It's been a truly amazing stretch of weather region-wide for New Englanders the past several days, and the stretch of mid-summer feel should extend through Wednesday! Our most recent and final surge of warmth in this stretch will come as a bubble of high pressure moving immediately south of New England on Monday settles to our southeast for the remainder of the week, ushering in deep summer warmth Tuesday and Wednesday that was responsible for 90+ degree temperatures on sunday from the Midwest through the Ohio and Tennessee Valleys and into the Southeast.
Though Monday won't bring deep summer heat, per se, temperatures will still climb some 12-14 degrees above normal for the date and resemble July readings rather than September. The difference, though, is the absence of humidity with a dry and pleasant airmass in place and our warming mechanisms being sunshine and a southwest wind, rather than any type of tropical air. That west-southwest wind will develop after winds begin from the northwest, with the shift in wind coming as New England shifts from the front side to the back side of a bubble of high pressure drifting from west to east, passing just south of New England's south coastline. Because of the clockwise flow of air around this high pressure cell, wind direction will shift and the new west-southwest wind not only will begin to carry deeper warmth to New England, but also will bring "downslope flow" - wind sloping down off the hills and mountains. Though you can always double click on any term in these discussions to find out what a word means (thanks to the folks at answers.com), downsloping flow is the term for wind that comes down from hilly or mountainous terrain, and as it descends, the pressure around it increases (the lower one is in altitude, the greater the atmospheric pressure). By the properties of physics, as pressure increases on air, so does temperature, and the result is a warming and drying of the air as it moves into lower elevations. So, on a day like Monday with a west-southwest wind, this means a lot of Central and Eastern New England will reap the mild benefits of this warming and drying of the air with sunny skies and just a few high altitude clouds mixing in from time to time.
Not only is dry air comfortable, and not only does it warm quickly with the aid of sunshine, but it also cools quickly when the sun is subtracted from the equation, meaning Monday night temperatures will be cool compared to the summer-like daytime highs, dropping into the 50s for most spots, though likely hanging in the lower 60s for the urban areas of Southern New England. The air will once again warm quickly on Tuesday as a southwest wind strengthens and continues the flow of summer air into New England, sending temperatures into the 80s beneath plenty of sunshine yet again. To our west, a deep feed of tropical moisture will continue streaming from the Gulf of Mexico into the Midwestern United States, where it will interact with a cold front pushing east, associated with a cold and energetic system that's moving east out of the Rocky Mountains. This energetic disturbance and its attendant cold front will march slowly east, but progress will be slow as it encounters the ridge - or large area of high pressure - situated over the Eastern Seaboard.
After areas of fog Tuesday night, Wednesday the squeeze will be on across New England between the area of high pressure departing to our southeast and the approaching disturbance and cold front marching across the Great Lakes. As the southwest wind gains in intensity and gusts over 25 mph Wednesday afternoon, the measure of the amount of moisture in the air will rise, as well, with dewpoint temperatures climbing into the middle 60s and a noticeably sticky feeling returning to New England after a recent hiatus. The increasing moisture will serve as fuel for showers and thunder that may develop in Northern and Western New England very late Wednesday ahead of the aforementioned incoming disturbance, though most of Wednesday should stay mostly dry.
From later Wednesday night through Friday, a confluence of energy and moisture will make New England weather unsettled. Determining the exact timing and intensity of periodic rain and showers associated with this new pattern will be tricky, especially from this far out, as the timing of rain depends on the speed of each upper level disturbance that will be converging into a larger upper level storm in the New England sky. In fact, with the new "trough," or dip, in the jet stream winds aloft developing over the Northeast as cool air dips southward out of Canada associated with the northern stream energy moving overhead, the steering flow will begin to orient from south to north near the Atlantic coastline, and this is likely to allow tropical waves and tropical moisture to ripple north along the jet stream current aloft and the associated frontal boundary that will sit stalled off the coastline. One big question here is how much of this moisture can make it far enough northwest to deliver rain to New England, though the potential exists for some of this moisture to create a swath of rain that may end up heaviest Thursday night, and then perhaps a round of thunder Friday afternoon. All the while, the increase in clouds and falling rain from time to time will keep temperatures rather cool, especially in the North Country where temperatures may not get out of the 60s Thursday afternoon, and most areas probably will hover around 70 in New England on Friday.
The good news for fairgoers and others with outdoor dreams for next weekend is that this storm center should - after slowing overhead for a couple of days - become progressive in time to sweep drier air across New England later Friday afternoon or evening, leaving behind a dry airmass for Saturday and Sunday. Though a chunk of somewhat cooler fall air seems likely as the new high pressure cell builds in Saturday, this air would be likely to quickly modify as the fair weather cell moves southeast while strengthening, setting up a similar scenario to the past several days, though unlikely to last as long. Nonetheless, temperatures may warm back above normal by Sunday or Monday.
Have a wonderful Monday.
Technical Discussion: Planned on getting one out this afternoon but tropical activity necessitated numerous updates to the Tropical Page, which you can check out by clicking here. Sorry for misleading on the techie for today, but will make time tomorrow and hopefully you find the tropical systems worthwhile.