Area Forecast Discussion
Issued by NWS Portland, ME

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308
FXUS61 KGYX 161130 AAB
AFDGYX

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
630 AM EST Fri Feb 16 2018

.SYNOPSIS...
A weak wave of low pressure will move east of the region this
morning...taking early morning shower activity with it. A
secondary front will arrive later today bringing with it a few
rain and snow showers...with much colder and drier air filtering
into the region for tonight and Saturday. Low pressure will move
south and east of New England Saturday night and Sunday bringing
the potential for an accumulating snowfall...particularly to
southern New Hampshire and the coast of Maine. As we move into
next week...a warming trend is expected with temperatures moving
well above seasonal norms...especially by Tuesday and
Wednesday. There will be multiple chances for rain showers
through next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
535 AM Update: Updated PoPs to better account for where
observations and KGYX 88D returns indicate rain is reaching the
ground. Fog has begun to thin at most locations...but there
remains a few spots with locally dense fog. Otherwise...have
matched near term temperatures/dewpoints to observations which
only necessitated minor adjustments.

Previous Discussion Below...

High Impact Weather Potential:

*  Locally dense fog coming to an end through daybreak.

Pattern: Surface map early this morning reveals weak wave of low
pressure along WSW-ENE draped boundary arriving over southern New
England...with a region of rain showers making a glancing blow over
the southern half of the forecast area in a region of modest WAA and
moisture advection at the leading edge of a 50-60kt LLJ at H8.  The
moist llevel airmass has resulted in fog over the cold
ground/snowpack as dewpoints have moved above the freezing mark. The
surface map also reveals another cold frontal boundary further to
the north...draped south and west of a larger region of low pressure
over the boundary between New Brunswick and Quebec.  Arctic air is
poised behind this boundary to advance towards the region today.
Thus...primary forecast concerns center around morning
precipitation chances and the impact of the arriving secondary
front.

Through Daybreak: Showers are finally arriving from the
west...but are not terribly heavy and expect QPF to remain
rather light. Do expect this activity to help thin the fog
some...but some patches of locally dense fog are possible
through the pre-dawn hours. Temperatures continue to be highly
variable...but in general expect them to remain in the lower-mid
30s north and upper 30s through the lower 40s south through
daybreak.

Today: Weak wave of low pressure will be over the Gulf of Maine at
12Z...with southern frontal boundary sinking south away from the
forecast area.  This will allow rain showers to gradually sink south
and out of the area...clearing the coast by mid to late morning.
Robust dry advection doesn/t really arrive until late in the
day...so we will likely be stuck with a decent bit of low level
moisture into the afternoon.  This will interact with arriving
secondary front and modest low and mid level lapse rates to bring
the potential for rain/snow /transitioning to snow/ showers in the
mountains with sprinkles to the south and east.  Thus expect skies
to remain mostly cloudy with temperatures not having much success in
moving...likely remaining steady from the mountains north...with
modest warming to the south...with some low 50s possible over
southeastern NH.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
High Impact Weather Potential: Gale force wind gusts over the waters
outside of Casco Bay...otherwise none.

Pattern: A quiet period looks to be in store as short-
lived...but robust shortwave ridging moves across the region
with a brief return to a continental polar airmass. Primary
concerns through this period therefore center around
temperatures as well as gusty northwest winds on the leading
edge of high pressure Friday night.

Tonight:  Shortwave ridging builds in from the west with surface
high pressure arriving on our western doorstep by daybreak Saturday.
PWATs rapidly drop below 0.1" overnight. Expect a few mountain
snow showers to linger into the evening...but otherwise expect a
precipitation-free night. Robust northwesterly winds will be
ongoing this evening as isallobaric component aligns nicely with
H9 winds of 35 kts to produce winds of 15-25mph with primary
limiting factor being time of day. Guidance has trended slower
in the arrival of high pressure from the west...with coupled
boundary layer flow likely to continue through much of the
night...with some valleys potentially decoupling just as we
reach daybreak. Thus...there has been a similar uptick in the
guidance temperature forecast and will trend the forecast just a
tad warmer...but still a much colder night is expected...with
single digits in the mountains and temperatures in the teens
over southern NH and along the coast of Maine. As for
skies...expect full clearing given the arriving subsident regime
and aforementioned dry airmass.

Saturday:  We see a very brief respite in the active weather on
Saturday as high pressure moves overhead during the morning hours to
a location south of Nova Scotia by Saturday evening.  While skies
should be clear to start the day...expect increasing upper level
cloudiness...particularly during the afternoon as top-down
saturation commences ahead of next fast-moving northern stream
shortwave.  T9s begin warming during the afternoon as WAA regime
sets up reaching the -5 to -9C range by afternoon. While we might
not realize all of this as these values will be reached late /with
increasing clouds/ in the day...20s north to lower/middle 30s south
look reasonable.  Expect light northwesterly winds in the morning to
shift southerly but remain light through the afternoon given the
proximity of departing surface high.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Low pressure moving northeast well south of the region will
bring a light to moderate snowfall to much of the region
Saturday night into Sunday morning. Although models have varied
on the exact track and snowfall amounts, it now appears that an
advisory level snow is becoming more likely across southern and
coastal sections. The snow should move in quickly during the
late evening with the heaviest snow falling between about
midnight and 6 am. The storm departs quickly Sunday with
clearing skies expected across southern areas by midday. Clouds
will likely linger longer across the mountains with a developing northwest
flow.

High pressure will build across the region Sunday afternoon and
move offshore Sunday night. This will allow a warming southerly
flow to develop across the region starting Monday and continuing
through at least mid-week. Initially, the warmer air will
override some cooler at the surface leading to a period of
overrunning showers Monday night into Tuesday. At the same
time, a very slow-moving cold front will be moving east. At this
time, it looks Maine and New Hampshire will remain on the warm
side of the front through Wednesday night leading to some rather
mild February temperatures and a continuation of showery
weather.

The front is expected to cross the region Thursday with cooler
air slowly filtering into the region behind the front. Still,
for February standards, Thursday`s temps will be well above
normal. High pressure will follow the front for Friday.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Short Term...

Summary: Improving conditions are expected today as a cold
front departs the region to the south and east. Full
clearing...however...will not arrive until tonight as high
pressure arrives from the west and remains in control of
conditions through Saturday.

Restrictions: Variable conditions attm...with IFR-MVFR
dominating. Expect improvement to VFR all sites this morning
with perhaps the exception of HIE where MVFR CIGS may linger. A
few showers/sprinkles this afternoon could bring an isolated
restriction LEB-CON-AUG....but most likely scenario is that VFR
dominates from late morning through the day Saturday.

Winds: Winds becoming NW this morning and will strengthen to
10g18kts for the late afternoon and evening with a few gusts in
the 20-25kt range possible surrounding 00Z. Winds gradually
diminish after midnight and through Saturday morning before
shifting light southerly for the afternoon Saturday.

LLWS: No LLWS is expected through Saturday.

Long Term...

IFR to LIFR conditions developing Saturday night in snow,
especially across southern and coastal areas. Conditions
improving to VFR Sunday through much of Monday. MVFR to IFR
conditions are likely again Monday night through Wednesday in
showers with IFR conditions possible during the each night in
showers and fog.

&&

.MARINE...
Short Term...Frontal system passes through the waters this
morning with northwesterly winds developing behind this front
this afternoon and strengthening through this evening with gale
force wind gusts likely outside of Casco Bay. Winds/waves
subside late tonight and through Saturday as high pressure
moves overhead.

Long Term...SCAs are likely Saturday night as low pressure
moves south of the region. Winds will diminish and seas will
subside a bit Sunday night but SCAs will likely develop again
Monday as a southerly flow strengthens across the waters. The
southerly flow will likely continue through Wednesday night.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory from 4 PM this afternoon to 7 AM EST
     Saturday for ANZ153.
     Gale Warning from 6 PM this evening to 7 AM EST Saturday for
     ANZ150>152-154.

&&

$$
SYNOPSIS...Arnott
NEAR TERM...Arnott
SHORT TERM...Arnott
LONG TERM...Jensenius
AVIATION...Arnott/Jensenius
MARINE...Arnott/Jensenius



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