As high pressure shifts off the Atlantic Seaboard, the clockwise flow of air around it is ushering a southwest wind into New England, pumping mild air northward. Though we'll find a taste of this mild airmass in New England for a short while, a cold front marching east will mark the leading edge to cooler air that will envelope New England on Friday and into the weekend, though some uncertainty remains in the forecast for Saturday as there will be an interaction between moisture associated with Tropical Storm Noel and a non-tropical energetic disturbance moving over the Eastern Seaboard.
For now, the forecast is fairly straight-forward - a squeeze between high pressure off the coastline and a cold front over the Great Lakes will allow for a strengthening barometric pressure difference to develop over the Northeast through the day Wednesday, meaning a strengthening wind especially during the afternoon, when gusts may exceed 30 mph in some spots by late afternoon and evening, especially along the South Coast. These winds are ushering in somewhat milder air from the southwest, though this air is not an explosion into summertime warmth, it's a gradual warming that will ease across New England for our Halloween afternoon, pushing temperatures into the lower and middle 60s in concert with filtered sunshine through variable patches of high and middle altitude clouds. These clouds come as a result of the parade of upper level disturbances that have been crossing New England over the past couple of days, today met with enough dry air in the lowest several thousand feet of the atmosphere that, once again, no precipitation will result. With an active breeze Halloween evening, temperatures will be slow to fall compared to the last few nights, hanging in the 50s for most trick-or-treaters before eventually falling to around 50 south, and 40s for many other communities.
Meanwhile, the jet stream remains active across the Northern Tier of the United States. As often explained in these discussions, the jet stream is the fast river of air aloft that steers our storm systems and acts as a thermostat for the atmosphere, separating cool air to the north from warm air to the south. This active jet stream will continue to steer energetic upper level disturbances from the Great Lakes across New England, and the strongest of them is moving across Wisconsin today, en route to New England on Thursday, driving a cold front at the surface. Behind the disturbance and its associated front is cooler air that will spill east and south, eventually arriving to New England with the passage of the cold front on Thursday. Ahead of the front, some sunshine will make an appearance Thursday morning and winds will still be out of the southwest, meaning mild air will still be in place and temperatures will rise into the lower to middle 60s for many spots ahead of the front yet again. As the front moves out of New York State and from west to east across New England from mid-morning to mid-afternoon Thursday, clouds will thicken in advance of this airmass change and scattered showers will fall with its passage. Behind the front, winds will shift to blow from the west, and eventually the northwest, ushering cool and dry air across the region, breaking up the clouds Thursday evening and clearing the sky Thursday night as temperatures fall into the 30s in many locales, and lower 40s in warmer spots of Southern New England.
The dome of cool air that establishes across New England on Friday will be anchored by an expansive area of high pressure that will move from the Northern Plains to the Ohio Valley, providing chilly Friday sunshine and a slowly diminishing wind. East of New England, the front that led the way for this airmass will encounter our previous dome of high pressure, chock full of warmth over the Western Atlantic that will cause the front to slow over the waters southeast of New England. This contrast between the front and the Atlantic fair weather cell will mean a strong southerly wind developing not only at the surface, but also through the middle levels of the atmosphere, and this will set up a corridor of wind through several thousand feet of atmosphere from the Bahamas, to a position west of Bermuda, and toward Nova Scotia. It's no surprise, therefore, that the forecasted track of Tropical Storm Noel is to follow this steering current, keeping the storm well southeast of New England. Still, there is a bit of uncertainty lingering in our forecast for Saturday, and the uncertainty is based on this: As Noel merges with the cold front stalling to become a stationary front off the U.S. East Coast, he will encounter a setup that will enable him to become extratropical (non-tropical) but to strengthen while making that transition - a clash of airmasses between warm and cool, lingering warm ocean water temperatures, strong winds aloft, substantial tropical moisture carried north with the storm, and a convergence of upper level energy with the combination of energetic disturbances from the northern stream of the jet stream (moving across the Mid-Atlantic) with the southern stream energy associated with Noel. This combination of factors will breed a storm that will crank up rather quickly as it moves north toward Nova Scotia, and while the storm center itself will remain well east of New England, the interesting point to ponder here is how much moisture and tropical warmth will wrap around the backside of the storm system. That is, with cool air in place across New England and the coastal waters, an intensifying counter-clockwise storm circulation with air of tropical origin is bound to wrap warmth and moisture westward into the cool air, creating clouds and a wraparound band of precipitation on the west side of the storm. If Noel were still a separate entity from the stationary front, this moisture would most likely pool along the front and never come farther west than the frontal boundary itself. If, however, Noel becomes a storm on the front itself, this wraparound precipitation will develop, which is what I expect to happen. The question for Saturday then becomes where the back edge to that wraparound precipitation will be - how far into the cool air the tropical warmth and moisture will penetrate - and whether showers or rain should be in the forecast for Eastern New England as a result. For now, I'm concerned that at least some of these clouds and showers will extend far enough west to bring showers to Eastern New England and that's why I'm keeping showers in the forecast for these eastern areas. You turn to this discussion for more detail and the honest assessment on the forecast, though, and that honest assessment is that I can clean up the Saturday forecast if the trend comes in over the next day or two to keep this wraparound precipitation farther east than I'm currently thinking. Either way, cool air will be in place at ground level across New England, holding temperatures in the lower 50s.
That cool air will still be in place on Sunday as the strengthening storm pulls through Nova Scotia while becoming extremely strong, forcing a northwest wind across New England and reinforcing the cool and dry Canadian air for all of New England. The fast jet stream flow will continue across the Northeastern United States, meaning repeated upper level disturbances will continue to move overhead, bringing increasing clouds later Sunday and Sunday night, and thickening clouds with another chance of showers - and quite possibly Northern Mountain snow showers - on Monday.
Indications are that a Northeast U.S. trough in the jet stream will persist for at least a couple of weeks, and probably through most of November while a ridge persists over the Western United States. Such a pattern would allow warmth in the west to bubble eastward through the Central U.S. at times, and warmth from the Southeastern United States, south of the trough, to bubble northward as storms organized to our west, but also implies that repetitive shots of cool air will continue to invade the Northeast through the period, with the most substantial shot of below normal temperatures coming around November 8/9 behind what may be a fairly sizeable rain-maker, or may just barely deflect rain to our northeast - I'll keep you posted on that. Keep in mind, though, that as we get deeper into the season the chances for periods and areas of snow on the backside of departing cold domes increases and shifts farther south, so this light snow in Northern Maine may only be the beginning of a trend that will bring oscillating warm ups and sharp cool downs.
Matt
Matt's Technical Forecast Discussion, Updated Wednesday, October 31 at 1:10 PM
I wish I had some really meaningful thoughts to offer on the forecast for Saturday, which right now appears to be the most challenging part of the forecast. I'll get there in a moment, but in the short term the forecast is fairly straight-forward with a strengthening pressure gradient kicking up the winds and with Richardson Numbers indicating good mixing to at least 900 mb and winds at that level at about 30 kts, certainly not unrealistic to expect afternoon 30-35 mph gusts, esp along coastlines with best chance at South Coast. Cold front over Upper Great Lakes was dry early in the day but has been exhibiting increasing convective activity as it encounters more moisture - especially evident over the Great Lakes. This narrow band of forced lift will continue to move east in advance of the instigating vorticity lobe that will swing thru NewEng on Thu. Given the sharp and well defined band of moisture, expecting some sun in the early AM ahead of the front, then band of clouds and showers, then perhaps some late breaks again giving way to eve and overnight clearing. Colder air streams in on shifting winds Thu Ngt.
Of course, while dry and cool advection secures quiet Friday, questions revolve around the evolution of Noel thereafter. Most of my thoughts are laid out in the General Wx Summary again today but this discussion allows more raw thoughts to pour out: Noel is strengthening once again and moving north and represents a lot of tropical moisture and energy. Front moving thru NewEng Thu represents baroclinic zone that will push east over the Atlantic waters, which are still warm. The combo of warm water and warm subtropical nature of air filling high pressure off the coast will cause cold front to stall while southerly winds strengthen around the back of the high and along the front. Meanwhile, the setup aloft is dynamic with upper low that came into California earlier this week now pieces across the Central Plains and MS River Valley that will move off the MidAtl coastline Fri and merge with Noel, who will be moving north. This sets up a confluence of a number of factors favoring cyclogenesis: surface/low level baroclinicity aided further by warm SSTs, upper level merger of energy and baroclinicity, plenty of available moisture with a continued southerly wind supplying a tap from the Bahamas, low level convergence and lowering heights north of the storm. This all combines for a transitioning storm to extratropical, but conditions are instantly favorable for quick and intense deepening of the system. This system, thanks to its baroclinic nature, should develop along the front and then may even push back into the cool slightly as it starts to mature, which is why the westward scenarios of the Canadian Ensemble members and some GFS members really can't be discounted. The big question for NewEng is how far west the overrunning precip assoc with wraparound tropical moisture and warmth makes it on Sat, and buying a western solution, I really have to keep clouds and showers in for Sat in Eastern New Eng - better conditions west - until and unless I see an eastward trend that is undeniable, or better agreement among the guidance on a more eastern solution. By Sunday, this powerhouse storm cranks north of Nova Scotia which increases the NW wind and solidifies cool air in NewEng.
Longer term laid out in General Wx summary, but bottom line is with shots of substantial cold, not only will manmade snow blasting continue, but one has to think that the active jet stream flow of northern stream disturbances will bring repeated shots of snow showers or light overrunning snow to the North Country at times throughout the month, and...if and when the right scenario arises with enuf lingering low level moisture...one of these cold shots should be able to wring out some upslope snow for some areas, tho tough to say right now which one may come with enuf llvl RH given the dry airmasses of late.
Have a great Halloween.
Matt
