Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 050503
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1203 AM EST Mon Dec 5 2016

.SYNOPSIS...
High pressure crests over our region this evening before gradually
moving off to the east tonight. A weak low pressure system will
bring some light snow to the region on Monday. High pressure will
settle in for Tuesday. Low pressure may affect the region with
more light rain or snow Thursday with colder air following for
Friday and over the weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
1203 AM Update...
Have updated the grids based on current conditions and latest set
of mesoscale models. Clear skies and light winds, as well as very
low dew points have allowed temperatures to crash late this
evening across Maine and New Hampshire. Have lowered mins a few
degrees with lower to mid teens expected over the next couple
hours over the interior valleys, one of the coldest nights of the
year.

Thick CS shield over southwest New Hampshire will spread north and
east, allowing temperatures to hold relatively steady during the
late night hours. Light snow to arrive over southwest NH around
the morning commute and a statement has already been issued for
this hazard.

945 PM Update...
Some cirrus is finally reaching SW NH at this time. Otherwise
precipitation is still trailing well to the southwest. Any snow
looks to be more than a few hours away...possibly reaching NH
towards daybreak. Have not changed PoPs or weather grids tonight
since new model data may have a better handle on timing. Have
issued a statement about the slippery morning commute for southern
and western NH including Nashua and Manchester. Otherwise...
temperatures are dropping into the teens and 20s with tonight`s
sounding coming in with just 0.13 inches for PW values. Therefore
lowered temperatures farther.


650 PM Update...
Minor changes to the forecast mainly to sky cover. Stratus is
diminishing across the Connecticut River Valley and the Canadian
border with dew points in the teens and 20s. While some clouds are
already arriving from the SW ahead of the next short wave, feel
the timing of the snow showers are fine right now with RHs still
in the 50s over southern NH. When tonight`s sounding arrives will
not be surprised to still see quite a bit of dry air which will
have to be overcome before we can see measurable snow. We should
have plenty of time for temperatures to radiate towards the low
dew points mentioned above before significant cloud cover/
moisture arrives.


Previous discussion...
High pressure will crest over the region tonight before moving
off to the east late. This will allow for clear skies for at least
the first half of the night along with rapidly diminishing winds
this evening. Strato-cu in the mountains will also dissipate this
evening. This will set much of the area up for good radiational
cooling conditions through at least half the night. Many folks
will drop into the teens to lower 20s.

A quick-moving short wave trough will approach from the west later
tonight, and aid in increasing clouds from west to east starting
after midnight. Light snow should begin to develop across
southwestern NH prior to or around dawn as moisture and forcing
for ascent increases ahead of the short wave trough.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
As a short wave trough approaches from the west early Monday
morning, light snow will overspread the forecast area from west
to east. The snow should begin in much of NH during the morning
commute. While the snow will not be particularly heavy, cold
overnight temperatures will allow what falls to stick to the
roadways. This will likely make the morning commute a slippery one
mainly across southern and central NH. The snow then spreads east
and northeastward during the balance of the morning, reaching the
mid coast of Maine and Augusta area by midday.

There is multi-model consensus that the short wave trough is
strong enough and just far enough south to induce stronger
pressure falls in the Gulf of Maine Monday afternoon. This will
aid in slightly enhanced inflow into a weak frontogenetic zone
extending northwestward to the Maine coastal plain. These features
should enhance the snowfall a little bit more than what we were
previously thinking, allowing for a 1-3 inch snowfall even on the
Maine coastal plain. So while much of Maine may escape a slippery
morning commute, a slippery evening commute is likely in the
cards.

The snow will come to an end from southwest to northeast around
midday across southwestern NH, finally shutting off during the
early evening hours on the mid coast of Maine. At this time, it
looks likely that the snow will have ended for the evening commute
in much of New Hampshire.

Gradual clearing is expected monday night.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Upper air pattern remains fairly progressive across North America
as larger scale waves move relatively easily from west to east
across the continent through the week. Several weak waves will
move toward New England with the biggest one arriving on Thursday.
Colder air pours out of Canada behind it through the end of the
week. The details:

High pressure crosses the area on Tuesday with seasonably cold
temperatures expected, generally in the upper 30s to low 40s. High
pressure pushes east Tuesday night, with the best radiational
cooling across the eastern part of the forecast area. Clouds will
begin moving in from west to east during the evening in advance of
the next minor shortwave trough. The increase in cloud cover will
limit nighttime cooling over most of New Hampshire and later into
western Maine as well.

The wave that moves into the area Tuesday night is a relatively
broad but weak feature which connects a northern and southern
stream wave. The southern stream wave moves east into the Atlantic
south of New England while the northern one moves towards James
Bay. Increasingly as this feature moves east, the connection
between the two will lessen, decreasing the overall precipitation
potential as it moves east into our area. Southwest New Hampshire
will have the best chance of seeing precipitation from this, with
chances decreasing toward the east. The temperature aloft looks
cold enough for this to fall as snow, with near-surface
temperatures also complying to a large degree. Amounts are likely
to remain in the Trace to 2 inch range, arriving just in time to
slicken up the morning commute.

The next wave is expected to arrive in our area on Thursday,
though models are still not agreeing on the degree to which the
southern and northern stream waves will merge. The GFS keeps the
southern stream wave separate until it moves into Nova Scotia
Friday night and is picked up by the broader northern stream
trough. The ECMWF continues to forecast a merging to take place
over eastern New England, which would develop a more robust
surface low tracking near the Maine coastline Thursday night. The
GFS solution would favor colder temperatures (at least aloft,
downslope warming might actually mean warmer surface temperatures
on the coastal plain) and less precipitation, while the ECMWF
produces an advisory level snowfall with the track close enough to
the coast to be concerned about a rain/snow line. A compromise
approach would favor a surface low track offshore of Cape Cod with
light snowfall for southern and coastal areas. This compromise
approach was used when preparing the forecast, with both rain and
snow mentioned all the way to the coastline and all snow inland.

As this wave merges with the broader northern stream trough, it
will pull it toward the east and into our area allowing cold air
from Canada to pour in behind it. Expect progressively colder
temperatures Friday and Saturday with a northwest flow over the
region allowing for a chance of snow showers in the mountains.
Saturday night could be the coldest night of the season so far as
both major models agree that surface high pressure will be moving
across the forecast area during the overnight hours, allowing good
radiational cooling conditions within a couple weeks of the
longest night of the year. In fact, raw ECMWF suggests subzero
temperatures in the far northeast of the forecast area. Current
forecast is for widespread lows in the teens with single digits in
the typical cold spots. While this would be the coldest of the
season so far, the cold temperatures available here are not that
impressive by typical winter standards.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
Short Term...VFR conditions are expected to be the rule overnight
tonight. Snow will advance northeastward across the region Monday
morning...likely reaching MHT/CON around 12z...and PWM by 14-15z.
MVFR to IFR conditions are then expected for several hours as
light snow falls. VFR conditions are likely to redevelop across
the region starting in SW NH later Monday afternoon, then
progressing northeastward from there during the early evening
hours.

Long Term...After a VFR day on Tuesday, expect another round of
light snow moving in from the southwest Wednesday morning with
MVFR to IFR conditions expected mainly in the southwest.
Precipitation will dissipate as it moves east, so places like LEB,
CON, and MHT are the most likely to be impacted while AUG and RKD
may not see any snow. Low pressure moving through the Gulf of
Maine Thursday will lead to another chance of accumulating snow
especially toward the coast.

&&

.MARINE...
Short Term...Conditions are expected to remain below small craft
thresholds tonight through Monday. However, visibilities are
expected to be lowered in rain and snow on Monday.


Long Term...Several weak high and low pressure systems cross the
waters through midweek, with winds generally variable at less than
20KT. Low pressure moving near the Gulf of Maine on Thursday will
be the strongest of these systems, and will pull in colder air
behind it through the end of the week. This will keep a brisk west
to northwest flow over the Gulf of Maine into the weekend, likely
near advisory level though possibly briefly reaching gale force.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.

&&

$$



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