Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 222245
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
645 PM EDT Sat Sep 22 2018

.SYNOPSIS...
High pressure will stay in control of the weather through
Monday. A warm front will approach from the southwest on
Tuesday and will lift into the region Tuesday night. A cold
front will gradually push in from the west Wednesday through
Wednesday night and will shift offshore on Thursday. These
fronts will result in a period of unsettled weather over the
region during the middle part of the upcoming week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Update...
Have updated the forecast based on latest satellite imagery.
High cloudiness continues to streak across northern areas. This
may effect the onset time of fog and more importantly, the
overnight lows. Still forecasting readings around 30 degrees in
the north with a freeze warning remaining in effect. However,
these min temperatures may need to be nudged upwards by a couple
degrees if the cloud pattern persists. Frost expected further
to the south across portions of Maine and New Hampshire away
from the coast.

Prev Disc...
High Impact Weather Potential: Frost/Freeze...otherwise none.

Pattern: GOES-16 visible satellite imagery shows lower clouds
clearing beneath a broad region of upper level moisture that is
only very slowly sliding south as of this hour. A significantly
cooler and drier /see 12Z GYX RAOB/ airmass has moved into the
region today with surface high pressure over the Great Lakes
building east into New England. Despite the cooler airmass
overall...surface temperatures over southern areas were just as
warm today as yesterday given the loss of yesterday/s stubborn
low level inversion. The stage is set for a significantly cooler
night with arriving high pressure bringing decreasing winds and
skies continuing to gradually clear. Primary forecast concerns
tonight center around temperatures and current frost/freeze
headlines.

Through this evening: Upper level cloudiness will continue...but dry
airmass and decoupling winds will allow temperatures to begin to
fall rapidly...nearing 40 over northern areas by 8pm with 50s
south of the mountains.

Tonight: High pressure pulling overhead from the west will ensure a
dry and wind-less night...so attention turns to the evolution of
upper level cloud cover and whether this will have any significant
impact on temperatures.  Flow aloft is westerly...so the band of
upper level moisture seen on afternoon water vapor imagery streaming
towards our area will continue to be pushed slowly south...but do
not expect it to fully clear over southern areas. Still...it is
upper level moisture and unlikely to have significant impact on
radiational cooling...with conditions otherwise being optimal.
Inherited temperatures looked good...and perhaps moved them up a
tad over southern areas...but do not have the confidence to
remove any headlines at this time. Warning vs. advisory area
looks very good and also lines up well with where we expect
skies to be clearest tonight.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
High Impact Weather Potential: Additional frost potential Sunday
night...otherwise none.

Pattern: Through the short term period...another...more expansive
area of high pressure will build into the region...ushering in
a slightly cooler and drier airmass...but nothing unusual for mid
September.  Primary feature of note is a secondary cold front ahead
of this advancing high that will weaken as it arrives over the
area Sunday evening. Primary forecast concerns center around
clouds and any precipitation chances with this front as well as
overnight temperatures Sunday night and the threat for
additional frost.

Sunday: High pressure overhead with clear skies to start the day
with robust westerly flow aloft with little/no temperature
advection. Expect the day to begin clear...with some increase in
clouds over northern areas in the afternoon as secondary polar cold
front arrives from Quebec.  This front will be encountering a dry
llevel airmass and thus models are not too bullish on any showers
reaching the forecast area. Sounding suggest some low level
instability ahead of the boundary...so an isolated evening sprinkle
or shower near the international border is not completely out of the
question.  T8s range from +3-4C north to +7-8C south.  This suggests
highs in the 60s over most locations with perhaps a few spots
reaching just above 70 over southern NH and SW Maine.

Sunday Night: The aforementioned cold front washes out overhead...
but some mid level moisture remains in what will likely
manifest itself as a layer of SCT-BKN mid clouds. High pressure
continues to build in from the north with these clouds likely
thinning overnight. Statistical guidance is insistent that
Sunday night will not be as cool as the night previous and this
appears reasonable given cloud cover and the lower tropospheric
airmass being a bit more moist than 24 hours previous. 1000 mb
geostrophic winds remain around 15kts suggesting enough gradient
for hilltop locations to see a bit of a breeze overnight while
the deeper valleys should still decouple. Overall...this means
that low temperatures in the frost range will be more localized
with bigger disparities from location to location. Will continue
with frost mention for the coldest location...but do not see it
being nearly as widespread as Saturday night.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
High pressure over Eastern Canada nosing SE over Northern New
England will result in a dry and seasonably cool start to the
work week. Models for several runs have had approaching warm
front with a strong upper level short wave along it pushing
toward the region Tuesday Afternoon and night. Looks like a good
over-running event that should yield a period of rain with some
embedded downpours. We will briefly be in the warm sector behind
this front late Tuesday Night into Wednesday before a cold front
sweeps through the region Wednesday Afternoon. Weak high
pressure tries to build over the area Thursday. The
complicating factor is that models try to develop a weak wave on
the cold front that will be just south of the area. This wave
may result in clouds and showers pushing back north as it moves
along the front. The GFS is further north than the ECMWF with
this feature. This could keep things somewhat unsettled over
southern areas. Another cold front will move through Friday
Afternoon/Friday Night...behind this front a larger area of high
pressure will push east for the weekend.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
Short Term...

Summary: High pressure builds in from the Great
Lakes tonight. On Sunday a secondary cold front arrives towards
evening with a stronger area of high pressure arriving out of
Canada Sunday night.

Restrictions: VFR throughout the period outside of valley fog
potential LEB/HIE both tonight and Sunday night. Expect fog to
be a bit a bit thicker/longer-lived tonight vs Sunday night.

Winds: Northwest winds 10g20kts continue to gradually subside and
will weaken to calm/light-variable overnight. Westerly winds 5-10kts
are expected for the day Sunday before diminishing again to less
than 5kts Sunday night.

LLWS: No LLWS is expected through Sunday night.

Lightning: No lightning expected through Sunday night.

Long Term...VFR to MVFR expected Monday. A warm front will cross
the region late Tuesday and Tuesday Night with a period of rain
and embedded downpours which should result in a period of IFR
to LIFR conditions. A Cold front will cross the area on
Wednesday which should produce a line of showers and
possibly thunderstorms which would have areas of IFR associated
with it. Weak high pressure will return MVFR conditions on
Thursday.

&&

.MARINE...
Short Term...Winds/waves are now falling below SCA levels and
will continue this way through Sunday. Behind a secondary cold
front Sunday night...northeasterly winds will briefly strengthen
with both winds/waves nearing SCA levels after midnight.

Long Term...Wind and waves heights may reach SCA conditions
Tuesday Night and Wednesday as a warm front followed by a cold
front crosses the waters.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Freeze Warning from midnight tonight to 9 AM EDT Sunday for
     MEZ007-008-012-013.
     Frost Advisory from midnight tonight to 9 AM EDT Sunday for
     MEZ009-014-018>020.
NH...Freeze Warning from midnight tonight to 9 AM EDT Sunday for
     NHZ001>004.
     Frost Advisory from midnight tonight to 9 AM EDT Sunday for
     NHZ005>007-011.
MARINE...None.

&&

$$
JC


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