Plain-language summary:
Today will be a classic late-summer day with slightly hot temperatures, moderate humidity, and some scattered afternoon thunderstorms developing mostly north of the U.S./Canada border. Showers and thunderstorms will spread into New England tonight into Monday. It then briefly turns drier and slightly hotter on Tuesday before a cold front brings showers and possibly scattered thunderstorms on Wednesday, preventing the previously expected hot spell from materializing. It will turn drier and cooler on Thursday, then warm up Friday and Saturday before another weaker cold front arrives and likely brings showers and thunderstorms on Saturday. Early September appears rather cool, in contrast to some of the recent summerlike Septembers, though it might also be sunnier and drier.
Meteorological discussion:
After the impressive 3-day cool spell, the most intense for August since at least 2014 for most, summerlike temperatures have returned. Today will start with partly cloudy skies, but as a very weak upper-level low approaches into central Quebec from the northwest, daytime heating and slightly higher dew points than the past few days will lead to some instability, enough to trigger scattered showers and thunderstorms especially north of the U.S./Canada border. Before the storms though, temperatures will rise into the low-mid 80s F (27-29C) in lower elevations south of the U.S./Canada border and up to 80F (27C) north of there, where it will be cloudier with earlier storm initiation due to closer proximity to the upper-level low. The upper-level low will sink south into New England tonight into tomorrow, triggering scattered showers and thunderstorms even at night, though most of the activity will be from the Champlain Valley on east, with it drying out to the northwest by tomorrow afternoon. The clouds and rain will keep temperatures in Vermont on eastward slightly cooler than today, though nowhere near as cool as earlier this week. This upper-level low is much weaker than the one this past week, and it will also move faster (not stall over our region) due to another disturbance bringing a cold front and faster jet stream winds approaching from the northwest. As the upper-level low moves to the east, westerly flow will return on Tuesday, with very hot and humid air lurking just to the southwest. The warm advection and sunshine will lead to temperatures warming back into the 80s F (27-30C) in lower elevations from the Champlain Valley on westward, with the hottest farthest west in eastern Ontario, closest to the hot air mass source.
 |
| Source: TropicalTidbits |
 |
| Source: TropicalTidbits |
The cold front arriving on Wednesday will put an end to the summer warmth and prevent the hot spell that was earlier expected at the end of the month to materialize. The upper-level ridge previously expected to encompass all of eastern North America will be mostly limited to the Great Lakes on south and west, similar to last August. This will ensure that average high temperatures in our region will be below average for August for the 2nd August in a row, though lows might be a little above average still due to the clouds and high humidity we have had for much of the month. Depending on the exact speed of the front, which depends on the strength of a low-pressure area to the west over the Great Lakes on Wednesday (stronger means slower front), it could be very cool over the entire region on Wednesday with clouds and showers (but probably not thunderstorms), or it could still be warm and humid south of the U.S./Canada border, with stronger showers and thunderstorms possible Wednesday afternoon. A cool Canadian high pressure will follow behind the cold front, though the coldest air mass will be aimed at Atlantic Canada and northeastern Canada. Still, with the stronger cool air masses as fall approaches, it will be noticeably cooler and less humid on Thursday, even if it turns sunny as it is currently expected to for most. The high pressure quickly moves to the east for Friday and Saturday, allowing the flow to switch to southerly, but that brings moderation off the Atlantic Ocean, especially east of the Champlain Valley, instead of the hottest air from the west. As such, the reservoir of true hot air will never reach our region. Another cold front will likely bring showers and thunderstorms Saturday (though the exact timing is not certain this far out), though the nature of the cold front, with surface low pressure staying to the northwest ensuring a westerly to southwesterly flow and only weak cool advection, could actually lead to similar or only slightly cooler daytime highs for the following Sunday if there is sunshine before the true cool air from central Canada gradually filters in.
 |