Plain-language summary:
This weekend will feature a strong contrast across the region, due to a slowly-moving cold front just northwest of the St. Lawrence Valley in Ontario and Quebec. To the southeast, there will be a spurt of record heat with high humidity in Vermont and New Hampshire, and to the northwest, there will be a series of showers and thunderstorms, especially on Sunday, some of which could be severe. It will cool down and clear out for Monday and Tuesday before another storm likely affects the region on Wednesday and Thursday.
Meteorological discussion:
After a cooler and rainy day on Thursday, today is the transition day, with deep southerly to southwesterly flow advecting hot and humid air from the Gulf of Mexico and Texas. With the warm air advection and abundant sunshine, high temperatures will be set late in the day and be much warmer than yesterday, except in Maine where southerly flow off the still cold Atlantic Ocean is causing low clouds and modification of the warm air mass.
This weekend's forecast has been more uncertain than usual due to the proximity of a slowly-approaching front just northwest of the St. Lawrence Valley with a couple of relatively weak low pressure systems riding along it. Models have finally gotten into better agreement though. Tomorrow, the hot and humid air mass will be fully engulfed in our region, except in eastern Maine with the continued maritime influence. Abundant sunshine will lead to hot temperatures in Vermont and New Hampshire, especially in southern areas where lower elevations could reach 95F (35C), breaking daily record highs and potentially challenging monthly record highs. Humidity will be much higher than with the mostly dry heat last week, and heat advisories have been issued for the southern Champlain and Connecticut Valleys and southern New Hampshire. Farther northwest, over northwestern New York, eastern Ontario, and southern Quebec, , some extra clouds and some scattered showers and thunderstorms will keep temperatures lower, though it will still be humid there.
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| Source: PivotalWeather and TropicalTidBits |
It is a similar story for Sunday, but a weak low pressure riding along the front will slowly strengthen over Quebec and help advance the cold front southeastward in the afternoon. Since the cold front will not arrive until evening (except early afternoon in eastern Ontario), temperatures will be similar to those on Saturday, but with a better chance of showers and thunderstorms with the cold front or terrain-induced pre-frontal troughs. It will turn much cooler and drier Sunday night behind the cold front.
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| Source: PivotalWeather and TropicalTidBits |
With the hottest and most humid air mass of the year, CAPE will be quite high in the hotter areas with more sunshine, from the St. Lawrence Valley on southeastward. There is a threat of severe thunderstorms both Saturday and Sunday afternoons. The threat is lower on Saturday afternoon with a lack of large-scale triggers for any storms to develop, but the few that do develop could be severe. The cold front will provide a stronger trigger on Sunday afternoon and evening. This is indicated by the SPC as well.
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| Source: PivotalWeather and SPC |
The cold high pressure responsible for the snowstorm in Colorado will have moved eastward into Ontario by Monday, with the cold air greatly modified. Temperatures will be below average but not by too much especially with some sunshine and no rain. The broad high pressure system will moved eastward and continue to provide clear weather through Tuesday night. A developing storm in the Midwest will likely provide unsettled weather for Wednesday and Thursday. For now, it looks like the heaviest rain will be mostly north of the U.S./Canada border, closer to a frontal zone, with south/southwesterly flow to the south causing another warm and humid surge, though not as hot as this weekend. However, that is 5-7 days out, so details are iffy.
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| Source: TropicalTidBits |
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