Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 211133
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
633 AM EST Wed Feb 21 2018

.SYNOPSIS...
Strong southwesterly flow ahead of an approaching cold front
will result in a very warm day across the region with record
high temperatures possible. The cold front will cross the region
this evening with cooler air filtering in for tonight and
Thursday. A weak weather system passing south of the region on
Thursday may spread some light rain and snow to southern areas
before high pressure builds overhead Thursday night through
early Friday. A series of weather systems will move through the
region beginning late Friday and continuing through the weekend
with mixed precipitation and slick travel possible.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
630 AM...Main focus of this update is on the dense fog, and to
extend the advisory until 15Z along the coast and in ME zone
just inland of the coast. Fog is diminishing in some of the
zones further west, but with the strong shallow inversion in
place, will likely have to wait until the core of strongest
winds abv the boundary layer move through closer to midday. Also
tweak temps a but based on current obs, which are a little
colder than forecast in may of the foggy spots early this AM.
Still uncertain of how warm it will get in areas that hang onto
the inversion and the fog thru the morning, but any changes to
high temps were minor at this point, and day shift will have to
monitor.

Previously...Trying to crack the sfc inversion is a struggle
early this AM, as warming aloft continues, but lack of any real
flow in the colder air near the sfc keeps that very shallow
inversion in place. Temps at elevations and in downslope areas
have been in the 50s, and even low 60s overnight. Fitchburg, MA
/KFIT/ was 63 at 08Z, while mesonet data shows temps in the 50s
above 2K feet in the White Mtns.

So, the big question remains is how much and how quickly does
mixing, even to 950mb, occur, as that would produce sfc temps in
the 60s at least, given those temps are 17-18 C. Srn NH, and
perhaps the CT valley is likely to be the first to mix down,
given that the strongest flow aloft reaches there earliest, and
they were in the warm sector yesterday. As you head N and E,
things becomes less certain. Early morning clouds and fog will
be a factor limiting breaking the inversion early. There should
be enough mixing to the top of the inversion to allow warming
into the 50s almost everywhere, but along the ME coast and into
areas just interior of the mid- coast, the marine influence
will temper some of the mixing. So, ultimately, highs range from
around 50, which will be areas right on the mid-coast and
Penobscot Bay, to the mid-upper 50s areas just inland of the
mid-coast, and into the ME mtns. Interior SW ME, and even the
coast S of Portland /except for the beaches/, will make a run
at 60, if not 65 further inland, which will also be the temp
range across much of NH. The exception will be srn NH,
especially Merrimack and Seacoast where upper 60s, and even some
low 70s are possible. By afternoon, should see breaks of sun
across most of the CWA as well.

The front crosses the CWA late in the afternoon, and should see
temps start to fall behind the front, which could produce a few
SHRA in the mtns. Actually, any place that is stuck in the
inversion through the day will likely shoot up to its high very
late in the day in the mixing right behind the front.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
As the winds shift to NW behind the front, will see T/Td fall
through the night. Some upslope SHRASN will possible this
evening, but these will be quite light. Some partial clearing
expected, especially in the N, but as the front stalls to our
south, look for mid clouds to linger over southern zones.
Despite temps falling through the night, mins will stay above
normal, generally in the 20s to low to mid 30s across the
warmer srn zones.

Weak wave track from west to east along the front will cross
the CWA late Thu, and this will produce some light precip over
the srn third of NH, and perhaps York county ME. Boundary lyr
temps will be on the milder and with sfc temps running in the
mid to upper 30s, will see a mix of snow and rain. If there are
any accums, the would likely occur in the hills of Cheshire and
western Hillsborough county, where some spots could see an
inch. Otherwise look for any morning sun in the nr half of the
CWA to fade to mostly cloudy, with highs in the mid 20s to low
30s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
High Impact Weather Potential:

* Some slick travel potential in the mountains and foothills
late Friday/Friday night.

* Snow/mixed precipitation possible Sunday-Sunday night.

Pattern: The highly amplified trough/ridge pattern across North
America early this morning will not persist long as energy ejecting
out of the western trough will work to flatten the eastern ridge
with an onslaught of energy rounding a ridge in the central/eastern
Pacific acting to re-invigorate the western North American trough.
At the same time...the vortex that has been centered in the vicinity
of Hudson Bay will continue to weaken and shift away from the CONUS.
Thus an active...and relatively mild period of weather is expected
in the long term.

The Details...

Thursday Night: Shortwave passes east of New England to open the
long term period with 1042+ mb high settling briefly into Maine with
frontal boundary south of the region.  Expect clearing to be
partial...with mid and upper level clouds continuing to stream
overhead ahead of next shortwave ejecting out of western trough and
riding into the Great Lakes by Friday morning.  The setup poses a
potentially challenging low temperature forecast with the clearest
skies in the north likely allowing these locations to fall into the
single digits.  Further south...where there is the potential for
thicker cloud cover...can see many areas remaining in the 20s.

Friday - Friday Night: Next shortwave arrives during the day
Friday...with precipitation gradually becoming less organized as
llevel convergence along northward advancing warm front weakens
given veering llevel flow.  Given that cool high pressure will be
centered over Maine to start the day...the warming trend during the
day will be slow in the low levels with some indication of a damming
signature.  The surface/H8 freezing line appears rather well aligned
such that a snow to rain transition with some freezing rain possible
Friday evening over northern areas where temperatures will likely
never reach above freezing.  QPF is not particularly high given
weakening forcing for ascent thus outside of some slick travel
potential in the foothills and mountains...see no very significant
impacts.

Saturday - Sunday:  The pattern repeats itself during this period
with high pressure cresting north of the region on Saturday before a
much stronger low pressure system moves into the Great Lakes late
Saturday night into Sunday...with triple point low forming in the
vicinity of the New England coast Sunday with warm advection-induced
precipitation shield moving overhead on Sunday and continuing into
Sunday night.  This setup lends itself much more favorably to cold
air damming /as indicated by previous forecaster/ and forcing
for ascent is more robust although still weakening somewhat as
primary low occludes well north and west of New England. Much
will depend on the details of the potential triple point
development as a stronger feature will help anchor cold air and
favor a snowier scenario...while a weaker low would likely lead
to more icing. A moderate QPF event appears most likely with
ensemble probabilities favoring in excess of one half inch
liquid...but little support for one inch.

Monday - Tuesday:  High pressure returns to the northeast to end the
long term forecast period with cooler...albeit still seasonable
temperatures aloft. Maintaining low PoPs /for snow showers/ in the
upslope flow areas of the mountains is prudent at this range given
residual moisture in the northwest flow.  Even with the cool down
aloft...temperatures will remain above seasonal norms with locations
south of the mountains likely reaching the 40s.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Short Term...Fog will linger through about 12-14Z, during which
time there should be gradual improvement to VFR by mid-morning
at all but KAUG/KRKD. KAUG will see fog linger through midday,
while KRKD will be stuck in fog through most of the afternoon,
although conds should improve to IFR to MVFR around midday. LLWS
will also continue into early afternoon. VFR is expected at all
terminals tonight, and all but KMHT/KPSM on Monday, where MVFR
in light SNRA will produce at least MVFR Thu afternoon.

Long Term...VFR through early Friday then deteriorating
conditions with rain and snow showers likely allowing for
restrictions late Friday and Friday night before VFR conditions
work back into the region Saturday. IFR restrictions likely
return in snow/mixed precipitation on Sunday as a strong low
pressure system approaches from the west.

&&

.MARINE...
Short Term...Borderline WSW SCA winds with seas of 5-6 ft in
the open waters can be expected today. The winds will shift NW
this evening and will diminish late tonight.

Long Term...High pressure builds north of the waters Thursday
night and into the Canadian Maritimes on Friday with quiet
winds/waves before weak low pressure moves north of the waters
Friday night with high pressure quickly building back in to the
north of the waters on Saturday. A stronger low pressure system
will move northwest of the region on Sunday with a secondary low
potentially forming along the New England coast. This will
likely result in renewed small craft advisories...with gales not
out of the question depending on the strength/location of
secondary development.

&&

.HYDROLOGY...
Flood watch continues for the threat of ice jams over western
New Hampshire. Rainfall amounts have been light...but dewpoints
continue to climb and do expect a significant melt in the
headwaters tonight and through Wednesday. Lesser threat exists
to the east where less rain has fallen...there are less existing
jams...and temperatures through tonight are cooler.

&&

.CLIMATE...
Record high temperatures are expected in the region today.

*** February 21 Records Temperatures***
Portland            Concord             Augusta
59F - 1953          64F - 1872           52F - 1981

***Warmest February Temperatures***
Portland            Concord             Augusta
64F - Feb 26, 1957   69F - Feb 24, 2017   60F - Feb 18, 1981

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Dense Fog Advisory until 7 AM EST this morning for MEZ007>009-
     012>014-018.
     Dense Fog Advisory until 10 AM EST this morning for MEZ019>028.
NH...Flood Watch through this evening for NHZ001>003-005-007.
     Dense Fog Advisory until 7 AM EST this morning for NHZ001-002-
     004-006-010.
     Dense Fog Advisory until 10 AM EST this morning for NHZ014.
MARINE...Small Craft Advisory until 5 AM EST Thursday for ANZ150>152-
     154.

&&

$$
SYNOPSIS...Arnott
NEAR TERM...Cempa
SHORT TERM...Cempa
LONG TERM...Arnott
AVIATION...Arnott/Cempa
MARINE...Arnott/Cempa
HYDROLOGY...
CLIMATE...



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