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810
FXUS61 KGYX 132030
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
330 PM EST Fri Dec 13 2019

.SYNOPSIS...
Low pressure developing along the East Coast will move north
through New England on Saturday. Expect warm conditions and rain
for most areas, though some light freezing rain is possible at
the onset tonight. Once the low moves away a cold front will
blow through and bring back colder conditions on a strong
westerly wind on Sunday. The next low pressure system tracks
near southern New England on Tuesday with a chance of wintry
weather for New Hampshire and Maine.

&&

.NEAR TERM /TONIGHT/...
** Storm will move through the region tonight and Saturday bringing
heavy rain south with mixed precip north ***

Cloud cover has steadily increased through the day as low pressure
begins to develop off the southeast coast. Pressure falls were
occuring through the Appalachians with good agreement for the
low to track northwards and up the eastern seaboard and west of
us through the Hudson and Champlain valleys.

Tonight expect light precip to develop well ahead of the low center
as a jet max moves in aloft paired with a strong theta-E ridge at
the surface, forcing moisture into our area. Isentropic lift begins
in the low levels with the result being a few hours of drizzle
or very light snow expected this afternoon before the whole
depth of the atmosphere is able to saturate.

Some light snow has been reported and temperatures remain
below freezing in areas of western New Hampshire. While the major
ASOS across SW NH are now above freezing, the dewpoint remains in
the 20s which gives a chance for some cooling and the wet bulb
remains below freezing.  Any icing will be quite light, but still
impactful and so for this reason will go ahead and keep the
Winter Weather Advisory in effect until the rain moves in later
tonight.


&&

.SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/...
Saturday the rain will become heavy at times through the
morning as the low intensifies and moves northwards. Good
agreement on the timing of the heaviest precip between 12Z and
18Z. Trends with the short term guidance has been to increase
precip amounts along the coast. Have trended this direction
bringing the midcoast to around 2" without going as high as some
of the 3"-4" high outliers. Strong frontogenesis oriented east
west as the warm front lifts northwards through the morning may
help to focus this precip along this front. If flash flooding
occurs it will likely be at the intersection of this with the
coast, which is already nicely contained within the Flash Flood
watch.

Temperatures will be warm with 50 degrees possible along the coast.
Have kept the temperatures a bit on the cold side of the model
spread, but still puts the entire area well above freezing through
the day. The rain will continue through the day on Saturday as the
low deepens to the west and we remain in the warm sector.

Overnight Saturday the cold front will push through the region.
Expect a brief changeover back to snow, possibly mixing with sleet
through the mountains, however temperatures at the surface and aloft
will drop at roughly the same time so limited sleet and ice is
expected.  Accumulations will be limited so no winter headlines are
expected to be needed for this.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Expect cold advection on Sunday behind the cold front which will
be surging eastward beneath the departing low. A tight pressure
gradient between this departing low and the approaching high
will lead to strong westerly winds. Winds at 850MB of near 50 KT
suggest the potential for strong wind gusts especially
considering the considerable cold advection through the day. In
fact, temperatures may be falling as the day goes on. This top
down cold advection leads to strong mixing within the lowest
layer of the atmosphere which will allow much of these strong
winds aloft to reach ground level. Current forecasts keep wind
gusts just a bit below Wind Advisory levels (46 mph gusts), but
would not be surprised to get a few gusts to that level. Winds
will likely continue into the night Sunday night before the
pressure gradient finally relaxes with high pressure arriving on
Monday.

The next wave tracks out of the Ohio Valley on Monday and near
southern New England on Tuesday. With our area remaining on the
northern side of this low and a cold air mass in place, expect
this wave to bring mostly snow to our area. However, there are
some indications that some warmer air may lift north into the
southern part of the area, at least aloft. This may make
precipitation type a bit more dicey, but for now will keep the
forecast tilted toward mostly snow until greater confidence
exists in any warm layer aloft. The consensus on precipitation
amounts at this point suggest up to 0.5 inch liquid equivalent
in southern New Hampshire with lesser amounts further north into
the cold air. It seems likely this will be a widespread advisory
level snow event Monday night into Tuesday.

Seasonably cold weather continues behind this wave. Another much
stronger shot of Arctic air arrives on Wednesday as a dry
clipper system scoots by to our north and opens the door for
this cold air to pour in behind it for the end of the week.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Short Term...Cloud ceilings are already beginning to decrease
to MVFR across southern New Hampshire as low pressure begins to
develop off the southeast coast. Expect patchy freezing drizzle
tonight becoming steadier rain after midnight. Fog combined with
heavy rain will keep all terminals at IFR throughout the day on
Saturday.

Long Term...Sunday will be windy but VFR at least on the coastal
plain. MVFR conditions could linger in the mountains into Sunday
night or early Monday. Wind gusts to 30 or 35 KT are likely
mainly during the day Sunday, gradually diminishing into Monday.
The next wave brings mostly snow to the region Monday night into
Tuesday with southern New Hampshire and coastal Maine most
likely affected.

&&

.MARINE...
Short Term... Increasing southeasterly flow ahead of this system
will peak at Gale force on Saturday. There will be a brief lull
on Saturday afternoon before winds once again return to stronger
Gales on Saturday night.

Long Term...Westerly gales are expected behind the front lasting
at least through Sunday night. Winds gradually diminish as high
pressure moves in on Monday.

&&

.HYDROLOGY...
QPF/Flash flooding: QPF amounts have increased through the
coastal plain with over 2" possible. With the frozen ground and
small creeks and streams already running high from the last
event expect this will result in some minor flooding through the
coastal plain. With the heaviest rainfall in the 12-18Z time
frame this would be the time for any Flash flooding if the very
high rain rates are able to remain in one spot.

Rivers: Greatest threat for river flooding is across southern
New Hampshire. Here around 2" of QPF is expected in addition to
the melting of the remaining 1-2" of SWE still on the ground
from the prior big snow event. The combination will lead to
minor flooding on several rivers including the Suncook,
Contoocook, Warner, and Piscataquog. This area remains in a
Flood Watch.

Across the White mountains the rainfall totals will be lower - 1
to 1.5 inches, which will be able to be at least partially
absorbed by the snowpack. Thus while there will be rises on the
rivers significant flooding is not expected.

&&

.TIDES/COASTAL FLOODING...
The combination of increasing onshore winds and building wave
action will coincide with relatively high astronomical tides
Saturday afternoon. The astronomical tide Saturday afternoon in
Portland is 10.6ft. At this point the addition of storm surge
look to push water levels right to the minor flooding
threshold, with building wave action likely to cause minor
splashover and beach erosion in vulnerable areas.

Another aspect of this tide cycle will be that most of the rain will
have already fallen by the time of high tide around noon. The timing
of the high tide will cause runoff to drain more slowly from the
marshes and tidal waterways, which could be enough to cause some
minor flooding issues on roads and drainage areas within the tidal
zone around the time of high tide.

Winds will turn offshore and waves will be subsiding in time for the
Sunday high tide cycle, and coastal flooding issues are not expected
at this time.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Flood Watch from 1 AM EST Saturday through Saturday evening
     for MEZ018>028.
     Coastal Flood Advisory from 11 AM to 3 PM EST Saturday for
     MEZ023-024.
NH...Winter Weather Advisory until 10 PM EST this evening for
     NHZ001>009-011-015.
     Flood Watch from 1 AM EST Saturday through Saturday evening
     for NHZ008>010-012>015.
     Coastal Flood Advisory from 11 AM to 3 PM EST Saturday for
     NHZ014.
MARINE...Gale Warning from 7 AM Saturday to 6 AM EST Monday for
     ANZ150>154.

&&

$$

NEAR TERM...Curtis
SHORT TERM...Curtis
LONG TERM...Kimble
HYDROLOGY...Curtis
COASTAL..CLAIR/TUBBS