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Area forecast discussion
National Weather Service Gray ME
1028 PM EDT Fri Apr 28 2017

Synopsis...
a cold front will move offshore this evening. Another cold
front will push across New England on Saturday before stalling
to our south. High pressure will briefly build in from the
northwest late Saturday then shift offshore Sunday as the
stalled frontal boundary lifts north as a warm front. The warm
front will stall over the region Sunday night and Monday. Low
pressure lifting northeast out of the Great Lakes will drive an
occluded front toward the region Monday night and will cross the
region on Tuesday. Low pressure will slowly move east through
southern Canada Tuesday night through Wednesday.

&&

Near term /until 6 am Saturday morning/...
1025 PM...have added patchy for inland areas of the coastal
plain in ME and into the Kennebec/Penobscot valley areas. Tds
remain higher here, as front went thru later in the day and
failed to mix out the moisture. For the most part, drier air did
make it to the sfc in New Hampshire. May see a little drying overnight, but
not enough to preclude some fog.

710 PM...after a couple cells flared up over interior wrn ME
this afternoon as cold front moved through, including one that
flared up for about five minutes, are dissipating with the loss
of heating. Otherwise should be a mainly clear evening, except
along the mid coast near Penobscot where some fog will linger.
Mid-high clouds will move in from the west after midnight.

At 18z...a surface cold front extended from Quebec province
southward through western New Hampshire. Along and ahead of this
boundary...National Weather Service Doppler radar mosaic showed widely scattered
convection developing. Expect this activity to exit the coast by
early evening along with the boundary and associated weak
shortwave impulse. We'll see a brief lull early tonight with
patchy stratus and fog developing before a shortwave impulse
over the upper Great Lakes races eastward along the U.S. -
Canadian border. This impulse and associated cold front will
approach northern New England towards dawn accompanied by
increasing clouds and scattered showers.

&&

Short term /6 am Saturday morning through Saturday night/...
the shortwave impulse and associated cold front will race east
and exit the region by afternoon. Clouds and scattered showers
should give way to clearing skies by afternoon as the system
exits the region and drier air pours in from the northwest.
Highs will range from 60s along the international border to the
70s elsewhere. A few 80 degree readings will be possible over
far southeast New Hampshire.

The cold front will stall to our south by Saturday evening as
surface high pressure builds by to our north across eastern
Canada. The boundary will try to inch back north after midnight
as a warm front marked by increasing middle and high clouds
across southern and western sections of the forecast area.

&&

Long term /Sunday through Friday/...
high impact weather potential: the pattern supports heavy rain by
the end of next week...but still way to far out for any confidence
that this will occur. No other significant impacts expected.

Pattern: a somewhat messy five wave pattern exists across the
northern hemisphere early this afternoon with a broad longwave
trough centered across the western half of North America...where it
has been stationed throughout this week. It is the season of cutoff
lows...with two meandering over the western Atlantic with another
one upstream over western Canada. Through the long term forecast
period...there is good deterministic/ensemble agreement in a
realignment of this pattern...with the eastward progression of the
western North American trough to the eastern half of the
continent with large scale ridging building in from the west. As
we reach the end of the forecast period...a -ao/-nao signal
becomes rather robust in the gefs/ec-eps...which would anchor
this trough over the eastern states.

Pattern implications:

Precipitation likely at or above normal with some threat for heavy
amounts late in the week depending on the details of nearing longwave
trough.

Temperatures below normal to start the period under the influence of
backdoor cold front...returning to near or above normal under deep
southwesterly flow through midweek before trending back to or below
normal as longwave trough arrives to end the week.

Model trends/confidence: deterministic/ensemble guidance agreement
is rather strong through about Wednesday of next week. Beyond
this...the ejection of the last remaining western trough becomes
much less certain in the guidance with substantial model to model
and run to run disagreements. Thus...confidence falls below average
for Thursday and Friday.

The details....

Sunday: high pressure noses in from the north with high precipitable water plume
shunted south of the region with temps aloft seasonal. Active
seabreeze under light gradient with a mix of sun and clouds with
warm front nearby to the south. Highs near or just below
seasonal norms.

Sunday night - monday: deep southerly flow between Bermuda high and
eastward moving longwave trough transports deep moisture plume north
with open Gulf of Mexico. Focus for precipitation along northward
lifting warm front with precipitable waters nearing 1.25"/99th percentile.
Strongest signal for precipitation is over the northern half of the
area given best low level jet support. Greatest threat window is 06z-18z
Monday. Ensembles support 0.5-1.0" basin-wide averages over the
northern half of the forecast area with some locally higher amounts
along the southeastern slopes of the mountains likely. Temperatures
likely steady or warming overnight Sunday night in the south.
Location of the front portends large temperature differences
southwest /near 70/ to northeast /near 50/ on Monday with confidence
lower than average on high temps.

Monday night-early tuesday: cold front crosses the forecast area
with narrow precipitable water plume reaching about an inch and a third /over the
99th percentile vs climo/ but front will be fast moving with forcing
in question. Speed of the front should limit precipitation amounts
to less than one half inch basin-wide...although given deep
southerly flow...the Southern Mountain slopes would be most likely
to see heavier amounts. Temperatures relatively mild in the deep
southerly flow.

Tuesday afternoon-wednesday: gradually drying cyclonic flow behind
departing low pressure system. Precipitable waters as well as t8s +1-2 sigma on
Tuesday decrease to seasonal norms on Wednesday. Expect scattered
to numerous rain showers in the mountains Tuesday with partly/mostly
cloudy skies to the south and east...with shower coverage decreasing
through Wednesday and a similar decrease in cloud cover. GFS
significant colder than gefs mean and the ec/ec-eps and will lean
toward the European model (ecmwf) solution which suggests less of a Stark temperature
difference between the days with temperatures above normal Tuesday
given temps aloft...and closer to normal on Wednesday with some
downsloping assistance. Robust gradient suggests breezy conditions
both days.

Thursday-friday: longwave trough sharpens/deepens as it pushes east
for the end of the forecast period. Orientation of trough suggests
an increase in available moisture along the East Coast...with broad
agreement that low pressure will form downstream of this
feature...but very poor agreement on the location and evolution of
this low. There is general agreement on increasing precipitation
potential for Thursday night/Friday vs. Thursday with deep
southerly flow suggesting the potential of a heavy rainfall
threat...but by no means is this yet likely or expected. Will
also refrain from moving temperatures too far from
climatological norms with temperatures aloft near normal and a
trend for increasing clouds/precipitation.

&&

Aviation /03z Saturday through Wednesday/...
short term /through Saturday night/...lingering MVFR/IFR in
stratus and fog vicinity of krkd otw mainly VFR through tonight.
Scattered MVFR btw 12 and 16z Sat in -shra with a cold frontal
passage. Northwest sfc wind gusting to 25 kt at times behind the cold
front Sat afternoon. VFR Sat night with LCL IFR psb in valley
stratus and fog.

Long term...
VFR Sunday with MVFR/IFR restrictions possible Sunday
night/Monday especially north as warm front lifts through the
region with rain showers and stratus/fog. Cold front bring wind shift
and continued restrictions in rain showers on Monday night. Improvement
on Tuesday with residual rain showers bringing potential MVFR
restrictions away from the coast. VFR likely Wednesday.

&&

Marine...
short term /through Saturday night/...Small Craft Advisory for seas remains in
effect through early evening for swells of 4-6 ft outside the
bays. Building winds and seas on Saturday afternoon with a cold
frontal passage. We could see brief marginal Small Craft Advisory winds and seas
outside the bays late Saturday into Saturday night.

Long term...
scas likely Tuesday-Wednesday in the southwesterly flow ahead
of...and westerly flow behind a potent cold front. There is some
potential for an East Coast low pressure system by the end of
next week...but confidence in occurrence is low at this time.

&&

Fire weather...
good relative humidity recovery is expected tonight. A cold
front will move across the region Saturday morning with
scattered showers. Otherwise warm dry weather is expected both
Saturday and Sunday punctuated by good relative humidity
recovery at night.

&&

Tides/coastal flooding...
we'll see an 11.7 ft MLLW tide at Portland once again tonight
(1:32 am saturday). However, swells are expected to be lower
tonight and winds offshore. Therefore...we are not expecting
any flooding at this time and will forgo any sort of coastal
flood advisory.

&&

Gyx watches/warnings/advisories...
ME...none.
New Hampshire...none.
Marine...none.

&&

$$
Synopsis...
near term...cempa
short term...

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