Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 240312

Area Forecast Discussion
National Weather Service Gray ME
1012 PM EST Sun Feb 23 2020

High pressure will shift offshore tonight. After another cool
night, increasing temperatures are expected for the start of
the week. By late Monday into Tuesday, increasing cloudiness
and very light precipitation may develop. Heavier precipitation
holds off until Wednesday night into Thursday morning with a
messy mix of rain, snow, and ice/sleet possible. Quiet and
cooler weather returns at the end of the week.


1015PM UPDATE...
Still some higher clouds over the area and this along with
higher dewpoints is keeping temperatures a bit warmer tonight.
Still expect the coldest valleys to fall well into the teens,
while the warmer towns and hills stay in the 20s. Have adjusted
the temperature forecast slightly.

A batch of cirrus is moving across the area early this evening,
although it is clear behind it. Expect just a few passing clouds
tonight with otherwise mostly clear and calm conditions once
again leading to widely variable low temperatures. Afternoon
dewpoints in the low 20s suggest lows tonight should be warmer
than last night in all areas, with the coldest valleys falling
into the low teens. Current forecast shows this fairly well so
very few adjustments were needed.

Sunny skies and southwesterly winds have allowed temperatures
to climb about 7-10 degrees above normal this afternoon. Mostly
clear skies and light winds will allow for another cool night,
with upper single digits to lower teens for the better
radiators. Much of the interior will be in the mid to upper
teens, with locations along the coast and over southern NH
bottoming out in the 20s, thanks to their warmer temperatures


Aloft, an open wave will shift offshore and towards Nova Scotia
by Monday morning. High pressure will be offshore, and this
will allow winds to increase Monday as the pressure gradient
increases between departing high pressure and approaching low
pressure. The jet stream to our north will steer low pressure to
our north through Monday. This will keep some cloud cover
across the far north initially, with sunny skies elsewhere.
Temperatures will increase Monday thanks to increasing SW
winds, with highs ranging from the mid to upper 40s north to the
50s along the coast. Southeastern NH will top out in the mid

The more concerning system will be in the central Plains and
eventually move towards New England in fast SW aloft Monday
night. PWATs/sky cover ramp up from the SE by Monday night,
keeping overnight lows relatively warm. Top down saturation and
onshore flow increases towards the early morning hours Tuesday.
This may make for a few flurries and/or sprinkles during the
morning commute.


Deep southwest flow overtakes the eastern CONUS by Tuesday,
developing a warm and moist sector ahead of long wave trough
amplifying over the Plains. At the surface, mature low pressure
over the Mississippi and Ohio Valleys is picked up by this
feature and driven NEward into the Great Lakes where it occludes.

Locally, weak high pressure erodes with increasing northeast/
onshore low-level flow. Subsidence atop saturated low levels
will lead to drizzle development over the southern coast,
eventually spreading up into the foothills where pockets of
below-freezing surface temperatures could create pockets of
freezing drizzle. Though, temperatures will continue to warm
during the afternoon which ought to mitigate significant
impacts. Models are split on producing QPF in rain (as opposed
to trace with drizzle), but with synoptic subsidence do think
light rain or rain/snow mix would be relegated to higher terrain
later in the day Tuesday when deeper moisture arrives.

The mid-levels gradually warm through Wednesday under continued
southwest flow with more widespread areas of precipitation
developing (rain over the south, snow over the mountains). Models
are again somewhat split on QPF with a weakening short wave
driving isentropic upglide, but given synoptic forcing have
included a chance of light showers through Wednesday. Some
drizzle may continue into Wednesday along the coastal plain, but
without confidence to include this detail quite this early.

Later Wednesday into the overnight, upglide and forcing
increase dramatically over New England as the trough aloft
tilts negatively into the northeast CONUS. At the same time, the
surface occlusion/ triple point reaches the Mid- Atlantic coast
and begins to amplify. The GFS has favored a slightly sooner/more
amplified solution vs the ECMWF, but then again the 12Z Canadian
came in with a much delayed yet stronger coastal system. So,
still plenty of uncertainty exists even in the depth of the
long wave trough, which is also reflected well in a high degree
of ensemble spread after about 12Z Wednesday (and the WPC model
diagnostic discussion). Following WPC logic, leaned more on
ensemble means and less on raw-GFS.

The result is a broad band of isentropic rain and snow crossing
Wednesday night into Thursday morning with extra amplification
across the south and coastal plain due to amplifying low
pressure over southern New England. A warm nose is likely to
develop over the south which may introduce mixed precipitation
at the transition between snow to the north and rain along the
coast...though temperatures aloft look to be cooler than other
systems this season. A period of strong onshore flow ahead of
the crossing low looks like it could produce wind gusts nearing
30-35 kts along the immediate coast with weak cold air damming
likely to mitigate winds gusts over land. Will trend gust forecast
toward that solution with plenty of uncertainty in system track
and coastal development yet to figure.

QPF wise, widespread amounts of 0.5-1.25" looks like a good
place to start for the event; highest totals are likely over
the S to SE-facing upslopes and southern Maine Coast with lower
amounts likely in downsloping regions of the north. Greater
coastal amplification would lead to stronger coastal FGEN and
greater rainfall so will need to pay attention to mesoscale
details as the system approaches. h850 temperatures will again
near freezing over the foothills to the coast, with the same
applying to surface temperatures, so possible precipitation
types run the gamut at this point in the forecast...will
continue to keep it simple for now with rain/snow mix, which is
actually looking like a more likely solution than recent events.
Still, the system looks similar to other events where the
mountains pick up headline-level snow with a mixture between it
and mainly rain along the coast.

A dry slot comes in behind occlusion quickly with a steep
decrease in PoPs from SW to NE starting 06-12Z. The parent low
and trough aloft meanwhile spin into eastern Canada with NW
flow aloft and CAA keeping a few snow showers going in the
mountains through the end of the work week. Troughing gradually
shifts eastward through the weekend with temperatures likely to
run a little below average


Short Term...VFR conditions expected at all terminals through
Monday under mostly clear skies. Clouds will begin to increase
at HIE/LEB/MHT/CON during the afternoon on Monday, with possible
flurries or sprinkles late Tuesday into Monday morning for
nearshore terminals.

Long Term...low clouds increase with onshore flow along the
coastal plain Tuesday into Wednesday, with gradually lowering IFR by Wednesday. Precipitation chances steadily
increase through this period with drizzle/sprinkles/flurries
Tuesday giving way to rain and snow (LEB/HIE) showers. East
winds increase late Wednesday with gusty conditions along the
coast...along with widespread IFR or worse in RA, SN, and a
wintry mix between. Restrictions gradually lift Thursday with
gusty winds turning westerly. Restrictions --shsn over higher
terrain may continue through the end of the work week.


Short Term...Seas and winds are expected to remain below SCA
criteria through noon Monday. The two northernmost outer water
zones may briefly flirt with marginal SCA conditions Monday
afternoon, but at this time did not think it was enough to
trigger an advisory.

Long Term...NE winds will steadily increase Tuesday through
Wednesday. Wednesday night, low pressure deepens over southern
New England which pulls in a period of strong onshore flow,
possibly to gale force. SWerly to westerly winds develop behind
the departing system Thursday through the end of the work week.





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