Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 270729

Area Forecast Discussion
National Weather Service Gray ME
329 AM EDT Tue Jul 27 2021

One more warm and humid day today before a cold front sweeps
through and ushers in a cooler and drier air mass. Ahead of that
boundary we can expect thunderstorms to cross the southern half
of New Hampshire and maybe spill over into far southwestern
Maine. Some of these storms may be strong to severe before they
exit to the coastal waters and weaken. High pressure is expected
Wednesday before another front crosses the region Thursday. The
cool temperatures continue through the weekend as several fronts
reinforce the cooler air.


Canadian wildfire smoke continues to reduce visibility this
morning. HRRR smoke guidance has a lot of this mixing out by
midday across Wrn ME...while the currently improving visibility
across NH may actually get worse as another plume of smoke moves
in. This will clear out with the convection this evening.

The large MCS over MI early this morning will decay as it moves
across the Ern Great Lakes. However convection is expected to
initiate along and ahead of its remnants as it moves thru the
Hudson Valley. This has consistently been forecast to move
across Srn NH and extreme SWrn ME across the suite of CAMs.
Given the forecast CAPE in the 500 to 1000 J/kg range...stronger
shear than yesterday...and better trigger I think thunderstorm
coverage is more likely than not. I have included gusty wind
wording along the path of the MCS. While isolated severe
thunderstorms are would not take much of an
increase in 0-3 km shear to see the threat increase even more.
0-3 km shear magnitudes approaching 30 kt brings mesovortex
generation into play...which can act to enhance winds in an
otherwise well-behaved line of storms.


There looks like some potential for the frontal boundary to
hang up near the NH/MA border with showers and thunderstorms
rippling along it thru the overnight. At this time it looks like
the deep convective potential will be any sort of
training heavy rain threat looks unlikely.

Much drier air will filter into the region starting overnight.
This will allow Nrn zones to drop into the 40s...especially if
they can completely clear out. During the day Wed...with near
normal high temps it will feel quite pleasant outside.


A series of cold fronts will bring fresh supplies of cooler air
through the weekend and into next week as general troughing
develops over the eastern US downstream of western ridging.
There will be at least a brief return of some warmer and more
humid air ahead of each front with the chance for showers and
thunderstorms accompanying the frontal passages. High
temperatures for the most part will be 5 to 10 degrees below
normal for midsummer.

High pressure crosses the area Wednesday night leading to a cool,
calm night in a dry air mass. We should see 40s in the cooler
valleys with 50s more widespread. Moisture returns on Thursday
ahead of the next upper trough and surface cold front. Chances
for showers and thunderstorms will be greater in the second half
of Thursday and into Thursday night as the core of the forcing
moves through. A lingering upper trough on Friday provides a
cold pool of air aloft beneath which daytime heating will tend
to trigger scattered convective showers and thunderstorms. But
overall a cooler and drier air mass will be moving in with fall
like temperatures expected through the weekend.

A deeper upper low drops south out of Canada and spins near
southern Canada or northern New England late Sunday into Monday.
This will bring yet another chance for showers and thunderstorms
when the leading edge of this trough arrives, although the
moisture return ahead of it may be a bit too limited.


Short Term...Lingering FU across the area will keep some areas
of MVFR VIS thru midday. Any FG looks like it will remain
confined to the CT River Valley...including HIE and LEB. TSRA
are expected to develop just W of the area late this
afternoon/early this evening. These will sweep across the Srn
half of NH and parts of SWrn ME...with MVFR or lower conditions
possible. I also cannot rule out some gusty winds within those
TSRA...mainly in the 21z to 01z time frame. VFR conditions
prevail Tue night into Wed.

Long Term...Mostly VFR conditions expected through the rest of
the week and weekend, although morning fog is possible most
mornings at LEB and HIE. Showers and thunderstorms are possible
late Thursday into Thursday night area wide, with more scattered
showers during the day Friday.


Short Term...Some lingering smoke reducing visibility this
morning is expected to clear out this afternoon. An area of
thunderstorms will cross the waters S of Cape Elizabeth this
evening...and gusty winds are possible over the waters. As drier
air moves into the region Wed...NE flow may freshen over the
water with some gusts around 20 kt.

Long Term...A series of fronts will cross the area later this
week and this weekend. The first arrives Thursday night into
Friday morning with a brisk northwest flow expected behind it
for Friday into Saturday.





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