Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 212248
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
648 PM EDT Fri Sep 21 2018

.SYNOPSIS...
A strong cold front will cross the region tonight accompanied by
gusty winds and a few showers and thunderstorms. High pressure
and drier air follows for Saturday. A secondary cold front will
push south through the region Sunday. High pressure and even
cooler air will build over the region Sunday night and Monday.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Update...
Have update the forecast based on current radar imagery and
surface observations. Showers and thunderstorms remain well to
the west of the region, mainly over the Saint Lawrence River
Valley this evening. Have lowered pops for the next couple
hours, and for several more hours over southern locations. Have
also lowered the chance for any thunder over southern areas this
evening.

Minor adjustments were made to temperatures with a non-diurnal
curve expected this evening. Wind advisories will remain in
place as the gradient continues to tighten.

Prev Disc...
At 18z...a 984 millibar low was vicinity of the
Ontario- Quebec border. A warm front extended southeastward into
northern New England with a trailing cold front through the
central Great Lakes. NWS Doppler Radar mosaic showed a band of
convection along and to the north of the warm front...namely
across our far northern and eastern zones. This activity should
exit to our east next few hours. Thereafter...mostly cloudy in
the warm sector with widely scattered convection and a
strengthening southerly flow ahead of the cold front. Short
range model suites show a broken line of weakening convection
crossing the area overnight with the front and 60 knot low level
jet. We`ll see gusty winds along and ahead of the front with
the strongest winds confined to high terrain...and along the
Maine mid coast where a wind advisory remains posted as we
should see some 50 mph gusts with the cold frontal passage
overnight. The convection ends from west to east in the predawn
hours as the front sweeps offshore and drier air pours into the
area on a strong post- frontal northwest flow.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Fair and seasonable Saturday as high pressure build east into
the area. Highs will be mainly in the 60s with some upper 50s
near the international border. Mostly clear and cool Saturday
night. The light winds and clear skies allow temperatures to
drop into the 40s...with 30s across the mountains along with
scattered frost and patchy late night valley fog.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
High Impact Weather Potential:

*Frost/Freeze Possible in northern areas Sunday and Monday nights.

*Some potential for heavy rainfall in the Tuesday-Wednesday period.

--Pattern and Implications--

Pattern across North America early this afternoon reveals trough
centered over central Canada...with spokes of energy moving rapidly
from west to east south of this feature near the US/Canadian border.
This general flavor to the pattern will not change through the
long term forecast period with current trough near Hudson Bay
moving east through the weekend but being pretty quickly
replaced by a deeper trough by the middle of next week. The
primary change next week is for anomalous ridging to develop off
the west coast of North America....helping lock in a ridge-
trough-ridge setup across the continent. Flow within this
amplified pattern continues to be robust through the period with
numerous shortwaves rotating through the longwave trough axis.

What does this mean for us? Given the initial trough ejecting to
our north this weekend...a turn to dry weather and seasonable
temperatures is expected before the re-developing central North
America trough forces downstream heights to build with this
favoring temperatures moving back above seasonal norms. Primary
precipitation chances surround the period of warm advection when
the more warm/moist airmass works back into the region in the
late Tuesday-Wednesday timeframe.

--Daily Details--

Sunday - Monday Night: Impressive area of high pressure /MSLP +2-3
sigma over climatology/ over Ontario strengthens as it builds south
and east driving a seasonably dry airmass /PWATs 0 to -1 sigma/ into
the region on northwesterly flow.  A cold front will lead this area
of high pressure...but guidance is consistent on the demise of this
feature as it pushes into confluent flow along with a dry llevel
airmass.  Only impact from this front will likely be a few
additional clouds in the mountains Sunday evening and perhaps an
isolated shower...otherwise expect partly-mostly clear
conditions through the period with primary concerns centered
around the potential for frost both Sunday and Monday nights
/see below/.

Tuesday - Wednesday: Mid level ridge pulls east of our longitude
Tuesday morning with warm advection overspreading the region.
Precipitation looks most likely in the period from roughly noon
Tuesday through noon Wednesday with some wiggle room on either end
given uncertainty.  Temperatures will warm through the two day
period.

Thursday - Friday:  Pattern remains progressive...so while high
pressure looks to build in Wednesday night and Thursday...the next
system rotating around the central North American trough may very
well arrive before the end of the forecast period.  Fast flow
suggests some uncertainty in timing...so will end the period with
chance PoPs.  Temperatures look to remain at or above normal given
only a brief shot of WNW flow behind departing Wednesday system
before warm air advection again redevelops.

--Hazard Assessment--

Frost/freeze potential Sunday & Monday nights: Dry airmass with
Canadian high pressure suggests frost/freeze potential from a
pattern recognition standpoint. This is in agreement with recent
MEX/experimental ECMWF MOS numbers and a look at the 12Z MET
and MAV. Potential concerns would be the frontal boundary
ushering in the cooler airmass on Sunday may leave some
cloudiness around Sunday night. By Monday night...southerly
winds will begin to strengthen aloft in the developing warm
advection regime...and may keep many locations mixed. For these
reasons...don/t have confidence to go any lower than MOS
consensus but will stick close to it.

QPF/Thunder Potential Tuesday-Wednesday: An early look at the
deterministic/ensemble guidance through this period indicates very
limited CAPE ahead of cold frontal passage on Wednesday.  Perhaps of
more concern is the fact that the cold front will slow as it arrives
/as it becomes increasingly parallel to the mid/upper level flow/
     with some guidance runs suggesting that another weak wave of low
pressure may ride along with through Wednesday night.  Ensemble QPF
probabilities do paint a decent signal at this range for 1" but very
little support for 2"+ despite some support for this from
deterministic runs /e.g. 21.12Z GFS/. Moisture anomalies are
not tremendous...but we/ll have to keep an eye on this period as
southern NH continues to have river levels running well above
normal given recent rainfall/flooding.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Short Term /Through Saturday Night...VFR with areas of MVFR
in cigs, shra/tsra. Sfc wind gusting to 30 kt then diminishing
following a cold frontal passage early Saturday. Bcmg VFR
throughout on Sat morning. Lcl IFR psb btw 09 - 12z Sun in
valley stratus and fog.

Long Term...
VFR Sunday through Monday outside of some morning valley fog at
LEB/HIE.  Conditions deteriorating on Tuesday with MVFR restrictions
in showers possible particularly by afternoon.

&&

.MARINE...
Short Term /through Saturday night/...South to southwest winds
increase ahead of an approaching strong cold front with Gale
force winds expected to develop for all waters except Casco Bay.
Winds shift to northwest behind the front late tonight with
gusty winds. Winds and seas will gradually subside behind the
front Saturday as high pres builds across the waters.

Long Term...
Northeasterly winds behind a secondary cold front on Sunday
night will bring winds over the outer waters to near SCA levels.
Next round of widespread flags likely arrives Tuesday night
into Wednesday as southerly flow strengthens ahead of an
approaching cold front.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Wind Advisory until 7 AM EDT Saturday for MEZ025>028.
NH...None.
MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ153.
     Gale Warning until 8 AM EDT Saturday for ANZ150>152-154.

&&

$$
JC



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