Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 210338

Area Forecast Discussion
National Weather Service Gray ME
1138 PM EDT Tue Mar 20 2018

High pressure will gradually move away from the region tonight
through Tuesday. A low pressure system will pass to our south
late Wednesday and Thursday and will likely bring some
accumulating snow to at least southern and coastal areas. An
upper level low will cross over New England this weekend. A
storm may form well out to sea early next week.


Have updated the forecast based on current conditions. Minor
adjustments made to temperatures and dew points before we shift
our focus to a developing system to our south. Plenty of
moisture will be heading north, while subbingly dry air remains
in place over northern New England per 00Z GYX sounding.

Prev Disc...
As of 930 pm...current temps not falling off like a
rock the last few nights. Some thin cirrus from roughly the
White Mountains in NH to south central ME keeping temps under
control somewhat this evening. Have made some adjustments to
temps overnight across the board bringing them up a few degrees.
Still a cold night, but not quite like the past few.

A couple notes on the 18z model guidance regarding the storm
across southern areas- 18z guidance suite has not brought any
more fidelity to the snowfall potential across southern Maine
and New Hampshire. Several inches still appear possible across
the southwest coast of Maine and coastal New Hampshire. Some
things to note that the 18Z guidance across the board is very
slow to saturate the low levels and not bringing measurable QPF
until late in the day tomorrow. Model diagnostics suggest a
strong signal, most notably the 18Z NAM, that a mesoscale band
will be near/just offshore at the peak of the event. The
frontogenetic forcing/stability parameters suggest it will be
there, yet the QPF associated with such a feature is just not
there. At this point I have no strong reason to make changes to
current QPF, but the area will be very close to an axis of much
higher QPF. Will hope that the 00Z guidance will help to build
a better consensus as to whether the band will remain offshore
or will come close enough to affect our coastal areas.

Previous Discussion...
Fair weather will continue through tonight with high pressure
ridging nosing into northern New England from the north. Went
lower than guidance for lows tonight since we should be able to
radiate at the better radiators. Otherwise, expect an increase
in high clouds from south to north.


The main forecast concern tends to revolve around potential
accumulating snowfall Wed night and Thu with low pressure
developing well to our south. The 12z model suite didn`t give
much clarity to snowfall amounts so therefore confidence remains
to low to issue any watches, warnings, or advisories at this

First things first, on Wednesday, the day will be dry. We`ll see
skies become cloudy, but any precipitation will likely remain
to the south of the forecast area during the day.

The real forecast problem involves how far north strong forcing
for ascent progresses in association with developing low
pressure to our south. Various 12z models have differing
solutions on the strength and positioning of forcing
mechanisms...and therefore big differences in precip amounts and

Several hi-res models such as the ARW and NMM including to an
extent the RGEM are too far south to bring our region much snow.
The 12z ECMWF has joined that camp. Otherwise, the NAM and GFS
are more robust. When uncertainty like this this continues at
near point blank range the best course of action is to stay on
the conservative side. We used a blend for QPF and snow amounts,
but kept them well below warning thresholds. We thought about
issuing a winter storm watch for portions of southeastern NH and
York County ME but confidence is not high enough as we feel
there`s not yet a 50/50 chance in verifying widespread 6+ inches
at this time. Updated snowfall map reflects our thinking and
doesn`t really offer too much change from the previous forecast.

Temperatures should warm up pretty quickly Thursday so whatever
falls Wednesday night and Thursday morning will melt on roadways
quite quickly.


The week ends with a fairly high amplitude pattern over the
CONUS with a trough over the northeast US and a strong ridge
building through 105W over the Rocky mountains. After stalling
over the Northeast the upper level trough tries to progress near
the end of the weekend. The high amplitude nearly blocked flow
means fairly low predictability as upstream blocks inhibit the
progression and suspect the upper low will linger into the start
of next week.

While the upper low will bring an extended period of below average
temperatures with some shower activity, overall no major storm
systems are in the future through the start of next week.

Friday the upper level low will be over head. Without much flow to
drive showers only a few very isolated showers are possible in the
mountains. Skies will remain mostly cloudy with temperatures topping
out near 40 degrees to the south, above 5 degrees below average.

Saturday the trough axis will shift east slightly. This will allow
the mountains to affect the weather. Upslope showers will increase
slightly through northern New Hampshire and along the Canadian
border in western Maine, while downsloping will bring decreasing
cloud cover to the coastal plain.

Sunday features a bit of a backdoor cold front as another upper
level vort max pushes through aloft and switches surface winds
around to northeast. The result is Sunday is the colder day of the
weekend. The same upper level forcing will help to destabilize the
atmosphere with widespread snow showers, or even brief snow
squalls as the colder air moves in early Sunday morning.

The northeasterly flow sticks around into the start of next week as
another oceanic coastal low develops well off the coast of North
Carolina. Currently the forecast progression of the upper level
blocking pattern would allow this storm to pass out to sea however
this merits watching as a slowdown in the eastward progression of
the pattern, which is fairly typical in blocked scenarios, would
result in a threat for rain/snow along the coast by the middle
of next week.


Short Term /through Thursday/...VFR tonight. MVFR conditions
develop across southern NH around 00z Thursday, with light snow
developing Thu night most areas, especially coast and southern
NH, with IFR conditions possible through early Thu morning.

Long Term... Upper level low will keep a cloud deck in the region
through the weekend, but ceilings should remain largely VFR. Sunday
showers will develop likely dropping to MVFR through the


Short Term /through Thursday/...Gales have been issued for
Penobscot Bay and the ocean waters for Wed and Thu. SCA for
Casco Bay.

Long Term...Winds and seas will subside on Friday in the wake of the low
passing out to the Maritimes. On Saturday night an upper level
vort max will help to push a backdoor front through the region.
Have increased the winds a bit over the waters as the core of
the upper level low moves over early Sunday when brief wind
gusts to Gale force are possible.


The highest tides of the month will combine with an increasing
storm surge and building waves Wednesday night to allow for
beach erosion, splash-over and possibly minor coastal flooding.
Total water level tables have been adjusted for this tide due to
the increasing northeasterly winds.

Models are suggesting the possibility of a large and intense
storm to develop well out to sea next week. The forecast
suggests this storm may form a small loop and retrograde Monday
night into Tuesday. This may produce large, long period swells
next week which may lead to some beach erosion. Note, astronomical
tides are relatively low next week.


MARINE...Small Craft Advisory from 7 AM Wednesday to 4 PM EDT Thursday
     for ANZ153.
     Gale Warning from 11 AM Wednesday to 4 PM EDT Thursday for
     Gale Warning from 4 PM Wednesday to 4 PM EDT Thursday for


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