The ridge is eroding, slowly yes, but doing so nonetheless. 500mb heights across the southeast US are trending downward, 10-20m over the past 18hr. Winds in Tampa and Melbourne have shifted from the south, signifying the ridge axis to its east, to out of the east, signifying the ridge axis to the north and east. This confirms what is seen on water vapor imagery as a northward movement of the ridge. It is building northward, but also being eroded on its southern side as Dennis gives the impression of eroding part of the ridge.
The shortwave trough to the west has not made a lot of progress to the east as of yet, but continues to dive southward. It has taken on a tilt from southwest to northeast , suggesting that there is still some digging left to do before becoming neutrally tilted (north-south) and then beginning to lift out (tilt northwest-southeast). A weak upper-low in the Bay of Campeche is likely to be caught up by this trough and tracked to the north and east, helping to push the entire trough towards the east. The longwave trough, currently situated in the lower Ohio River valley, has pushed southward for the past two days and is nearing a point where it can phase with the trough along the Texas coast. The ridge in the west is building, as evidenced on GOES-10 water vapor imagery, and is being helped by the progressively westward movement of the remnants of EPac TC Dora away from the Mexican coastline. This building of the ridge will naturally help to dig the trough a bit in the east, allowing it to move.
The timing of everything and how strong the response is of the storm to it all will determine where it makes landfall. Recently, with the slowly eroding ridge, the storm has started to move more northwesterly -- but also slow down. Dennis may well slow for a period of time in the SE Gulf, as hinted at a few days ago, though it should not stall. Any slowing will likely signify a substantial northward turn to come as the storm will have reached the extent of the ridge axis at that time; the other scenario would be for it to lose all steering currents whatsoever, but given the current setup, I view that as highly unlikely.
I do still believe that a Florida Panhandle hit is the most likely scenario, still a bit east of the NHC position, and hope to be able to refine that with time. I believe the models may be overdoing the westward trend as a result of weak initialization/forecast of the storm; this results in a reduced Beta drift (explained a few days ago; results in a weakened natural tendency to move northward) as well as a strong susceptibility to the low-level flow, which currently is relatively weak and largely out of the west or southwest across the entire Gulf. As a vertically deep & well-developed storm, Dennis is going to feel winds aloft moreso than the lower-level winds; these winds aloft are moreso out of the south and southeast across much of the Gulf, signifying the possibility of this storm moving to the east of the model tracks.
Bottom line: landfall is still up in the air, but everyone along the central and eastern Gulf coasts needs to be prepared for this storm now.
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