HOT AND HUMID FOR MANY NEW ENGLANDERS TODAY...THUNDERSTORMS ERUPT ALONG LEADING EDGE OF NEW COOL AIR

Please keep in mind that tropical activity is now discussed on the Tropical Meteorology page, rather than in this discussion.  Also, lots of posts on the homepage and many more under construction that will follow today, tomorrow, and early next week, so you may want to check back on the main page from time to time if you're interested.

As for New England's forecast, a series of weather systems will make for a period of complicated forecasting for New England in the next 36 hours, but when all is said and done, a dry, pleasant and comfortable airmass will take hold of all six states through the upcoming weekend, and most areas should enjoy a decent summery day on Labor Day, as well.

Our Thursday weather map features an area of low pressure hundreds of miles southeast of Nantucket, swirling winds around its center in a counter-clockwise fashion and bleeding warmer and more humid air toward the Cape and Islands on a developing southeast wind.  Although this system is expected to stay well southeast of New England, the tropical warmth and humidity associated with it will keep banks of fog parked east of Nantucket Thursday and Thursday night, with some clouds likely to back into the island from the east late Thursday, perhaps dropping a few showers on Nantucket Thursday night and otherwise sparing New England from any direct effects.  Meanwhile, our bubble of high pressure that has provided fair weather to all of New England that past three days is relenting, moving slowly east and breaking down with the approach of a strong cold front dropping south out of Canada and into New England.  This cold front features a 30 degree temperature difference on either side of the front, and has brought that type of temperature contrast to Southern Canada and the Northern Tier of the United States as it's been heading toward New England in the past couple of days, so there's no reason to doubt we'll find a strong temperature difference between the North Country and the remainder of New England today, with strong thunderstorms likely to erupt near the battlezone.

There will be two triggers for thunderstorms Thursday, as well - the aforementioned cold front dropping slowly south across New England, and an upper level energetic disturbance that will cross Northern New England as it rides the fast moving jet stream winds near the Canadian border.  The jet stream - the river of air high in the sky that steers our storms and acts as a thermostat for the atmosphere, separating cold air to the north from warm air to the south - has been sailing well north of New England this week, keeping the deepest cool air locked securely north of the Canadian border.  Beginning today, however, the jet stream will begin to buckle as substantial energy south of the Aleutian Islands of Alaska digs southward to a position off the Western United States coastline, carving out a dip, or trough, in these winds over the Eastern Pacific.  Farther east, this will have a see-saw effect of raising the jet stream into a "ridge" of warmth over the Western and Central United States, and another reactive "trough" over the Northeast by Thursday night and Friday.  Of course, while this dip in the jet stream winds indicates cool air moving into the Northeastern U.S. high in the sky, there will be a reflection at the surface, as well - the cold front mentioned above that's been slicing southeast from Canada and the Great Lakes.  With increasing heat and humidity throughout the day Thursday in Central and Southern New England thanks to a persistent southwest wind, dewpoints will climb into the middle 60s which not only means the air will feel sticky, but also means fuel for thunderstorms will be in place.  Morning showers will therefore gain intensity dropping south out of Canada, and areas near the front but in relatively warm air - far Northwest Connecticut, Western Massachusetts including the Pioneer Valley, most of Vermont, most of New Hampshire except far southeastern areas, and most of Maine - will see strong thunderstorms with heavy rain, frequent lightning and perhaps some locally damaging winds for a few communities.  Farther north across the North Country, the arrival of cool air will mean pesky clouds most of Thursday and highs only in the 60s and lower 70s, while immediately on the other side of the front, temperatures will warm considerably.  This could leave a Thursday afternoon temperature range of as much as 20 degrees from the northern to southern White Mountains, or from the Northeast Kingdom to the Upper Valley.  Farther south, the areas that need rain most will miss out on anything substantial, as any afternoon thunderstorms in Central, Northern and Western New England will mostly fall apart with the loss of daytime heating and the passage of the instigating jet stream disturbance, leaving only a few wandering showers across Central and Southern New England amidst areas of fog and lots of clouds Thursday night.

Skies will gradually clear across Northern New England overnight Thursday night as Canadian air spills south across the border and temperatures will fall into the 40s and 50s in response.  The remainder of New England will remain considerably milder thanks to the slow passage of our cold front, though we'll all see drier air on the move during the day Friday.  The biggest trick for our Friday forecast continues to be the speed of this front and determining how quickly the front can clear Southern New England, but I'm expecting to find mostly sunny skies within just a few morning hours across the northern half of the six-state region, with clouds stubbornly hanging on longer in the southern half, only begrudgingly giving way to increasing sunshine from late morning onward.  The farther south one is Friday afternoon, the closer to the slow-moving front you'll be and the better chance of a few scattered showers you'll have Friday afternoon, especially from Connecticut through Rhode Island and into Southeastern Massachusetts.

Behind the cold front, cooler and drier, very pleasant air will return for the start of the Labor Day holiday weekend.  This shot of drier air moving into place for especially Central and Northern locales on Friday will only gain further footing in New England for Saturday, for a dry, pleasant and sunny day.  By Sunday, we'll see only an extremely gradual return of deeper warmth and moisture to the Northeastern United States, and while a few high-altitude clouds of little consequence may stream overhead, it's unlikely that showers would get any closer than Western New York State and Southern Canada.  Temperatures will climb again, however, as the jet stream winds aloft skate gradually northward again, to a position near the Canadian border.  By Labor Day Monday, a new disturbance aloft will traverse these jet stream winds, driving its reflection at the surface southward as a cold front, bringing a chance of showers and thunder back into the forecast by Labor Day afternoon.  With the jet stream so far north, however, it seems likely that the best chance of showers would be scattered in nature, occur during the afternoon and be most prevalent across Northern New England on Labor Day.  Cooler and drier air will sweep across New England on Tuesday before warmth attempts a return for the middle and latter portion of the week.  As this warmth moves north, so will moisture tapped from the Gulf of Mexico, though right now it looks to me like the corridor of moisture capable of producing widespread showers would remain to our west...enough to deliver a pleasant and relatively mild midweek next week, but also keeping rain from reaching the areas of Southern New England that need it most.

Enjoy your Thursday.

Matt

Matt's Technical Meteorological Discussion: Updated Thursday, August 30 at 1:30 PM

Decent bow echo ongoing as of this writing in mountains of ME will continue E but probably is not the last of the show across Northern NewEng, where CAPE will build to 1000-1500 J/kg and as LIs drop to -4 ahead of approaching front and with weak height falls assoc with buckling jet.  Additional severe weather likely ahead of cold front, but while these tstorms will favor most unstable areas from Wrn MA to SW NH to Central ME points north to the front, farther S it will be tough to get precip in, as mentioned yesterday, due to limited height falls.

No real change to my thinking laid out in yesterday's techie discussion beyond that except that it's a bit of a wait and see with the timing of the front for tomorrow, dependent partially upon the action of the subtropical low that is trying to take shape several hundred miles south-southeast of Nantucket.  Though this feature will remain removed from NewEng and seems likely to throw the bulk of its precip onto Nova Scotia, the western edge of the tropical envelope wrapping north of the storm center in the counter-clockwise circulation is creating a band of fog and low clouds that will probably skirt Nantucket from time to time, esp Thu eve and night when some showers are possible on the island.  The front can't really clear the rest of NewEng until this feature begins to move, but that happens Fri so I still like the idea of gradually morphing light scattered showers N to S early Fri and leaving the best chance for afternoon shra/tsra in far Srn NewEng from CT to RI to SE MA.

This weekend, some lows in the 30s seem likely in the valleys of the North Country.  Longer range still looks in line with ideas laid out here yesterday.

Have a great Thursday.
Matt

Wednesday's Discussion

2:45 PM:  Quiet weather for now with high pressure shifting southeast but return flow has already begun as is evident in dewpoint rise thru Upstate NY - actually a bit surprised to find pool of mid 60s dewpoints already in Southern Hudson Valley!  This air will continue to advect northeast overnight Wed night with areas of fog, especially in Central, Western and Southern NewEng valleys, and where decoupling occurs in Nrn valleys.  Has been a daily event for past two days of early AM fog lifting into clouds for awhile before burning off, largely thanks to subsidence inversion and limited mixing.  That's not expected to be the case Thu thanks to increased wind and therefore increased mixing. Surface theta-e plot shows warm and moist tongue extending into the Northeast and this air will be in place on Thu afternoon with noticeably more humid conditions as Tds rise into the mid and upper 60s in Srn and Central NewEng.  Should be a rather impressive difference in sensible weather along the cold front settling south across Nrn NewEng on Thu - today's difference in temps either side of the front in Ontario feature quick drop from 80s to only near 60, and tho temp diff shouldn't be so pronounced with a slow displacement of warm air on Thu in NewEng, still expect to find low 70s for max temps at Canadian border, lots of clouds N of frontal boundary, and middle to upper 80s with perhaps a few 90+ readings in deep humidity Central and Southern Neweng.  Huge forecast issue here is regarding the sub-tropical type low the NMM is spinning up southeast of Cape Cod and carrying periods of heavy rain onto the Cape on Thu, esp late in the day.  I basically disregarded the 00Z and 12Z versions of the NMM to this regard.  I'm never comfortable with entirely disregarding a solution, but both water vapor imagery and 500 mb progs don't indicate nearly the amount of retrogression the NMM is offering up, and the GFS appears to be handling the scenario much better, also in line with the ECMWF and UKMET.  In the end, a scenario similar to the GGEM may end up verifying, with heavy rain falling over the fish just E of the Cape, but only increased clouds and perhaps a few widely scattered showers on the Cape and Islands late Thu.  If course, this retrogression of the warm ocean system will also have a big impact of speed of the cold front, but right now if I'm not going with as much retrogression as the NMM, I have to buy a steadier (tho still slow) frontal progression, bringing strong thunder focused on Central NewEng Thu afternoon with CAPE likely to climb to 1000-2000 J/kg and surface based lifted index falling to between -4 and -6, a corridor of severe weather seems likely ahead of the cold front, though with limited lowering of heights farther south, these storms may actually end up training in the same corridor rather than becoming mobile and moving south.

High pressure in charge thru the weekend and next week it looks like above normal temps return but with trof axis to our west, string of moisture tapping at least some broken Gulf moisture looks like it, too, will stay to the west coming as close as New York but perhaps not delivering any rain to NewEng and that will be a problem for places that are seeing low water tables.

That's all for today.
Matt

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