Plain-language summary:
A broad high pressure area will provide mostly sunny weather and record warmth through Thursday, but especially west of the Appalachians from Monday through Wednesday. It will turn cloudier by early Friday ahead of a low pressure system and cold front that will bring the first rain to many in two weeks on Friday and Saturday. Much cooler will follow, with the first snowflakes at higher elevations and first frost at lower elevations possible. A changeable weather pattern is likely afterward with occasional storminess and much more day-to-day temperature changes than the region has seen recently, more typical for October, though it may still be generally on the warm side.
Meteorological discussion
Today will be the warmest day since
the early September heat wave for most, with sunshine and very light winds dominating
for almost everybody with a broad surface high pressure in control. With the
clear skies, drier ground, and slightly lower relative humidity, it will reach
or exceed 75F (24C) for most lower-elevation areas. A big ridge building in
eastern North America will provide warming and continued sunny and dry weather
through Wednesday and possibly Thursday. Tomorrow will be a bit cooler with
slightly lower dew points from Vermont northeastward, especially in Maine, due
to a subtle shot of slightly cooler air from the northeast, but by Tuesday,
everyone will be even warmer with higher dew points due to the westerly flow
bringing in the really warm air. Lower elevation temperatures up to 85F (29C) are
likely. Another shot of slightly cooler air from the northeast will keep
eastern New England cooler on Wednesday, but it will continue to be hot farther
west, perhaps even hotter than Tuesday. Increasing southerly flow will provide
some maritime moderation by Thursday, especially east of the Appalachians. Clouds
will start to increase later Thursday as well, causing cooler but still
unseasonably warm temperatures for most, with the warmest being west of the
Appalachians. Even nighttime lows will be warm by Wednesday night, possibly up
to 60F (16C), due to the increasing southerly wind and dew points hindering
radiational cooling.
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| Source: TropicalTidbits |
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| Source: TropicalTidbits |
However, big changes are on the way
later in the week. The potent trough/upper-level low in the western U.S. will
be quickly replaced by a pronounced ridge, leading to a downstream trough in central
and eastern North America. This is associated with a rapid shift from a negative to positive PNA while the NAO remains negative. A cold front and an associated low-pressure system will
inevitably arrive from the northwest by late Friday or Friday night, leading to
the first rain for many in two weeks or more, and heralding the shift to real autumn
weather. How much rain falls depends on the location, speed, and strength of
the storm, all of which are still uncertain. There is the potential for a widespread
>1” (>25 mm), especially east of the Appalachians, with the upslope
southerly flow and arrival of deeper Atlantic moisture. Much cooler air will
follow, possibly with secondary cold fronts accompanied by showers, and it will
be mostly cloudy and breezy, which is not uncommon for October. A lot of leaves
will be changing color and falling off the trees. With temperatures at 5,000 ft
(1,500 m) dropping below freezing, especially northern Vermont on northwestward,
the first snowflakes could fall in higher elevations! Widespread frost could
also occur if the surface high pressure comes close enough, causing clearing
skies and calming winds, though it could remain too cloudy and windy for most
if the high pressure stays to the west, as often happens early in the fall,
especially with the low-level moisture enhanced by the flow off Hudson Bay and
all the unfrozen lakes in Quebec.
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| Source: Climate Prediction Center |
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