Area Forecast Discussion
Issued by NWS Portland, ME

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341
FXUS61 KGYX 170434
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1234 AM EDT Thu Aug 17 2017

.SYNOPSIS...
High pressure will continue to build into New England through
Thursday bringing dry conditions and seasonable temperatures.  On
Friday...a warm front associated with low pressure over the Great
Lakes will lift through the region followed by a cold front
Saturday. These features will bring the threat of showers
throughout the region. High pressure will build back into the
region this weekend with warming temperatures.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
midnight...Good radiational cooling is ongoing and some of the
usual suspects such as Sanford and Berlin have already fallen to
the forecast minimum. Therefore have dropped the low
temperatures slightly through valley locations. Dewpoints in the
CT river valley are slightly higher and they should fog in
before dropping into the 40s. At elevation, winds will help keep
temps up above the valley locations.

0137Z Update... Temperatures continue to rapidly
drop across the forecast area as clear skies, light winds and
low dew point values are leading to radiational cooling.
Readings will be in the 40s across portions of the mountains in
the next hour or two. Have made minor delays in the onset of
high surf advisory to the region as swells currently only remain
in the 2-4 foot range.

Prev Update...
Have updated the forecast based on current conditions across the
forecast area. Mainly clear skies continue with just a few Cu in
the mountains and over southern New Hampshire. Ci outflow from
departing hurricane well of the coast continues to push east
with time.

The drier air that has entered the region on northwesterly winds
will allow temperatures to fall rather rapidly after sunset.
Patchy late night fog will form as well with the Connecticut
River Valley fogging in initially shortly after midnight.

Prev Disc...
High Impact Weather Potential: Dangerous surf
conditions along the midcoast. See marine section for details.

Current Pattern: Early afternoon water vapor imagery reveals an
impressive vortex north of northern New England over Quebec...
gradually moving south and east with time. Brisk westerly flow
aloft is the result over much of New England...with yesterday/s
cold front well south and east of the region and satellite-based
total precipitable water products showing a significant pool of
drier air gradually sinking south into northern New England.
Drilling down to the surface...high pressure is located over the
eastern Great Lakes...making gradual progress to the east.
Thus...a dry period is foreseen through the near term forecast
period as high pressure gradually arrives from the west. Our
only forecast concerns center around valley fog
development...and the coastal impacts of waves originating from
Hurricane Gert...now well south and east of New England...but
making its closest approach before racing north and east into
the northern Atlantic.

Through this evening...Dewpoints continue to fall under robust dry
advection with cloud cover thinning as a result.  Expect the
clearing trend to continue with the dry airmass promoting a pretty
rapid temperature fall this evening with temperatures likely at or
below 60F in the mountains by 8pm...with 60s elsewhere outside of
the MHT/ASH corridor where temps will remain at or a bit above
70.

Tonight...PWATs fall to around one half inch...representing a -1-2
sigma departure from climatology.  This...and building heights will
ensure a clear and quiet night.  A modest surface pressure gradient
will remain in place behind surface low associated with mid level
low over Quebec and arriving high pressure.  1000 mb geostrophic
winds fall to 15-20kts...suggesting that only the deeper valleys
will fully decouple.  This will likely limit low temperatures in
some spots...and also reduce the amount of fog development.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/...
High Impact Weather Potential: Dangerous surf conditions along the
midcoast. See marine section for details.

Pattern Summary: Flow aloft increasingly becomes northwesterly
through Thursday as mid level low over Quebec settles south and east
into the Canadian maritimes.  Surface high crests over the region
Thursday before shortwave ridge arrives Thursday night...with warm
advection pattern developing overnight. Thus...very quiet weather
will continue through the short term forecast period. First
potential of precipitation associated with developing warm
advection regime will be at our doorstep as this forecast period
ends.

Thursday:  Surface high pressure crests overhead with dry wedge
remaining centered along the eastern edge of the forecast area.
Surface pressure gradient looks at least as impressive as today
through the morning...with wind gusts 20-25 mph again likely before
diminishing some as the high arrives in the afternoon.  Ample sun
is expected throughout the forecast area. Mixing to T8s around
+10C suggests mid 70s as the average high for much of the
area...with southern NH/southwestern ME likely to reach right
around 80...and the mountains likely closer to 70.

Thursday Night:  Surface ridge axis begins to pull east of the
region with shortwave ridge axis arriving around daybreak Friday.
Top down saturation will commence with westerly flow at H2 likely
bringing in some cirrus clouds in the evening before warm
advection pattern and continued advection brings thickening/
lowering high and then mid level cloudiness especially after
midnight. Mesoscale guidance /NAM...RGEM and SREF/ all support
the idea of shower activity making a run at southwestern NH
towards daybreak Friday...with the global guidance a bit slower
on it/s arrival. Will hold off on introducing QPF before
12Z...but there is enough model support for a chance PoP around
daybreak.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Models in fairly good agreement for the end of the week and the
weekend. Low pressure lifting northeast out of the Great lakes will
swing a warm front toward the region on Friday. Expect a mostly
cloudy day with occasional showers. Increasing onshore flow
will bring a fair amount of ocean stratus into the mix by late
in the day as the front remains south of the region. Highs on
Friday will top out in the lower to mid 70s.

Friday night will feature a fair amount of low clouds...areas of
fog and drizzle along with a few stray showers. Lows will range
from the mid 50s north to the mid 60s south.

Warm front will slowly push east through the region on Saturday.
As boundary layer winds switch into the southwest expect low
clouds to gradually shift out of New Hampshire and far western
Maine during the day. With a slow moving cold front approaching
from the west will likely see some afternoon convection in
western zones depending how much heating is achieved by early to
mid afternoon. Midcoast and interior central portions of Maine
likely remain stuck in marine layer and ocean stratus for much of
the day. Highs on Saturday will be in the lower to mid 70s in
midcoast and interior central sections of Maine. New Hampshire
and western Maine will see highs in the upper 70s to mid 80s.

Showers and some thunderstorms Saturday evening will gradually
end as the front pushes east of the region. Expect variable
clouds with areas of valley fog overnight. Lows will range from
the mid 50s north to the mid 60s south.

High pressure will build in from the west on Sunday delivering
cooler and less humid air on a brisk northwest flow. Upper trough
swinging in from the west will result in variable cloudiness
through the day with a few stray showers in the mountains. Highs
will range through the 70s to near 80.

Fair weather will continue Sunday night and Monday as high
pressure builds south of the region. High pressure will slide
offshore south of New England Monday night and Tuesday bringing
warmer and more humid air back into the northeast. A cold front
approaching from the west will bring the next chance of rain to
the region on Wednesday.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Short Term...
Summary: Patchy to areas of fog expected in the CT River Valley
tonight, especially in areas such as LEB. High pressure will
build into the region through Thursday...before moving east of
the area Thursday night. This will allow for generally good
flying conditions through the period.

Restrictions: VFR conditions will dominate the TAF period outside of
overnight fog potential HIE/LEB tonight...with somewhat lesser
potential again on Thursday night. Solid agreement from the
guidance and climatology for a period of LIFR/VLIFR fog in the
8-13Z timeframe at HIE/LEB early Thursday. Otherwise...expect
mostly clear skies to dominate through Thursday before
increasing high and mid level cloudiness arrives Thursday
night. This cloudiness...if it arrives fast enough...may help
preclude additional fog development Thursday night.

Winds: Northwest winds 10g18kts will continue through the
afternoon before diminishing to 5kts or less tonight. Northwest
winds will again strengthen to 10g18kts /strongest AUG-RKD/ for
the day on Thursday before likely going calm/light-variable
Thursday night.

LLWS: No LLWS expected through Thursday night.

Lightning:  No thunderstorms expected through Thursday night.

Long Term...IFR/LIFR ceilings developing Friday and persisting
through Friday night. Conditions gradually improve to VFR in New
Hampshire and western Maine on Saturday with areas of MVFR/IFR
ceilings in midcoast and central interior sections of Maine.
Becoming VFR with areas of Valley fog Saturday night. VFR Sunday
and Monday.

&&

.MARINE...
Short Term...
Forecast generally remains unchanged for tonight through
Thursday night as long period swell originating from Hurricane
Gert reaches the coast of Maine. However, have delayed the onset
of swells. Around a 5 ft swell with a long period of 12-15
seconds will bring high surf conditions to south facing aspects
primarily along the midcoast through downeast. Will continue
high surf advisory as a result.

A small craft advisory due to seas will continue over the adjacent
outer coastal waters for the same period. Westerly winds will
near 25 kts for a time Thursday afternoon.

Long Term...SCA`s may be needed Friday through Friday night and
again on Sunday afternoon.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...High Surf Advisory until 8 PM EDT this evening for MEZ025>027.
NH...None.
MARINE...Small Craft Advisory for hazardous seas until 8 PM EDT this
     evening for ANZ150-152.

&&

$$
SYNOPSIS...
NEAR TERM...Curtis
SHORT TERM...
LONG TERM...Sinsabaugh
AVIATION...
MARINE...



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