Area Forecast Discussion
Issued by NWS Portland, ME-- Remove Highlighting --
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FXUS61 KGYX 230125
Area Forecast Discussion
National Weather Service Gray ME
925 PM EDT Thu Sep 22 2016
Mild conditions will continue through tonight south of a strong
frontal boundary draped north of the region. This boundary will
move south as a cold front tonight and Friday...spreading some
showers across the region. Canadian high pressure will arrive
Friday night Saturday...remaining in control of our weather
through the weekend and into early next week. This will bring a
much cooler airmass to the region...with a frost possible for
northern areas both Saturday and Sunday nights.
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
-- Changed Discussion --925 PM Update...Have adjusted timing of PoPs a little bit
overnight as well as sky cover. Nice night out there for the most
part as skies are still mostly clear through some cirrus, even in
a large portion of the north. Clouds will eventually fill in from
N to S overnight with rain showers arriving in the mountains
around or after midnight.
615 PM Update...Little change to the going forecast. Have adjusted
PoP timing and little bit for tonight and backed off on cloud
cover a little bit for this evening. Otherwise, forecast remains
High Impact Weather Potential: Minimal.
Current Pattern: Early afternoon water vapor imagery showing a
fairly healthy shortwave over western Ontario sharpening somewhat as
it digs south and east. Ahead of this feature is a region of brisk
zonal flow atop a surface frontal zone that extends from the Great
Lakes eastward into far northern Maine. Closer to home...south of
this boundary temperatures aloft and at the surface are well above
normal...with T8s +1-2 sigma yielding temperatures in the 80s over
my southern zones. Changes are in the works...however...as the
aforementioned shortwave interacts with the downstream frontal
system...with a wave of low pressure forming along it and pushing
east. This weak low will move just north of the forecast area
overnight...with primary forecast concerns centered on precipitation
potential just ahead of the surface boundary as it slowly edges
Through Early Evening: Very little weather to speak of through this
evening...with some SCT/BKN mid level cloudiness in WAA regime south
of the quasi-stationary surface boundary. Very warm temperatures
will continue into the evening with 60s and lower 70s the rule even
through 8pm. No precipitation expected through at least 7-8pm.
Tonight: With developing wave of low pressure upstream...
southwesterly jet at H8 strengthens overhead this evening...likely
keeping frontal boundary north of the forecast area through at least
the first half of the night. Can/t rule out a shower in the
moistening area of warm advection over far northern areas...but
expect that we/ll have to wait until after midnight for
appreciable shower activity to arrive. Once the weak wave of low
pressure passes east of our longitude /around 6Z/ ...the boundary
will be able to sink south...with height falls ahead of the
upstream shortwave spreading south and east coincident with the
right entrance region of a 120kt jet streak on the 1.5 PVU
surface. This should help bring showers further south through
daybreak Friday...but still likely confined to the northern half
of the forecast area. With only modest dynamics...and residing
south of the surface low...expect QPF values to remain relatively
light...generally less than a quarter inch...perhaps a bit higher
over extreme northwestern ME and Coos county, NH. Lows
temperatures should remain mild...with modest gradient south of
the front as well as increasing cloud cover. Thus...expect most
sites to remain in the upper 50s.
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.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
High Impact Weather Potential: Very low risk of a rumble of
thunder far southern NH Friday afternoon. Elevated rip current
risk over coastal areas as long period swell builds.
Pattern: The period starts with shortwave north of the region
making its closest approach...with flow through the column turning
increasingly northwesterly through the short term forecast period.
This will drive the cold front and it/s associated moisture plume
south and out of the region by Friday evening...with a change to a
cooler/drier than normal airmass Friday night. Forecast concerns
center around just how much precipitation will be experienced along
the front Friday...turning to just how chilly it will be
/particularly in northern areas/ as the cool/dry airmass arrives
Friday: Cold front over the northern zones to start the day will
take much of the day to press south. Am not too excited about
precipitation potential as right entrance region jet forcing exits
along with surface low...with weakening llevel convergence along the
southward sinking front. Thus...expect a line of morning showers to
become more scattered in nature as it pushes south...and thus QPF
amounts will /unfortunately/ lessen as you head to our most drought-
stricken areas. Do expect a mostly cloudy to cloudy day for just
about everyone...with some clearing arriving over northern areas
towards evening behind the departing front. With the front
bisecting the area for much of the day...expect a pretty large range
in temperatures. Southern NH could again make a run at 80 with T8s
near what they are today...but with a good bit less in the way of
sun. Behind the boundary with robust CAA developing...areas from
the mountains north will likely not reach much above the lower 60s.
QPF: Total QPF through this rain event looks to reach the quarter
to half inch range north of the mountains...with amounts likely
closer to one tenth of an inch to the south of this.
Thunder: Can/t completely rule out a rumble of thunder with any
showers that survive into far southern New Hampshire during the
afternoon as MUCAPE plume of 200-400 J/kg is present /better values
to the south of the CWA/ Have included a slight chance of thunder
in this area.
Friday Night: Surface front will be south of the region to start
the period Friday evening...sinking south overnight as northwesterly
flow aloft strengthens...with surface high pressure building into
the region. Fairly robust cold and dry advection is expected
through the overnight with PWATs falling below seasonal norms /-1 to
-2 sigma/ after midnight. Certainly a dry period...although some
lingering cloud cover is possible beneath developing subsidence
inversion. 1000 mb geostrophic wind remains 15-20kts overnight...so
most locations will likely remain coupled. This should keep
temperatures and fog from getting too out of hand...but some patchy
river fog in the CT valley is certainly possible. Consensus lows
weighted more toward MOS appear appropriate...with some upper 30s
likely over the far north...and upper 40s to around 50 along the
coast and in southern NH.
.LONG TERM /SATURDAY THROUGH THURSDAY/...
Models are in disagreement over the long term period. Canadian and
GFS in one camp and ECMWF in another. All models do agree that
high pressure will be in control Saturday through Monday. Cold
high pressure at that with a chance of frost in some areas Sunday
morning and again Monday morning. The models diverge for Monday
and the remainder of the week. The GFS closes off an upper level
system well to our west and never does drive a cold front through
the region as high pressure aloft keeps the system at bay. The
ECMWF brings a cold front through the region Monday night with
about a third of an inch of precipitation. For the now will
introduce a low chance POP over the forecast area Monday night
and will continue low chance POP through Wednesday.
.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
Summary: A cold front will slowly settle through the region tonight
and on Friday bringing deteriorating flying conditions. High
pressure and drier air will arrive from the north Friday night with
Restrictions: VFR at the moment all sites. As the front arrives
from the north tonight...expect deteriorating conditions
/some IFR fog possible LEB ahead of the front overnight/...with
shras possible HIE/AUG/LEB/RKD by daybreak Friday...spreading south
during the day. This will likely bring a period of MVFR
restrictions as early as late tonight HIE...spreading to LEB/AUG
through early Friday. Confidence decreases further south...where
restrictions may be more brief as shower activity weakens.
Improvement to VFR is likely all sites Friday night...with
additional restrictions possible HIE/LEB in fog development.
Winds: Southerly winds less than 10kts today /with sea breezes
developing at all coastal sites/. Winds will go light/variable
tonight before gradually shifting northwesterly and increasing to
12g18kts for the day on Friday before decreasing to less than 10kts
from the northwest Friday night.
LLWS: West/southwesterly winds ahead of the front will strengthen
tonight...with 1-2kft winds increasing to around 30kts overnight
with a period of LLWS. No LLWS beyond this.
Thunder: Very slight chance of a thunderstorm Friday afternoon over
far southern NH /MHT/...otherwise no thunder is expected.
Long Term...Expect VFR Saturday through Monday. Except chance
MVFR/IFR in patchy valley fog Saturday night and again Sunday
night at HIE and LEB. MVFR is possible in Showers Monday night
Short Term /through Friday Night/...
Winds/waves look to remain below SCA levels through Friday...with
strengthening north/northeasterly winds Friday night potentially
bringing seas to marginal SCA levels.
Long Term...SCA criteria winds and seas are possible Saturday
into Sunday behind the exiting cold front.
NEAR TERM UPDATE...Ekster