re: Hurricane Irma - Spinning Down

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The models are going to struggle with 93L. Wave breaking across the North Atlantic is going to spin off some ULLs, not to different than what we saw during 92Ls trip across the Atlantic. Then there is a massive WPAC system that is going to pump a ton of energy into the high latitudes and that always gives the models fits. As that energy is distributed to the downstream waves it will determine the large scale setup across America and the Atlantic as 93L approaches. Stronger Eastern trough and weaker Atlantic ridge closes doors to the Gulf. The reverse certainly causes concern.

Another issue, the outflow from any potential Gulf system could play a role. So, take any model solutions or speculation on what might happen based on model runs with a grain of salt.

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i was just about to type all this same thought just like that.

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The models are going to struggle with 93L. Wave breaking across the North Atlantic is going to spin off some ULLs, not to different than what we saw during 92Ls trip across the Atlantic. Then there is a massive WPAC system that is going to pump a ton of energy into the high latitudes and that always gives the models fits. As that energy is distributed to the downstream waves it will determine the large scale setup across America and the Atlantic as 93L approaches.

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So last night, I posted a bit about the ULL, PV streamer, and ridging over the N. Atlantic that would all go into playing a role in the SW dive and the eventual turn back towards the north. The models are still struggling with that but are starting to narrow down the possibilities

So another challenge to focus on is the progression of Sanvu to the high latitudes. As it transitions from tropical to extratropical it will release a ton of energy that will excite the down stream waves (troughs and ridges). This produces a low predictability regime for the weather models as they struggle to with how to transition this energy into the extratropical flow.

For simplicity reasons, think of the black arrows as energy that is helping to pump up ridges and dig troughs (amplifying the downstream pattern)



The transition of Sanvu to the high latitudes plays a role in determining the orientation and strength of the downstream trough that digs into the East Coast as Irma is approaching the US.



In the image above, the red line tracks the Rossby wave dispersion into N. America. Red is ridging and blue is troughing, notice how this is setting up to be on the deepest East Coast troughs in the recent period (red circle) and the dark red is the West Coast ridge amplifying all the way up into Canada. The only problem is that the models struggle with this process and while the timing of the trough may be somewhat pinned down the development of it is still up in the air. Before the models can pin that down they have to figure out the upstream pattern across the Pacific and Western N. America. This is what the last few runs of the GFS have done:



TL, DR version - we are still 4 or 5 days away from pinning down the track of Irma with some certainty.