The storm off the Mid-Atlantic coast has become very well defined this afternoon and continues to ramp up. There are a few impressive features on this cyclone and I wanted to take a moment to contemplate them and work them out in a post this afternoon.
First, have you checked out the buoy observations on the western side of the storm circulation? Wow! 58 mph gust just east of Hatteras, and 54 mph gust at the buoy east of Virginia Beach! Seas correspond, registering at 14-18 feet with an 11 second period. Meanwhile, winds near the center of the storm are much lighter, which corresponds well to an extratropical storm with a wind field removed from the center and driven primarily by pressure gradient forces, in this case the product of the bridged surface ridge from Nova Scotia over New England then down the Appalachian Mountains. For now, the storm looks like it will remain extratropical in the next 12-18 hours, which is certainly supported by the dry air wrapping around the southern and eastern side of the storm and directly into its core. This has suppressed any deep convective activity near the center, and instead an impressive swath of mostly stratiform rain has been thrust north and west of the circulation. This is the band of rain that will affect New England in the coming 36 hours.
There appears to be decent agreement among the guidance on the storm reaching peak intensity tonight, well south of New England, and expanding its wind field far enough north to get Cape Cod and the Islands into an easterly wind with gusts to 40 mph - perhaps a few 45 mph gusts in the most exposed east-facing locales - then the wind field diminishes rather quickly on Thursday. Meanwhile, aloft, the slug of warm and moist advection responsible for the rain outruns the surface/mid-level center (the storm becomes stacked) and therefore weakens as it drifts north and northeast in the steering flow on Thursday. From Northern VT to far Nrn NH, little precip is expected, with heavier amounts farther south and east. The ample tropical and Atlantic moisture feed, coupled with a mid-level baroclinic zone, will provide sufficient lift for as much as 2" of rain Wed Ngt and Thu in some of Eastern MA - esp on Cape Cod - with 1"-2" more widespread in Eastern New England. Even along the Mid-Coast of Maine, though the rain may be weakening as it drifts up the coast and the Maine Turnpike on Thursday, a secondary max of precipitation may occur with a re-infusion of tropical moisture later Thursday and Thursday night.
This isn't quite the whole story on this storm, though, as there may be a few surprises with it Thursday. Though the agreement among the guidance is to weaken the wind field, there is also remarkable agreement on transitioning the core from its present state of asymmetric warm core to a symmetric warm core storm later Thursday. This is a big deal! One of the traits we look for in a tropical transitioning storm is this symmetry and warm core. Though the storm will be passing over water that's still 75 to 80 degrees over the Gulf Stream, it's unlikely to remain in such favorable conditions for long. Nonetheless, later Thursday we may see a flare up of thunderstorms around the center with a renewal of wind, this time contracted near to the center. Definitely something interesting to watch from a scientific perspective, and important for mariners and surfers, as well, as this may enhance and prolong waves on the southern waters/South Coast into Friday.
As for the sensible weather, the most successful viewpoint on the next few days is to view the airmasses as fluids - something we should always do in the field of meteorology, but an outlook that will really help to grasp the forecast for the coming days and latch onto what I think will be the right solution. The bottom line is that we have one fluid from the Mississippi Valley to the Southeast US (warm air), another fluid from Eastern Canada and New England through Ontario (cool, but not cold air), and another over the Rockies (chilly). It's interesting to note that the dewpoint of the air north of New England today is in the middle and upper 40s, so while it may be cooler than the stuff producing highs in the 70s and 80s over the Ohio Valley and Midwest, it's not a really cool or dry airmass, and this is part of the reason that, even with a northeast wind, I'm not going exceptionally cool for most of New England - not below the middle 50s - on Thursday, and by Friday the wind abates as a weak trough between the Upper Midwest low and our coastal low very sluggishly moves to and through New England from the southwest. Of course, with very light winds either side of this trough as New England finds ourselves in a col between systems, clouds will be stubborn on Friday and low level differential advection will probably bring drizzle and areas of light rain, especially east ahead of the trough, which comes with weak warm/moist advection on the leading edge of the Southeast US air. Saturday may start gray even though Central/Southern/Western areas are transitioning to the warmer side of the trough, but an increasing southwest wind ahead of the approaching occlusion/cold front will break the clouds up, and with warm air in place, at least Central and Southern areas will see enough breaks to warm considerably into the 60s. By Sunday, cold advection is underway in installments, with the first installment not all that chilly, and the combination of drying for sunshine, and downsloping flow, resulting in a fair day with bubbling instability Cu. Cool air continues building in for the start of next week.
Matt
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