A late post today and that may be the case again on Wednesday as I'll be out of the office and up in Maine, flying with the Blue Angels, but will post upon return, as this is an important week to get ideas out there.
We'll start at the beginning, which is quiet. Low altitude clouds developed over Cape Cod thanks to cool air aloft over relatively warm water, enhancing the northerly flow of air that was picking up ocean moisture along its fetch. Mid-level clouds rotating around the backside of the stacked low over the Scotian Slopes. That stacked low is deep and is going nowhere in a hurry, which means similar weather will be found on Wednesday, though slightly cooler low level air advects southwest across New England and means surface temperatures will be a few degrees cooler Wednesday afternoon than they were on Tuesday afternoon. Winds will similarly be from the north and northeast, though slightly lighter in intensity. The upper low lifts out on Thursday, which in turn allows the longwave pattern to move once again, meaning ridging takes hold of New England briefly, though a shortwave ejecting east out of the Great Lakes keeps 500 mb heights from rising all that much after their initial surge Thursday morning, and the passage of the shortwave brings a chance for afternoon convection later Thursday. The best dynamic forcing will be found in Northern New England, though the surface reflection of falling heights over the Great Lakes in association with the Central US trough will be a cold front pushing east, into the Northeast on Thursday. Ahead of this front, a southwest wind will carry very warm near-surface and low level air into New England, supporting Thursday afternoon maximum temperatures in the 80s...and in ideal downslope spots, upper 80s seems possible. The longwave pattern features a very rapid strengthening and retrogression of the Western Atlantic ridge, extending west over Bermuda and to the Eastern Seaboard by Friday. This building ridge has impacts as soon as it builds for us here in New England - and implications for the weekend and the evolution of Tropical Storm Hanna. As for Friday's meaning, the building ridge and rebounding heights will mean the cold front pressing through Northern and into Central New England on Thursday will stall. This means a north-south temperature gradient on Friday, with 80s again in Southern New England and 60s/70s north, with central/southern thunderstorms Friday afternoon near the enhanced convergence zone. The other meaning of this building ridge is a northwest track for Tropical Storm Hanna, and as she feels the pull of a northern stream shortwave rounding the base of the trough, she will accelerate. There has been a definite difference between the tropical guidance and foreign guidance - mostly in the same camp, along with the NMM, of taking the storm inland quite a ways and up the Appalachian Mountains - and the GFS along with all of its members, which has favored a track much less inland, then shooting east along or just south of the South Coast of New England. This is a huge difference between an east/southeast wind to 50 or 60 mph gusts, or a northeast wind of less intensity. It also may spell the difference between a relatively short-lived burst of warm and moist advection, or a longer-lived band of rain that would be found north/west of Hanna's track. I was leaning toward a GFS weighted compromise and the 12Z and 18Z guidance is coming around to a blend solution, as well, which is close to the NHC official track. There are some obvious variables here - the effect of dry air on both the strength, organization and longevity of Hanna, the rate of acceleration into the Carolina coastline, how quickly the ridge builds, and the speed of the storm in the coming days. Briefly, the meanings of each of these parameters include: whether Hanna can not only survive, but eventually thrive off the environment once the dry air, which has teamed with shear to drastically disorganize Hanna, loses some of its grasp on her. It should take at least 18 hours for Hanna to really get back on track, as she is going to recover but has to fill in the new convection and jumpstart the near-strorm environment, to create her own tropical envelope first. Rate of acceleration will have a big impact on how far north Hanna gets, as she needs to be moving quickly at the Carolinas to make it over New England, but the northern stream shortwave will assist. The building of the ridge will help to determine the track - and how far inland she goes...especially how close to the mountains...will play a huge role in her structure. If my hunch that an eastern solution - inland but not too far inland - verifies, that will mean a storm that still may bring 50 or 60 mph gusts to the south coast, and a warm advection shot of heavy rain, with longer lived rain in NY and Northern New England. In tomorrow afternoon/evening's post, we'll look more closely at Hanna's environment.
Of course, there's a sleu of storms over the Atlantic to follow. Each appears to follow the path of its predecessor to a point - near the Bahamas - then the path will depend upon timing of northern stream shortwaves, much like the interaction that Hanna depends upon. We will continue to examine these storms in the coming days.
Matt
