because Georges was a pussy and Ivan wasn’t projected to hit New Orleans. That was a voluntary evacuation out of caution. Katrina was the first mandatory evacuation ever for New Orleans.

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The real hero is peej. Flo was a Cat 4 when he said it would be horrible.

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This is true. Works every time

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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.

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.

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Above is an older post that laid out some of the early thinking on Florence. Basically all the models showed this development and some GFS runs showed an even better placement of the ULL that allowed Florence to get to sub-920mb (unrealistic but the GFS showed a really good setup for strengthening). However, reality turned out differently and the ULL has slipped into a position that has actually helped check Florence.

Here is the actual setup from this morning showing the ULL in an unfavorable location and the upper level anticyclone isn't as large & is offset from Florence vs being in a prime time location. This setup imparts shear on the system vs. establishing a favorable environment for strengthening.



There are many possibilities for how this came about but one possible option is that the early ERC disrupted the core of Florence just enough to throw off the -PV/+PV balance preventing the upper level anticyclone from setting up shop in the most favorable location for additional strengthening? One thing is clear, Florence has never fully recovered from that first ERC cycle.

Does it look like the the storm might be starting to look better in the last few frames? Looks like the dry air is not there .

It’s notable the gfs has been on point with track and intensity. It showed Flo having a pressure of 956 I believe yesterday when she reaches where she is now. I remember thinking no way she weakens that much

May as well look at a dart board after peej has killed 24 bud heavies and taken aim.

A burst of deep convection with lots of lightning is surging back towards the center of Isaac tonight. The key for Isaac will be how organized it is when it pass through the Islands and into the Caribbean. The low level flow accelerates in the Caribbean Sea and that makes it very hard for systems to strengthen and can even kill off a system.

GFS is slower to move her inland on this run. Only near Rock Hill, SC early Sunday then finally starts moving out afterward.