Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 180515
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1215 AM EST Wed Jan 18 2017

.SYNOPSIS...
Low pressure will move east from the Great Lakes overnight, then
redevelop off the mid Atlantic coast by early Wednesday. This
system will bring widespread snow, with highest amounts in the
south and some mixed precipitation along the coast. Another weak
front will pass through the area early Thursday. This will be
mainly dry with just clouds across the region and a few upslope
snow showers along the Canadian border. High pressure will build
in for the remainder of the week, with above normal temperatures
expected into Monday. A storm system may affect the region Tuesday
and Wednesday.

&&

.NEAR TERM /UNTIL 9 AM THIS MORNING/...
midnight update... Adjusted snowfall to reflect current reports.
Southern NH has seen around 2-3 inches with western Merrimack
around 4. Further east the snow has yet to move in and so that
results in a slightly lower total than the last update.

Pressure falls continue to be around 3mb just southeast of long
island as the low develops. Best dynamics will be for the next few
hours as the low intensifies. The low will move east a bit too
rapidly for the northern portion of the CWA and I expect snow
totals may be lower across that area. With no snow observations
and the strongest period of precipitation yet to develop have
held off on any major changes.

920 PM...Precipitation shield continues to spread north and east
across the forecast area at moment ahead of the 1009 millibar low
over Lake Ontario. Three hour pressure falls are centered
southeast of New York City and I expect the secondary low to
develop in this area over the few hours. Current forecast is in
decent shape and for this ESTF update I ingested the current
mesonet and made minor adjustments to sky grids to reflect
satellite trend.


Prev Disc...
Overall, not much change to the previous forecast. 12z forecast
models and latest ensemble data are in pretty good agreement with
forcing for ascent and subsequent QPF and dendritic growth. The
result was a high-end advisory to low-end warning snow event
starting this evening, with the heaviest snowfall later this
evening into the early overnight hours. Most areas will fall into
the 4 to 7 inch range with locally higher amounts possible in the
warning area. Snowfall rates this evening may exceed 1" per hour,
particularly across southern and central NH as a band of strong
waa/frontogenetical forcing moves through...with the resultant
omega co-located with the dendritic growth zone. This signal
weakens as it enters Maine, and therefore snowfall rates are
expected to be less intense there, with final totals lower as
well.

In addition, the boundary layer will be close to or above
freezing at the onset of the precipitation in many coastal and
central/southern zones. This may allow the precipitation to begin
as rain and possibly even some sleet in some areas. However, as
dynamics take over, a change to snow is imminent an hour or two
after onset. The will be a few hours either side of midnight
before tapering off late. Overall, the period of heavy snowfall is
expected to be relatively brief.

The main uncertainty in snow amounts is along the seacoast of NH
and the immediate coast of Maine where an onshore flow will
prolong any mixing with rainfall. However, with in-situ cold air
damming developing overnight, snow may be the predominant type
even there. Therefore, advisories remain in effect. Some
uncertainty also exists in amounts close to the MA border as some
sleet may mix in which could keep snow amounts down some.

&&

.SHORT TERM /9 AM THIS MORNING THROUGH 6 PM THURSDAY/...
The heaviest snowfall should be over with by 6 am Wednesday. Much
of the day will feature cool, cloudy northeast flow with
occasional light snow. The mid levels dry out later in the morning
and afternoon. However, with the low level remaining moist, some
light freezing drizzle and drizzle is possible. A short wave
trough will once again approach Wednesday night and may once again
bring a period of light snow or snow showers to the region,
especially north.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Fairly quiet weather is expected through most of the long
term period with unsettled weather early next week.

The overall weather pattern will be changing very slowly during
the long term period. The main features will be weak northwest
flow as an upper level trough remains just east of the
region...with several areas of high pressure dropping
southeastward from central Canada. At the same time, an area of
low pressure over the central U.S. will try to move northeastward
toward the surface ridge. As a result, any moisture from the low
will be slow to move into the area with the possibility of mixed
precipitation early next week.

More specifically, a trough will pass through the region
early Thursday and could bring some snow showers to the region
Thursday morning. The trough will move east during the day
Thursday allowing some partly cloudy skies across southern areas
Thursday afternoon. In the mountains, cloudy skies and scattered
snow showers will persist into the evening. As the trough moves
slowly east Friday through Sunday, a ridge of high pressure will
build southeastward from central Canada.

As the high moves off the coast late Monday and Tuesday, a
southeast flow will start to develop across the area ahead of the
approaching area of low pressure. This will allow temps to warm
aloft...but models continue to show some surface cold air damming
across the region. At this time, it looks like we`ll see mixed
precipitation across the region Monday night into Tuesday...with
precipitation gradually changing to rain during the day Tuesday
across southern areas. However, there remains much uncertainty in
the progression of these systems, so there is low confidence in
the details of the forecast for early next week.

Models are generally in agreement on overall pattern,
although details vary a bit.

&&

.AVIATION /05Z WEDNESDAY THROUGH SUNDAY/...
Short Term...VFR conditions will deteriorate to MVFR then IFR in
many areas this evening and overnight in snow. Precipitation
intensity will lighten after 09z tonight but low clouds, light
snow, and some drizzle should keep conditions predominantly IFR
through Wed and possibly Wed night.

Long Term...MVFR conditions expected Thursday morning improving
to VFR late in the day except in the mountains where mvfr
conditions may persist into Friday. Mainly VFR conditions
expected through the weekend.

&&

.MARINE...
Short Term...SCA in effect as storm system moves to our south
tonight through Wednesday and allows for building seas and
stiffening northeasterly flow.

Long Term...SCA criteria (for seas) possible Thursday. Winds
and seas should be below SCA criteria Thursday night through
Sunday. Increasing southeasterly flow will allow seas to build
early next week.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Winter Storm Warning until 5 PM EST this afternoon for MEZ012-
     018>020.
     Winter Weather Advisory until 5 PM EST this afternoon for
     MEZ013-021-023>026.
NH...Winter Storm Warning until 5 PM EST this afternoon for NHZ004-
     006-008>010-015.
     Winter Weather Advisory until 5 PM EST this afternoon for
     NHZ002-003-005-007-011>014.
MARINE...Small Craft Advisory from 7 AM this morning to midnight EST
     tonight for ANZ153.
     Small Craft Advisory until midnight EST tonight for ANZ150>152-
     154.

&&

$$
SYNOPSIS...
NEAR TERM...Curtis
SHORT TERM...Ekster
LONG TERM...Jensenius
AVIATION...Ekster/Jensenius
MARINE...Ekster/Jensenius



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