Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 260245 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Gray ME
1045 PM EDT Sat Sep 25 2021

The cold front will remain stalled to our east into tonight, and
will bring some scattered showers to areas along the coast East
of Portland. Another round of moderate to heavy rain is possible
early Sunday into Sunday after noon mainly across SW and
central Maine. Cooler, more seasonal are expected Monday through
the middle of the week. There is a chance for scattered showers
Monday and Tuesday as an upper low sits and spins jut to our


1035 PM Update...The going forecast remains largely unchanged at
this hour as latest observational data and model information
continues to line up with forecast PoPs and QPF overnight. A SFC
trough oriented on and near the coastal plain continues to be
the dividing line between a tropical air mass to the east and a
drier air mass to the west. As a long wave trough with embedded
short waves impinge on the forecast area tonight we are
continuing to expect the development of showers and isolated
thunderstorms on the coastal plain northward into portions of
central Maine after midnight. There is still uncertainty where
exactly the max QPF axis will be, as it is expected to be quite
narrow. It`s even possible that the heaviest rain stays just
offshore or over the islands. However, current model consensus
and meteorological observation trends support a general S-N
swath of heavy showers with 1-3" possible in sub-swaths from
roughly Portland to Augusta to Skowhegan. Heaviest precipitation
expected 3 hours either side of 12z. Localized flooding will be
possible in this area.

650 PM Update...Have updated the forecast mainly to tighten up
forecast PoP gradient for the overnight hours and shift the
highest PoPs a smidge to the east following the latest hi-res
guidance. This lowers PoPs significant in places like southern
and central NH overnight, except for the immediate Seacoast.


A front remains draped across the eastern edge of the forecast area,
with coastal York county, Cumberland, Androscoggin, Kennebec,
and the midcoast remaining in a warm moist airmass. Dewpoints
are in the 60s here with cloud cover. Meanwhile a completely
different world across NH where skies are clear and dewpoints
are in near 50. This dichotomy will continue through the next
day or so as the front stalls, and even retrogrades slightly.

For the eastern areas, radar and satellite imagery show a band of
showers developing along and just east of the surface boundary and
these showers will move north and fill in bringing rain to Penobscot
bay and points just west. While the individual cells are small they
will continue to move along the frontal surface and this training
presents a small threat for flash flooding through evening.

Now for something completely different; west of the front will
remain dry with clear skies to start off overnight. These clear
skies will allow temperatures to drop into the upper 40s with
widespread fog once again forming in the river valleys.


Sunday a new player arrives to join our existing boundary. A
short wave over the Great Lakes will approach from the west, and
this interaction will pull the moisture back further west,
backing right up into the White mountains. The CT river valley
will likely still remain on the `dry side` but overall expect
coverage of showers to increase due to this interaction. Have
kept the highest amounts from Portland east, where up to 2
inches is possible. However expect the drop off to nearly
nothing to be quite abrupt and most of the area will see less
than a quarter inch. This tight gradient is well represented in
the HREF and HRRR forecasts and have leaned on those for the
placement of the boundary.

Most of the rainfall will move through early Sunday morning,
with the front starting to push out by mid day. By Sunday night
the westerly flow aloft will finally kick out our lingering
surface boundary and make way for the drier air to move in
across the CWA. With so much moisture in place it`ll take more
than a single evening to dry out and thus fog is likely across
the region on Sunday night.


Latest models showing a little less amplifies 500 MB pattern
across N Hemisphere for next week, but still with some
significant blocking in place, and dominant Atlantic ridging.
Still, downstream blocking over the N Atlantic and Rex block to
our N extending to E of Hudson Bay will allow for 500 MB low to
close off just to out N, and stay in the vicinity of the CWA
through the week. Models have been fairly consitent with the
forecast for the first half of the week, but change from run to
run as to how that closed low interacts with energy coming in
from the NW, and upstream and downstream blocking, so confidence
beyond Tue begins dropping.

Mon/Tue will see that 500 MB close off to out N, allowing cooler
air to work in, along with some clouds, especially in the mtns,
and maybe a chance showers Monday afternoon/evening and Tuesday
evening. Monday will see a sfc cold front work through, so this
will be the better chance for showers across the nrn 2/3 of the
CWA. Highs Monday range from 65-70 in the N to the low to mid
70s in the S. On Monday the cold front will have settle S of the
mtns, but will likely still be crossing srn zones, so lows in
the N will be in the mid 40s, but in the mid to upper 50s in the
S. The from stalls a bit near S NH and the Me coast, and there
may be a few showers here on Tuesday, and more in the way of
clouds, with highs 60-65 in the N, and in the upper 60s to
around 70 in the S.

For Wed/Thu and maybe into Fri, will be dealing with the closed
low as it shifts S across the region, with cooler temps /maybe
even a hair below normal for highs/, and the possibility of
bouts of showers, but timing where and when at this point is
really a game of chance. We`ll call it unsettled, but not
expecting any day would be a washout. Depending on where that
closed low ends up could see ridging developing over the top of
it and a warming trend for next weekend.


Short Term...The tale of two airmasses is most noticeable in
the aviation impacts. The eastern portion of the CWA remains
shrouded by clouds, as some locations struggle to even exceed
MVFR ceilings. Meanwhile the western part of the CWA is clear
with VFR conditions. Overnight will see flight category
decreases across the area, but for opposing reasons. In the
east, the clouds will slightly to mainly MVFR with scattered
showers and even a rumble of thunder in heavy rain briefly
resulting in IFR. Meanwhile across the western portion of the
area, clear skies will give way to IFR and valley fog overnight.

Long Term...Mainly VFR in the extended from Monday through Thu.
Will see some MVFR cigs in the mtns at times along with some
showers. These are possible in the south as well es[especially
Monday night.


Short Term...Seas remain below 5ft currently with the front draped across
the eastern portion of the waters. However, on the outer waters,
we expect 5 footers to arrive after midnight, and gusts around
25 kt in CAA Sunday morning. Therefore, a SCA has been issued
for the outer waters, albeit a low-end one.

Long Term...Winds/seas stay mostly below SCA levels through


MARINE...Small Craft Advisory from 4 AM Sunday to midnight EDT Sunday
     night for ANZ150-152-154.



NEAR TERM...Ekster
LONG TERM...Cempa is the U.S. government's official web portal to all federal, state and local government web resources and services.