A weather pattern with a bit more sanity has settled into New England for the time-being. Pulse convection (thunderstorms that grow strong in pulses, then weaken) have been ongoing in Central and Northern Maine this Tuesday afternoon, but the remainder of the area finds dry enough air and a weak enough trigger to prevent storms from developing in most spots. The action in Maine is the result of a spoke of vorticity pivoting around the Eastern Canada upper low, and farther removed from the cyclonic vorticity advection there's only a weak surface trough in Central and Southern New England. That weak trough is evidenced by a shift of surface wind from southwest over the coastal waters to west over land. Though this trough provides enhanced surface convergence today, it will not be enough to initiate convection outside of the terrain-assisted locales. Speaking of terrain assistance, the Greens, Berkshires and Whites may spark their own isolated convection, but that should be about the extent of any action farther south/west than ME.
Before we go much farther, it's worthwhile to note that this break in the action expected for the midweek comes as a strong upper level low digs into the Pacific Northwest, digging south as an extension of the mammoth upper low near the North Pole, north of Siberia, which has been a persistent feature. Beneath that large upper low has not been phenomenally cold air for where it's placed, at such a high latitude, but the pools of cold air have been migrating southward, contributing to both the Eastern Pacific and East Coast troughs. When an especially vigorous upper low digs deep enough across the Eastern Pacific, it can dig into the Pacific Northwest, which is what will bring the rain and wind to Eastern Washington and Oregon. At the same time, this allows a brief eastward expansion of the large Western/Central U.S. ridge - or perhaps more accurately stated, a relaxing of the Eastern U.S. trough, because the jet stream shift comes more from the quick ejection of energy eastward from the Pacific NW upper low, rather than any act of expansion by the ridge itself. In fact, it's the combination of the stubborn refusal by the Western ridge to break down and the lingering Eastern Canada/East Coast trough that encourages the upper low to migrate east across Hudson Bay by the first days of August, and back to what seems to be a seasonal semi-permanent home for upper level lows in Eastern Canada. This also means the East Coast trough will dig again for the beginning and middle of August.
Lots of concern has also come from viewers with regard to the recent pattern of frequent thunderstorms and why such severe weather has been recurring. Keep in mind what an active tornado season we had in the nation's midsection, then recall why that was. It was the confluence of many types of air coming together - in fact, during that outbreak, it was almost every type of airmass possible. Cold Canadian, cool Pacific, warm tropical Pacific, dry continental tropical out of Mexico, warm and moist tropical out of the Gulf of Mexico, all meeting up. The driving force was frequent Canadian cold pools and associated upper level lows, digging deep into the nation as winter and spring cool air refused to abate. It wasn't the warmth coming in that was so unusual - that's simply called summer - but rather, it was the clash with a lingering cool air that would not abate. A few months ago in this discussion, I conjectured that an active severe weather pattern with occasional tornadoes would head toward New England for much of the summer as that Canadian cold would still refuse to abate, but the summer warmth absolutely HAS to move north with the increasing sun angle and warming of the earth's surface with the summer months. Suffice to say, that not only has come to fruition but will continue into and through much of August, meaning the threat for recurring severe weather in New England and the entire Northeast remains high. I'm quite certain the F2 tornado in New Hampshire, and F1 tornado at the South Coast, will not be the last vortices we see this season. We've also looked in this discussion at the role the recent "sink" in New England may play with regard to the tropics. That is, the persistent below normal heights along the Eastern Seaboard, sandwiched behind an Atlantic subtropical high that remains anomalously far north and quite expansive, puts New England in not only a favorable position for fronts to die overhead, leaving lots of boundaries for thunderstorms, but also puts New England in a veritable weather sink for tropical systems, too, as they will recurve around the western side of the subtropical Atlantic high, and that means timing of northern stream shortwaves will be the key to tropical strike potential this season.
Of course, there's still the day to day weather to contend with, as well, and while the next couple of days will be quiet, there are some challenges in the foreseeable future. I expect cirrus clouds to start moving into New England from west to east Tuesday night into Wednesday morning, then thicken on Wednesday in advance of a northern stream vorticity maximum that will be dropping southeast across the Upper Great Lakes. That shortwave will be responsible for Wednesday night/Thursday morning rain with a weak wave of low pressure moving east across New York State at a triple point. The attendant warm front will lift across New England Wednesday night, with the trailing cold front Thursday afternoon/evening. This does put New England in a precarious spot Thursday afternoon as morning rain will taper, sun probably will break out, and the atmosphere will destabilize, especially in Southern New England, ahead of the vort and the cold front. The result may be a very quick rise in temperatures where the sun comes out, but strong thunderstorms Thursday afternoon.
The GFS has been somewhat behind the 8-ball for Friday, but is catching on in its 12Z run. The ECMWF, GGEM, and both American and Canadian ensemble members have featured fairly good agreement on shifting the surface low east of New England on Friday, leaving a northwest flow. It's not common for Southern New England to see thunderstorms on a northwest flow, even with a cold pool aloft. There's also dry air forecasted to wrap into the system on Friday, so for now it seemed wise to keep Southern New England dry with scattered orographically assisted convection the farther north one goes. That said, the cold pool aloft is about -10 to -15 C, which isn't frigid but is enough to warrant the scattered thunderstorm potential for the northern half of New England.
Broadscale warm advection doesn't really take place over New England until Sunday, which should leave a beauty on Saturday before thunder returns to the forecast later Sunday under the warm advection regime, though the best chance would come late Sunday when diurnal destabilization peaks, and warm advection peaks, as well. Of course, this warm advection must lead to a warm day somewhere along the way, and it does appeaer as though deeper warmth will run at New England by Monday. The question about just how far north the warmth makes it before being beaten back down by an active northwest flow under the belly of the upper low north of the St. Lawrence River mouth is an important one in the early week forecast next week, and the uncertainty surrounding this issue is what would keep me from biting hook, line and sinker into the upper 90s that the GFS advertises. While that may be a possibility in Southern New England, it's far less of a likelihood in the North.
Matt
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