Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 140139
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
939 PM EDT Wed Sep 13 2017

.SYNOPSIS...
High pressure now over the region will give way to increasing
moisture and a weak low pressure system associated with the remnants
of Irma for Thursday and Friday.  This will bring an increase in
humidity to the region...with the threat for some showers and
possibly a thunderstorm.  High pressure will build back into the
region for the coming weekend. Temperatures will remain above normal
for September.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
---939pm Update---
Mid level moisture is only producing some scattered cloudiness.
With the boundary layer being more moist than last night, expect
radiational cooling to generate patches of fog in valley locations
again. As expected, isolated shower activity over southern New York
state has fallen apart as it approached the area. Will update to
adjust temperatures a little bit for current trends, as well as
making some small changes to cloud cover.

---620pm Update---
Another quiet night shaping up across northern New England this
evening. Watching a few showers over southern New York state that
will try to work their way northward overnight. Expect them to
fall apart before reaching southern New Hampshire though, or be
sprinkles by the time they get here. Temperatures trends in pretty
good shape as well. Still some pretty warm temperatures across the
region with Manchester being the warm spot at 80F degrees at 6pm.
Expect temperatures to fall into the 50s and 60s overnight. Will
update to make only very minor changes to temperatures in a few
locations.

/Previous Discussion.../
High Impact Weather Potential: Minimal.

Current Pattern: Skies have been almost completely clear for much of
the day as a ridge of high pressure dominates the pattern along the
eastern seaboard.  Some high cloudiness has started to encroach from
the south and west as the first signs of a change in the pattern as
the mid level circulation over the Ohio valley and remnant Irma
moisture begins to lift north and east in response to a trough
digging into the northwestern United States.  Regional radar imagery
shows some showers moving north over the Mid Atlantic region along
the leading edge of better thetae at H8. Thus the story through
tonight will be gradual top-down saturation as mid level height
falls arrive from the south and west...with forecast challenges
centered on the potential for fog redevelopment...and any shower
activity along the leading edge of deeper moisture.

Through this evening: Dry and mild with increasing high cloudiness
likely making for a nice sunset. South of the mountains...expect
temperatures to remain in the 70s for most locations except falling
into the upper 60s from the mountains north.

Tonight:  Band of showers will approach southern New Hampshire this
evening...but will weaken as it nears downstream confluence zone in
the mid levels.  Near term high resolution guidance suggests the iso-
sct shower activity will reach a LEB-LCI-SFM line before breaking
up. I don/t expect much in the way of QPF...but increasing PoPs are
warranted in these areas this evening.  Otherwise...should see
clouds increase for all areas.  Fog will likely not be as thick
given increasing clouds...but with a moister low level airmass...
patchy valley fog is again possible.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
High Impact Weather Potential: A few thunderstorms possible Thursday
afternoon.  An isolated severe storm can/t be ruled out.

Pattern Summary: Northeastward moving H5 low will weaken as it
approaches from the west on Thursday and crosses the region Thursday
night.  The primary forecast challenge through this period include
the potential for shower/thunderstorm activity associated with this
wave...then turning to fog development Thursday night in the
increasingly moist low level airmass.

Thursday: A pool of somewhat colder air H5 just ahead of the mid
level shortwave will arrive during the afternoon.  At the same
time...cold front north of the international border will settle into
far northern NH and northern portions of western ME during the
afternoon.  Broad low level convergence will be favored associated
with this feature.  Poor lapse rates will exist in the H8-7
layer...but with the cool pool aloft...are rather robust above this.
While the poor H8-7 lapse rates manifest themselves as some capping
in the afternoon...given what we/ve seen upstream today and
yesterday ...see no reason to disagree with latest high resolution
guidance /including the NCAR ensemble/ which shows isolated-
scattered shower and thunderstorm development south of the
aforementioned cold front in the afternoon.

While deep layer flow is very light...MLCAPE values approach 1000
J/kg south of the mountains which suggests that a few storms could
produce gusty winds...and locally heavy rain given slow motion.
Severe threat is low...but not zero as a result.

Temps:  T8s will warm about 1C from today /+14-15C/ and with some
filtered sun in the morning...expect temperatures to again make a
run through the upper 70s to around 80 in southern NH/southwestern
ME.

Thursday Night: Mid level wave moves overhead with a weak surface
reflection moving north and east along the stalled frontal
boundary.  All of these features are weak...and with the loss of
daytime heating expect just a few remaining showers for the
overnight.  Bigger story will likely be the development of fog given
a significantly moister low level airmass and light winds.  Will
mention fog all areas and will have to keep an eye for the potential
for this fog to become dense.  Lows will again be mild in the moist
airmass with upper 50s to upper 60s from north to south.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
High Impact Weather Potential: Minimal.

Pattern: Feature dominating water vapor imagery across the Northern
Hemisphere early this afternoon with the mid level system now
associated with the remnants of Irma that covers nearly the eastern
half of the CONUS.  This feature is separated from a rather
impressive band of westerlies across Canada with the Polar jet
centered in this location.  Changes to the pattern are in the works
however...as energy is now digging into western North America and
will carve out a trough across the western United States through the
end of this week.  This will first serve to eject the Irma remnants
north and east with heights building across the eastern half of the
country.  The initial core of western troughing ejects north and
east this weekend in response to an upstream kicker trough arriving
along the west coast.  The downstream feature will serve to flatten
the eastern ridge as we move into early next week.  However...by the
end of the forecast period next Wednesday there is strong
deterministic/ensemble support that the western trough will deepen
with building heights returning to eastern North America.  This
discussion intentionally leaves out the Jose circulation...which
we/ll discuss separately below.

Pattern Implications:  Not surprisingly...with a ridge building
across the eastern CONUS...temperatures aloft will be above normal
through the entire long term period...generally +1-2 sigma.
Statistical guidance follows this trend with highs through the
period 3-10F above seasonal norms...particularly away from the
coast.

With Irma/s remnant moisture overhead to start the long term
forecast period...PWATs will be in the +1-2 sigma range...nearing
1.5 inches.  This will likely be the period with best precipitation
potential as the ejecting mid level shortwave provides modest
support for ascent of the warm and moist airmass. Beyond
this...precipitation chances look rather minimal as PWATs settle
back to around one inch for the remainder of the long term forecast
period which is pretty close to normal for the middle of September.
In addition...deep layer ridging settles overhead providing for
large scale descent.

Forecast Confidence:  Confidence is at or above average through the
coming weekend with good deterministic/ensemble support.  Early next
week...confidence trends downward as western energy works to flatten
the eastern ridge...and then we watch the evolution of Jose off the
east coast.

Jose:  The system continues to struggle due to northerly shear early
this afternoon.  As we discussed yesterday...run to run changes in
the models have continued to be large with this system.  With the
building ridge over the eastern US...there remains little to steer
Jose through the weekend into early next week...reducing confidence
in the forecast. Of importance next week is the first wave ejecting
from the western trough and how/if it interacts with Jose.  The
status of the Jose circulation itself is in question as well.
Primary impact continues to appear to be the potential for increased
swell next week with a heightened risk of rip currents. Continue to
monitor National Hurricane Center products for the latest forecasts
and details.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
Short Term...
Summary: High pressure will gradually give way to a weak wave of low
pressure associated with the remnants of Irma.  This will bring an
increased risk for a few showers and a thunderstorm.

Restrictions: VFR through this evening.  Tonight...expect
redevelopment of valley fog most likely impacting HIE/LEB. VFR again
on Thursday with a few restrictions possible due to isolated showers
and potentially a Thunderstorm.  More widespread restrictions
possible Thursday night as increasing moisture leads to fog
development.

Winds: Outside of local southerly seabreezes this afternoon and
again Thursday afternoon...expect light winds /less than 10kts/
through the period with winds going calm/light-variable throughout
the region each night.

LLWS: No LLWS expected through Thursday night.

Lightning:  A very low risk for an isolated thunderstorm during the
afternoon hours on Thursday.

Long Term...
With increasing moisture from Irma/s remnants arriving...morning fog
Friday may be more widespread and persist as stratus through the day
bringing the threat for restrictions.  Showers and a few
thunderstorms will also be possible.  This increased moisture
remains into Saturday with gradual drying Sunday through Tuesday.
Thus...expect some fog/stratus into early Saturday before VFR
conditions become more dominant for the Sunday through Tuesday
period /with a daily exception being valley fog at HIE-LEB/.

&&

.MARINE...
Short Term...
Very quiet conditions over the waters with no flags expected
through Thursday night.

Long Term...
High pressure remains over the waters through the period with
light winds expected. There is some threat for marine
stratus/fog in the Friday-Saturday period as low level moisture
increases. Beyond this...headline free conditions are expected
through the weekend...with some building swell possible early to
mid next week depending on the evolution of Jose over the
western Atlantic.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.

&&

$$
SYNOPSIS...Arnott
NEAR TERM...Pohl/Arnott
SHORT TERM...Arnott
LONG TERM...Arnott
AVIATION...Arnott
MARINE...Arnott



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