First, the short range forecast is straight-forward and features a well-defined front crossing New England this afternoon. The front isn't producing much convection, owing to a lack of deep moisture and a lack of significant instability. Lifted indices are barely negative and mid-level dry air has retarded vertical development. A few showers and an isolated downpour or rumble of thunder will remain possible with the frontal passage through this evening. Thereafter, clearing as subsidence and drier air builds in with huge high pressure moving into James Bay taking over. Fall feel to rule tonight with lows in deepest Northern Valleys dropping into the upper 30s, but generally 50s for most of us. This cool start sets us up for an autumn preview on Thursday. I expect excellent mixing with a northwest downsloping flow and dry air allowing for adiabatic warming, but even that will put most of us in the mid 70s. Another cool night Thursday night and Friday isn't much warmer with sfc ridge moving overhead. This isn't the primary ridge, but rather a breakaway bubble of high pressure that still is moderate in strength and moves off the coast and into the Maritimes, essentially forming a bridge of ridging from the Western Atlantic across Maine to Central Canada. This is part of the problem that may keep Danny close, though there are two other factors. One is the trough digging into the Great Lakes and the other is the shortwave trough over the Gulf Coast (noting damage reports coming in from Louisiana this afternoon). The latter will encourage southerly flow up the Southeast US coast while the former keeps a primary low north of Lake Erie that will encourage an inverted trough to develop from Danny to that low. In essence, Danny becomes a hugely glorified wave on a warm front.
Of course, there is the question of what Danny is now, and what he will be. He has been classified a Tropical Storm but those of you watching my live stream yesterday remember that I was looking at the likelihood that the storm really is subtropical in nature. I still think that is the case today, but...
I understand and appreciate the NHC thinking that the FSU cyclone phase analysis deems the system more tropical than not (symmetric, warm core). Still, this is a classic subtropical setup, and I do think it's going to come back to bite us because it has the potential to be a hybrid when it surges up the East Coast and now the tendency is going to be to call it extratropical when it's still tearing things up through New England. But, we'll focus on what's now and what's to come.
What's going on now is that the surface circulation is clearly evident on visible satellite imagery and has become removed from the convection, which has been left behind to the east of the circulation. Now, these circulations can survive without convection, but only for so long because there's no method to keep the surface pressure low. So, new convection needs to fire or the storm won't survive. It's possible that this never happens and the circulation falls apart, which means Danny dies quickly, but I'd say only a 20% chance that happens. Much more likely is that the storm struggles with sporadic, struggling convection near the center next 36 hours, then flourishes in the tropical feed out of the Gulf of Mexico, over the waters east of NC and over the Gulf Stream, by early Friday. Now, there is the possibility that the storm takes on so much of this tropical moisture that it becomes entirely infused with tropical moisture and becomes a system with well-balanced convection, though I'm not sure that ever really happens. The truly tropical air is east of and along the dying stationary front off the Carolina coast, with slightly less tropical air farther inland across the Carolinas. Then, we'll have the deep fall air in New England that will be moderating Friday night as the bubble high breaks east and an onshore flow begins, with a westerly flow slowly coming around to southwest and then southerly aloft. This starts increasing moisture up the seaboard into New England late Fri, then precip likely develops as overrunning later Friday night into Saturday. But consider what we're looking at here - fall air, pressure gradient, overrunning precipitation...this is not a tropical environment in New England on Friday night into Saturday. So, we end up with dry and cool air to the north of the storm and tropical air around and south/southeast of the storm. This is a no-brainer for heavy rain in the area of greatest warm and moist advection over New England on Saturday. But much more importantly, this is a setup for the start of a hybrid system. Of course, there are a few possibilities here, including that the storm simply is a truly tropical creature that therefore shoots south of New England while staying tropical. I'm skeptical that will happen. We have the intense shortwave ejecting out of the Gulf Coast and up the Eastern Seaboard that will infuse this system with energy, and with mid and upper level baroclinicity. We have a jet streak moving across New England Friday that will be exiting to the east on Saturday, leaving a ridge axis at 500 mb that slowly moves east and drags a right-rear quadrant diffluent zone over New England. Meanwhile, the storm is still progged to have a small upper level anticyclone for ample evacuation of its own, meaning we're doubling the fun on divergence aloft over New England. Of course, the low level baroclinicity will be enhanced as tropical air encounters our antecedent fall airmass, and pressure gradient will be increasing all the while. Add to this the fact that the vorticity maximum coming out of the southeast is moving from southwest to northeast, and I think it's going to be harder than forecasted for this thing to actually get to the same latitude as Danny, and this means an extended "transition" (or, hybrid period, I'd argue) spent with a warm core at the lower and mid levels over water that is some 8 to 9 degrees above normal up the Mid-Atlantic seaboard to Southern New England! This water is running over 80 in some spots, and the MIT derived hurricane potential indicates water warm enough to fuel development of a Category 2 hurricane as far north as Long Island. And that's for storm development and maintenance, but I'd argue the baroclinicity and diffluence aloft will do its own dirty work.
This all leaves us with a scenario where I wouldn't be at all surprised to find a strong tropical storm or weak hurricane off the coast of North Carolina that will actually STRENGTHEN while moving north up the coast. A hybrid beast that will feature an expansive wind field as it attempts to make an extratropical transition (which normally features wind spreading out from the center) but simply can't finish the job as the perfect setup for dichotomy exists both surface and aloft. The result very well may be a maintained core of wind near the center with an expanding damaging wind field as the storm moves north, with flooding rain north and west of the track.
And what of the track? What guidance do we trust? I understand and agree with the NHC track to take this thing over the Cape. My reasoning for this isn't because I necessarily would put money on it verifying. It's because I truly believe it's the best forecast you can offer right now. If this storm goes hybrid, like I'm thinking, the forecasts of the ECMWF and GGEM (Canadian) will verify on track, though the ECMWF is horribly underdone on strength of the system. If the storm stays totally tropical, the hurricane guidance products will verify in taking this storm southeast of New England (which still throws heavy rain our way because of the airmass clash, but no damaging wind). As mentioned above, I don't think this will ever go totally tropical, and certainly not once it's north of Virginia Beach, so this favors leaning into the cool air just slightly and a farther west solution. Nonetheless, the uncertain status, let alone future, of the system would make it unwise to bite too hard on this western/inland track and the right balance is a track over the Cape and Islands, a la NHC. Of course, regardless of the track, big waves return to Northeastern coastlines Friday and especially Saturday.
After the storm, the weather is a bit difficult to decipher, because if the storm does a successful job of becoming a hybrid, there will be more cool air pulled down behind it. At this point, I think the fact that it serves as a glorified warm frontal wave should leave relatively mild conditions in its wake, though the Great Lakes upper low starts drifting east and some convection seems likely.
For now, we wait to see how the convection does around the center, and I will continue searching for clues on how this system will develop as it nears the coast.
(This post also appears on my personal website, mattnoyes.net)