Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 271425 AAB

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
1025 AM EDT Tue Oct 27 2020

Cool high pressure builds into the area today and moves to the
east on Wednesday. Expect drier conditions with seasonably cool
temperatures. Later this week, low pressure tracks eastward to
the south of New England just as an Arctic cold front brings
much colder air into the area from the north. Rain is likely to
begin in southern areas on Thursday with rain changing to snow
on Friday as the temperature cools. This will be the first
accumulating snow of the season for many areas. Cold weather
lasts through at least Saturday before high pressure shifts east
and allows temperatures to slowly rebound.


1030 AM Update...
Made a few changes to mainly the sky cover and temperature curve
this morning. Overall the forecast is in good shape with highs
in the upper 30s/lower 40s north to upper 40s/lower 50s south.

630 PM UPDATE...
What I expect was the last band of light rain has exited the
coast early this morning, so precipitation chances are on their
way down. Made a few adjustments to the forecast based on this

Winds have already shifted to the northwest across the area
this morning and expect cool, dry air to continue moving in from
the northwest today. We should even get rid of a good deal of
the clouds for at least part of the day today especially over
the coastal plain where downsloping conditions aid in mixing in
the drier air. Temperatures will be a bit warmer behind the cold
front today due to the deeper mixing. Southeast of the
mountains the temperature should warm into the 50s, while
northwest of the mountains the cold air will pile in and less
mixing will occur, keeping temperatures in the low 40s.


At the surface, high pressure will build in from the west
overnight with calm conditions expected. This would normally set
up ideal radiational cooling conditions as winds go calm and the
temperature craters toward the dewpoint in the freshly drier air
mass. However, we continue with a westerly flow aloft and
embedded in that flow we expect a subtle shortwave trough to
bring in some clouds and maybe even a few sprinkles to southern
New Hampshire overnight. These clouds will be the dominant
factor in low temperatures tonight. Areas to the north and east
are more likely to stay clear and reach their full potential for
cooling... into the low to mid 20s. Further to the south and
west, clouds will be more widespread and buoy temperatures in
the 30s most of the night. In between is where more uncertainty
exists, with the temperature likely falling during the clear
periods and holding steady or rising a bit when clouds pass by.

High pressure shifts east on Wednesday with some weak warm
advection bringing in some warmer temperatures aloft. We won`t
be as well mixed on Wednesday as we were on Tuesday, though,
with nearly all of the mixing coming as a result of the late
October sun. Thus expect temperatures once again in the 40s and
50s for highs with dry conditions expected. Periods of
cloudiness will also be around as the warm advection aloft


Overview: Subtle but significant shift in guidance this evening
has brought flatter solutions into play and introduced more
uncertainty regarding potential snowfall. Chances for measurable
snowfall still remain fairly high for late Oct...but guidance
seems to falling into two with just minor measurable
snow amounts and another with more significant accumulation.

Impacts: Again the largest impacts of this forecast period
revolve around how much and where snow accumulates. Trends this
evening were decidedly towards lower snow amounts...but there
remains plenty of support for more significant snowfall amounts
and there is plenty of time for guidance to trend back in this

Forecast Details: Thu afternoon precip looks to pretty quickly
enter SWrn portions of the forecast area. How quickly it spreads
N...if at all...remains in question however. The most noticeable
thing with guidance this evening is that the forecast appears to
be bifurcating...between a nominal snowfall and a more
substantial snowfall. In general the deterministic NWP featured
a faster Zeta that sneaks off the East Coast ahead of the
ejecting upper low...and ahead of the cold air. The result is
a more strung out and flatter system...and less QPF. Less QPF
means less QPF around when cold air surges into the system from
the N. As a result snowfall amounts across guidance is generally
lower due to unfavorable overlap between strongest lift and
coldest air.

The bifurcation also shows up in ensemble guidance...with the
EPS generally more in favor of amplified solutions and the GEFS
and CMC more in favor of faster and flatter. That being
said...members of all ensembles show some larger snowfall events
embedded among lighter events. Fewer and fewer members are
showing no snowfall at all. This is again supported by the NBM
probabilistic guidance...which has shown chances for measurable
snow increase to 60 or 70 percent across the Srn half of the
forecast area.

The NBM has also shown chances for 4 to 8 inches of snow...a
significant event for late Oct...increase as well albeit still
less than 30 percent chance. This makes sense to
guidance does show cold air rushing into the system late Thu.
This scenario would be favorable to flash precip over to heavy
snow for a time...and the difference of an hour or two on the
earlier side could mean several inches of snow.

Much of this is dependent on the upper air pattern as it
evolves when Zeta nears the Gulf Coast. We should have a better
idea of this heading into Wed morning...and extra soundings from
Gulf forecast offices will ensure it is well sampled.

For now the most favorable overlap of all variables occurs
across Srn NH. So I have the most snowfall there...generally 1
to 3 inches in the valleys and less as you head towards the
Seacoast where there may be more mixing. I also have up to 3 to
5 inches in the higher terrain from the Whites down thru the
Monadnocks as colder air will arrive there first.

Beyond this system Fri and Fri night will be some of the coldest
air of the season so far. Highs will likely struggle to reach 40
Fri afternoon...and we will fall well into the teens and 20s Fri


Short Term...Conditions improve to VFR rapidly this morning as
northwest winds bring in drier air. The exception to this will
be on the northwest side of the mountains where those winds will
be locking in the low level clouds for a while longer before the
dry air finally breaks these clouds up this evening. VFR
conditions expected tonight and Wednesday area wide as high
pressure moves through the region. Some passing mid level clouds
and a stray shower are possible in southern areas overnight.

Long Term...As precip moves into the area the second half of Thu
I expect widespread IFR conditions to overspread the area. Most
of the area should start as RA given the relatively mild
antecedent air...but precip is expected to transition to SN Thu
night. LIFR conditions will develop S of the mtns especially in
any snowfall. Precip tapering off Fri midday and warming temps
will help to change any remaining precip back to SHRA and
conditions will improve as flow becomes N and NW.


Short Term...Northwest winds behind a cold front may
occasionally gust to 20KT or so today over the outer waters in a
relatively well mixed environment. However, pressure gradient is
rather weak so it is unlikely that a Small Craft Advisory will
be needed. High pressure builds in from the west tonight and
crests over the waters on Wednesday with light winds.

Long Term...Low pressure developing S of New England will
increase NE flow across the waters into Fri. A period of gale
force gusts is possible outside of the bays very late Thu night
into Fri afternoon.





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