Plain-language summary:
After a cloudy and rainy day for many yesterday,
it will be somewhat drier and hotter today but still with isolated thunderstorms,
with hazy skies due to wildfire smoke near and north of the U.S./Canada border.
More widespread showers and thunderstorms with continued high humidity will
dominate Monday to Wednesday, with wildfire smoke pushing off to our north.
Severe thunderstorms are unlikely, but heavy rain with isolated flash flooding
is likely, especially in southeastern facing slopes of the Green, White, and
Laurentian Mountains. It will slowly dry out later in the week, possibly with a
day or two of slightly lower temperatures and humidity, before warming back up.
Warmth and humidity will likely dominate afterward, but continued frequent
chances of showers and thunderstorms along with the wet ground may prevent any
true heat waves from occurring.
Meteorological discussion:
In stark contrast to earlier in the
summer, deep southerly flow is transporting subtropical humidity into our
region, with widespread dew points of >65F
(18C), and even >70F (21C) in areas with very recent rainfall. After a storm
brought widespread clouds and rain yesterday, there will be a break for most
today ahead of another slowly-moving and almost vertically stacked low pressure
system approaching from the Great Lakes. With a very subtle cold front bringing
in slightly drier air, especially north of the U.S./Canada border, promoting
fewer clouds and more sunshine, it will be hotter than yesterday for most, with
a few low-elevation locations possibly reaching 86F (30C). However, scattered
thunderstorms could still form given the heat and some lingering humidity, especially
over the higher terrain. Given the lack of synoptic-scale forcing for storms,
the storms are unlikely to be widespread. Also, thick smoke near and north of
the U.S./Canada border will keep temperatures cooler than modeled, given that
the models do not take the smoke into account in its temperature forecasts,
which combined with the drier air, will mostly prevent convection.
Synoptic-scale forcing for storms will
be higher from Monday to Wednesday as the low pressure system over the Great
Lakes slowly moves into our area. Southerly flow will increase ahead of the
storm and push the wildfire smoke north of our region. Also, it is not just
humid near the surface, but throughout almost the whole atmospheric column. The
atmospheric profile is generally nearly moist adiabatic and with high relative
humidity (saturated at times), with weak instability during daytime due to modest
daytime heating. With mostly cloudy skies and wet ground, daytime heating will
be limited but not completely eliminated, and will be just enough to spark rather
widespread afternoon showers and thunderstorms. The rather weak instability
will limit any chances for severe thunderstorms, with limited updrafts for hail
generation, deep warm layer for any hailstones to melt in before reaching the ground,
and lack of dry air aloft to promote strong cold pools that would produce
strong gust fronts. However, the deep warm, moist layer will lead to a deep
layer of efficient warm rain generation and limited evaporation of rain before
reaching the ground. Therefore, any storm will be able to easily produce very
heavy rainfall rates, heavier than radar reflectivity might suggest given the
small raindrops, and isolated flash flooding could occur, especially since
these storms are going to occur for 3 days in a row and make the ground
increasingly wet. However, given the hit-and-miss nature of the storms, there
will be substantial spatial variation in rainfall amounts. There will be also
terrain modulation in rain amounts given the southerly flow, with the southern
or southeastern slopes of the Laurentians, Green Mountains, and White Mountains
likely to receive especially heavy rain (locally up to 6" or 150 mm total), while downsloped areas like the
Champlain Valley and the eastern St. Lawrence River valley will receive less
(perhaps as little as 1" or 25 mm total) and will also have fewer clouds and be hotter.
As the low-pressure system weakens and
moves off to the east, it will mostly dry out Friday and into the weekend,
though scattered terrain-induced thunderstorms could still occur given the lack
of a strong Canadian cold front to transport drier air. It will likely turn
subtly cooler and drier for a day or two with brief northerly flow behind the
storm before turning warmer. Though there is obviously uncertainty in the
weather pattern evolution afterward, ensembles agree on a general ridge forming
near the North American West Coast and another one in Quebec or Atlantic Canada.
This will lead to a continuation of the incredibly hot, fiery, and smoky summer
over Quebec, but the deep southwesterly flow will lead to more warmth and
humidity dominating in our area with the wildfire smoke being blown to the
northeast mostly away from our area. This could also be a rather stormy pattern
with disturbances from the southwest running into the warmth and humidity in our
area, though there will likely also be days of ridging with quieter weather.
The frequency of disturbances and wet ground could prevent true heat waves from
occurring despite the warmth and humidity.
