Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 181437
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
937 AM EST Sat Nov 18 2017

.SYNOPSIS...
High pressure will briefly be in control of the region today as
we sit in between storm systems. Strong low pressure is forecast
to move through New York and into the St. Lawrence tonight into
Sunday. Strong southerly flow ahead of it will keep temperatures
on the mild side...which should keep the bulk of the
precipitation falling as rain. Some frozen precipitation is
possible in the higher terrain and northern valleys though at
onset. That low pressure will drive a strong cold front through
the area Sunday night. Gusty winds are expected right through
Monday...along with snow showers in and northwest of the
mountains.

&&

.NEAR TERM /TODAY/...
930AM UPDATE...
Have updated the forecast primarily to adjust onset time of
precipitation. First wave moving through upstate New York
currently is expected to bring primarily cloud cover as it
arrives in about 3 hours with precip drying up as it arrives.
Better shot at rain comes this evening. Have also tweaked high
temperatures today based on morning sounding analysis. Steep low
level inversion exists this morning, and with cloud cover
increasing as the day goes on, it may be difficult to warm out
of the mid 40s.

7AM UPDATE ...
Minor update. Have dropped temperatures a few degrees in
sheltered valleys to reflect the morning observations.

ORIGINAL DISCUSSION...
Weak high pressure is over the region early this morning, which
combined with the clear skies has resulted in temperatures in
the teens across much of the area. Low pressure over the Great
Lakes will intensify and move up the St Lawrence valley later
today into tonight. The first signs of the approaching system
are already present as a warm layer begins to nose with
observations above 2,000ft reporting temps in the mid 20s and
rising. Temperatures will continue to warm today with all but
the highest elevations above freezing by later afternoon. Clouds
will also move in throughout the day in advance of the next
system.

&&

.SHORT TERM /TONIGHT AND SUNDAY/...
As the warm frontal precipitation approaches the region this
evening it will be a race between the approaching precipitation,
radiational cooling and warm air advection to determine the
precipitation type. For most of the region from the foothills
through the coastal plain temperatures will be above freezing
and light rain will begin early in the evening and continue
through the night. The forecast is not so clear cut further
north. In the mountains, the warm air will begin to come in
aloft with the peak of the warm nose expected around 6,000ft.
This still leaves a portion of our elevated terrain below
freezing with a mix possible through about 2,000ft. In the
northern mountain valleys such as Berlin, Rangeley, and Jackman
the temperatures may struggle to reach freezing at all during
the day, which would result in a brief period of freezing
drizzle and light icing as the precipitation approaches. Without
any blocking high pressure to hold the cold air in place the
warmth is expected to take over and transition everything over
to rain by daybreak. With so much uncertainty in the
precipitation type for the north have opted to hold off on
issuing an winter headlines as the first few hours of sunshine
today may largely determine tonight`s precipitation type.

Moving into the day on Sunday the core of the system aloft
drives a cold front through the region by midday and there will
be a quick switch over to snow and temperatures drop rapidly
behind the front. The precipitation will end almost as fast as
the temperatures drop so little accumulation is expected south
of the notches. Along the Canadian border, the front will bring
upslope flow which will result in continuing snow into the
evening on Sunday.

Along with a switch to snow the arrival of the front and cold
air will bring gusty winds becoming west to northwest through
the afternoon on Sunday. Wind gusts to 40mph are expected
through much of the area Sunday afternoon.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Sun night will be a strong CAA regime across the region behind
the departing cold front. Steadily dropping temps...gusty
winds...and upslope snow showers are expected. Wind direction is
not the most favorable for significant snow totals in upslope
zones...but a couple inches are possible where WNWly flow can
be maximized. Winds should also stay gusty even
overnight...though the best chance for strong wind gusts looks
to hold off until Mon.

Mixing should increase to around H8 during the day Mon...where
ensemble forecasts have a 1 to 2 standard deviation wind speed
anomaly tracking thru Srn and central New England. Those mixing
depths may allow for some 40 kt wind gusts to mix down to the
surface...especially across Srn zones closer to the speed max. I
increased wind gusts beyond the typically wind gust factor to
account for this.

The pattern becomes more zonal thru midweek after Mon. Fast
moving flow aloft will limit precip chances...though any passing
s/wv may increase coverage of upslope snow showers.
Deterministic model guidance and ensembles favor Wed night/Thu
time period for the next trof to cross the area. There is pretty
significant model difference in the depth and impact of this
wave. The 18.00z ECMWF favors a stronger phase between Nrn and
Srn streams...and so brings larger impacts to the local area.
The CMC and GFS favor a passing front with low pressure farther
offshore. With lean towards that scenario...with higher PoP in
the high terrain.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
Short Term /through Saturday Night/... VFR conditions will erode
to MVFR from west to east by afternoon as increasing cloud cover
approach and lower in advance of the warm front. Light freezing
precipitation is possible across the north tonight before
switching over to rain. IFR conditions will continue overnight
and into Sunday. An increasing LLJ may result in wind shear for
a period of Sunday morning before the winds are able to mix to
the surface.  Sunday a cold front crosses the region with gusty
winds.

Long Term...With Wly flow dominating thru midweek...VFR
conditions will prevail in downsloping. At HIE upslope flow will
keep the clouds around for most of Mon...some local IFR is
possible in SHSN...but not likely. Surface wind gusts during the
day Mon at all terminals may gust to near 40 kts...with
widespread 30 kt gusts expected.

&&

.MARINE...
Short Term /through Saturday Night/... High pressure will move
east away from our waters as low pressure intensifies over the
Great Lakes and passes to our north. Southerly flow in warm
advection will move over the waters but is expected to remain
below advisory level. A cold front will cross the waters on
Sunday with wind gusts to 45mph expecting in cold air advection
behind the front and a Gale watch has been issued for Sunday
afternoon.

Long Term...Strong CAA continues Sun night...with
gales likely outside of the bays. Gales may continue on the
outer waters thru Mon...before gradually diminishing Mon night
into Tue. A transition to zonal...fast moving flow may keep
conditions near SCA thresholds for much of the period.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Gale Watch from Sunday morning through Sunday evening for
     ANZ150>152-154.

&&

$$

NEAR TERM UPDATE...Kimble
SHORT TERM...Curtis
LONG TERM...Legro



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