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. No updates are planned through Labor Day - yours truly will be away from internet access through the holiday, but the tropical page is set up to monitor a possible Felix, including the Windward Islands webcam.
Friday is the finale of the transition back to dry and comfortable air - poised to take hold of New England through the Labor Day weekend and locking in fair weather right through the holiday! In the final step of the transition, the last of our mild and in some spots a bit sticky air will be overturned in the form of scattered showers and thunder with the southward surge of our new Canadian airmass.
It's a rather straightforward forecast from a complex weather scenario: sun and billowing clouds with scattered showers and thunder. But the weather setup that dictates our Friday weather is complicated - a cold front that was responsible for significant severe weather in Central and Northern New England on Thursday afternoon, still draped across New England, though losing definition - still evident with cooler air in place across Northern New England contrasted to areas farther south, but the original wind shift defining the front has washed out as a northeast wind has taken charge of the region. Of course, the reason for the northeast wind is also an interesting one - the combination of building high pressure out of Canada with an impressive storm center passing southeast of New England en route to Nova Scotia, though the skeleton surface circulation has been slower to depart the Gulf Stream waters and will be watched in coming days, though now is losing organization of thunderstorms around its center.
That storm came quite close to attaining tropical characteristics overnight Thursday night, developing a solid area of thunderstorms clustered around its center of circulation and surrounded on most sides by tropical air. On the western periphery of the storm, drier and somewhat cooler, non-tropical air was still feeding the storm center, and prevented a transition to a purely tropical system, but buoy reports close to but displaced from the storm center showed 30+ mph wind gusts and seas to 9 feet! One could infer that the actual center of circulation was even more impressive than the buoy recorded, but as Friday continues, that storm has taken on a decidedly non-tropical appearance, wrapping some of the drier air more completely around its center and interacting with the jet stream winds, dropping south over New England.
These jet stream winds have been the focus of this discussion over the past few days, especially the fact that this fast corridor of wind high in the sky will be dipping south as we head into the weekend. Knowing that the jet stream steers storms and acts as a thermostat for the atmosphere with cool air to the north and warm to the south, the emergence of a "trough," or dip in the jet stream over New England implies cooler air will follow suit, and this is already evident not only aloft, but also here on the ground, with the leading edge defined as the aforementioned cold front. With the semi-tropical disturbance off the coast, and given the storm-steering capabilities of the jet stream, these fast winds that dip over New England will also serve to pick up most of that ocean storm and carry it northeast, where the moisture associated with it will deposit several inches of tropical rainfall to Nova Scotia and send waves as high as six feet for waters southeast of Georges Bank on Friday afternoon. Additionally, the surge of tropical air associated with the ocean storm is wrapping around the center in the counter-clockwise flow, pulled up from the Gulf Stream, wrapped around the east side of the storm and then spiraling around the northern half of the storm center. In this case, that's into Cape Cod and Nantucket, which is why periodic bands of showers and downpours have been observed at times on Friday. As the bulk of moisture aloft that's associated with the storm center races northeast Friday afternoon, the bands of rain associated with it will retreat over the Atlantic, though the general increase in moisture through the atmosphere will help to feed showers and thunderstorms developing in the overturned air of our sluggish cold front, and as mentioned above, a skeleton circulation of the ocean storm will remain over the waters a couple of hundred miles south of Nantucket, and at least deserves some casual monitoring in the coming days to ensure it doesn't develop new thunderstorms near its center.
By Friday middle to late evening, any showers and thunderstorms will be quickly waning, giving way to the drier and cooler air across Northern New England that will clear skies regionwide overnight Friday night, and hold firm for a pleasantly perfect, dry, sunny Saturday. With such dry air in place and the center of high pressure, or fair weather, cresting overhead Saturday night, skies will remain clear, winds will be light and temperatures will tumble, falling into the 40s for many New England communities, and even into the 30s in colder locales, like the valleys of Northern and Western New England! For as cool as it may become on Saturday, sunshine will bring a rapid rebound on Sunday, though with dry air warming very quickly, an isolated dust devil would be possible in especially the valleys of Northern New England. Dust devils form without any clouds - caused by a spiraling vortex of air that develops due to uneven heating of the land early in the day, warming one spot much quicker than another and creating a "thermal," or a pocket of rising air. These dust devils can sometimes produce winds as high as 60 mph, and though damage is rare and dust devils themselves often are so isolated they're hard to find, there have been instances where one hits the wrong place at the wrong time (usually a weak and unstable structure) and results in damage. For most of New England, though, dust devils will be the farthest thing from our minds as we know only that the temperatures will be rising quickly, the sun will be shining brightly and the air will be dry and delightful.
All the while, the jet stream winds aloft will be gradually moving northward again, to a position near the Canadian border, providing for a gradual warmup through Sunday afternoon and into Labor Day Monday. Though a new disturbance aloft will traverse these jet stream winds late Labor Day, driving its reflection at the surface southward as a cold front into Southern Canada and toward Northern and Western New England, the trend for this system has been to slow it down the closer we get to Monday, to the point where my thinking is our Labor Day is simply another delightful day regionwide with sun and clouds, temperatures into the middle 80s and a slight increase in humidity but still a comfortable day. With the approach of the disturbance late, an evening thunderstorm would be possible in Northern and Western New England, with showers more likely to fall Monday night, leaving lingering clouds early Tuesday. With Tuesday morning clearing, though, will come yet another shot of dry and comfortable air - another string of great, sunny, comfortable days, but also enough dry air that it seems likely any tap of moisture from the Gulf of Mexico that may set up would be directed to our west, keeping New England dry, including the abnormally dry areas of Southern New England that need rain the most.
Have a great Friday.
Technical Discussion: None today.
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
