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FXUS61 KGYX 080338

Area Forecast Discussion
National Weather Service Gray ME
1038 PM EST Sat Dec 7 2019

High pressure builds east and will crest across the area
tonight with light winds, mainly clear skies, and cold
temperatures. The high retreats offshore on Sunday allowing a
milder return flow to develop. By Monday, temperatures in the 
40s will be common. But along with the milder weather will come
rain as a low pressure system crosses the region Monday and 
Tuesday. Much colder air follows this system for the remainder 
of the week.


1035 PM...It's a night where forecasting temps can really hurt 
a forecaster's brain. Some sheltered areas are plummeting 
/KHIE/, some that normally do, are not, thanks to some 
wind/KIZG/, while others have gone gone back up at bit after 
falling quickly post-sunset. I've played for mostly falling 
temps thru the night but some slowing in spots. Some valleys 
across NH will likely see lows close to zero or below in many 
spots, while lows in parts of ME will likely be more in the 
single digits above zero. 

655 PM...Temps falling off quickly in sheltered areas, and have
lowered mins by several degrees in these areas. Meso models do 
want to slow the fall as cirrus moves in from the west, but 
it's a ways off to the west, so may need to lower the mins yet 
again later. Certainly it's clear, dry and not very mixy on the 
boundary lyr through most of the night, so could see good rad 
cooling thru most of the night. 

 At 18z a 1031 
millibar high was draped across the eastern Great Lakes. Upslope
flow and the downstream moisture plume off the Great 
Lakes...along with a passing impulse was producing a few cloud 
streets along with scattered light snow showers/flurries at 
moment. For tonight...the surface high will build east with 
skies becoming mainly clear even across mountain sections by 
late tonight. The fresh snowpack, clearing skies and light winds
will allow the mercury to tumble overnight. Lows tonight will 
be mainly in the single numbers with a few subzero readings 
possible across the normally colder mountain valleys.


On Sunday the surface high center will slide offshore to our
south with a sunny and cold start to the day. Then...warm air 
advection mid and high clouds will arrive across northern and 
central sections by afternoon along with the chance for a brief 
snow shower. Milder return flow should produce temperatures several
degrees warmer then today. Mostly cloudy tomorrow night...with
some light rain possible after midnight over southern New Hampshire
and southwest coastal Maine...and a light wintry mix further
inland as surface temperatures remain at or below 32F while the
column aloft warms overnight.


Main story will be a mild storm system moving thru the region
Mon into Tue. With a deep trof digging into the central CONUS 
and high pressure departing off the East Coast...deep SW flow 
will be supported thru the region. The best chances for precip 
breaking out continue to be around midday Mon...which likely 
gives us enough time to warm above freezing before it starts. 
The dry air initially may support a little frozen precip due to
evaporative cooling...but that should quickly be overwhelmed by
warming at the surface and aloft. Model forecasts move the 
system along fairly quickly and cut it far enough W to bring the
dry slot thru the area...both of which should limit overall 
QPF. Ensemble guidance has been fairly consistent around 1 
inch...focused mainly along the coastal front and another area 
in the SE upslope regions of the White Mtns. Guidance is 
forecasting anywhere from a 0.5 to 1.5 inches snow water loss 
thru the storm...due to both warm temps and rainfall assisting 
in melting. That does leave a fairly large range of 1 to 2.5 
inches of run off possible thru the confidence is 
still fairly low on impacts. Ensemble river forecasts bring a 
couple gauges above action stage...see more in the hydro section
below. Another concern will be winds Mon night as the strongest
LLJ moves overhead. The greatest concern will be if we can 
sneak the surface warm sector into parts of the forecast area 
overlapping with the LLJ. The highest probability outcome is the
warm sector delays until after the LLJ has lifted NEwd...but 
there is enough of a threat that I increased wind gusts along 
coastal Wrn ME to around 30 to 35 knots.

The cold front will be left trailing as forcing moves into
Quebec. This may keep cloudiness around for a longer period of
time for coastal zones into Tue night and even Wed. A trailing
SW/V trof will move along the boundary Wed...and may allow for
some weak amplification thru the day. To varying extents model
guidance tries to bring some precip back to the NW...but most
likely the forecast area will be too far NW to reach. I have
kept chance PoP around coastal zones...but not willing to push
it any higher than that.

The remainder of the week will be quiet under high pressure.


Short Term /Through Sunday Night/...Sct MVFR in mtn cigs and
-shsn...then VFR throughout aft 05z Sun. Areas of MVFR dvlpg 
 aft 06z Mon in -RA southern New Hampshire and southwest coastal
 Maine with -SNPL north and mountains. 

Long Term...Still expecting an extended period of IFR conditions
beginning Mon as much warmer and more moist air moves into the 
region over a cold snowpack. Widespread IFR will continue thru
Mon night with clearing from the NW Tue. MVFR CIGs will linger
in the mtns with upslope flow around HIE. A window of LLWS is
also expected Mon night as the strongest part of the LLJ moves
overhead. MVFR conditions may move back into coastal terminals
Wed as a weak wave may try an form along the stalled front. Most
likely this remains far enough offshore that VFR is expected.


Short Term /Through Sunday Night/...Decreasing winds and seas
through Sunday morning as high pressure builds overhead. Winds
and seas increase Sunday afternoon as the high retreats offshore
with SCA conditions likely outside the bays. Winds could briefly gust
to near gale outside the bays Sunday night. 

Long Term...LLJ continues to increase thru the day Mon and will
be at its strongest Mon night. A period of gales is looking more
and more likely outside the bays Mon night...with possible gale
force gusts inside the bays during the overnight as well.
Confidence was still too low for a gale watch at this time.
Whatever winds and seas do build into Tue morning will gradually
diminish thru Wed as high pressure builds in behind the
departing storm.


Mild temperatures, high dew points, and periods of rain will 
impact the area Monday and Tuesday and erode snow cover across 
the region, particularly over southern New Hampshire and 
coastal western Maine. While the fresh snow will initially 
absorb rainfall, the snow will likely reach capacity and begin
melting. This will lead to an influx of water into local 
waterways. At this time, southern New Hampshire has over a foot 
of snow in places with liquid equivalent likely exceeding 1", 
with some areas possibly closer to 2". Should moderate rainfall 
occur in areas with high snow water equivalent, instances of 
localized flooding will be possible in poor drainage areas, as
well as some main stem rivers and tributaries rising above
action stage.




NEAR TERM...Cempa/Schwibs
SHORT TERM...Legro/Schwibs

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