Alton Bay, New Hampshire Weather Forecast Discussion

486
FXUS61 KGYX 171729
AFDGYX
Area Forecast Discussion
National Weather Service Gray ME
129 PM EDT Wed Sep 17 2025
.SYNOPSIS...
High pressure remains in place across New England through
Thursday with seasonably warm and dry conditions. Low pressure
passes well south of New England tonight. A cold front moves
through Thursday night, with dry and breezy conditions behind
the front on Friday. High pressure follows for the weekend,
bringing a fresh cool and dry airmass to New England. Mainly dry
conditions continue for at least the next week.
&&
.NEAR TERM /THROUGH TONIGHT/...
High pressure remains across New England overnight, as low
pressure slowly tracks off the Mid Atlantic coast and south of
New England. With the system`s closet pass comes more cloud
cover overnight, as well as the chance for some scattered
showers across the Seacoast and along the Maine coastline. These
would bring little in the way of rainfall, and quickly
dissipate as they move northward. More breaks in the clouds
allow for radiational cooling and fog across the northern
valleys again tonight. Some patchy fog is also possible along
the coast again tonight as moisture continues to increase on a
light easterly flow.
&&
.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
Increasing sunshine is expected through the day on Thursday as
the low pressure center moves out to sea. A cold front
approaches from the northwest through the day, helping to bring
warmer air in on a southwesterly flow. Along with the sunshine,
highs warm into the upper 70s to low 80s across the region for
the warmest day of the week. The MidCoast will be a bit cooler
with a SSW wind flow off the water.
The front then crosses through during the overnight hours on
Thursday, reaching the coastline by daybreak Friday morning.
Nothing more than some scattered showers are expected to reach
the Canadian border as the front moves through. These quickly
dissipate through the mountains, with a dry frontal passage
expected south of the mountains. Cooler and drier air arrives
into the mountains through the overnight as temps fall into the
40s, with 50s generally expected south of the mountains.
&&
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Pattern Overview: Broad troughing develops at the 500mb level
as a potent upper low moves into the Hudson Bay Region, but with
high pressure building in at the surface this will only act to
advect a cold dry airmass into the Northeast through the first
part of the weekend. Shortwave ridging then builds in for the
latter part of the weekend, and the beginning of next work week,
bringing warmer temperatures. A disturbance may bring our next
chance of precipitation midweek, but uncertainty is high.
Impacts and Key Messages:
* The first widespread frost of the season is possible Friday
  and Saturday night, as well as a freeze for northern zones.
* Continued dry weather will likely lead to worsening drought
  conditions and elevated fire weather concerns.
Details: Friday: A potent upper level low moves into Hudson Bay
Friday bringing broad troughing to the Northeast. At the same
time high pressure builds back in behind a departing cold front.
The upper troughing will act to advect this cold dry airmass
into the region bringing cooler daytime temperatures (in the low
to mid-70s south of the mountains and in the low to mid-60s to
the north). The pressure gradient remains tight during the day
Friday, so post frontal winds will be breezy, and with falling
dewpoints relative humidity values will also be lower leading to
elevated fire weather concerns (see the Fire Weather section
below for more details). Clear skies and calming winds look to
lead to an excellent night of radiational cooling. This airmass
is going to be of arctic origin so it looks likely that we will
see the 850mb 0C isotherm creep into northern New Hampshire and
northwestern Maine overnight Friday. This may lead to the first
widespread frost of the season as temperatures in the low to
mid-30s extend down into the foothills. This may also be the
first freeze for northern zones as well as the cooler guidance
brings temperatures as low as 30F into Coos county and the
Western Maine Mountains. The coastal plain may stay in the upper
30s and low 40s depending on if a breeze sticks around.
Saturday and Sunday: Northwesterly flow continues with high
pressure continuing to build in to the region so expect daytime
temperatures to be even cooler on Saturday with highs only
recovering into the mid to upper 60s south of the mountains, and
in the upper 50s to low 60s to the north, after a very cold
night. Dewpoints continue to fall as dry air continues to work
into the region as well, making for even lower relative humidity
values and continued elevated fire weather concerns (see the
Fire Weather section below for more details). It looks to also
be another very cold night as high pressure becomes centered
over the region overnight. Temperatures will be similar, but
frost and freeze potential become a little more widespread as
the freezing mark pushes toward the foothills and upper 30s push
to the immediate coast. Sunday begins a warming trend as
shortwave ridging builds in and high pressure becomes centered
over the Gulf of Maine. High temperatures look to climb into the
low to mid-70s areawide, with noticeably warmer lows Sunday
night (low to mid-40s for much of the area, with upper 40s in
the coastal plain).
Monday-Wednesday: Shortwave ridging, and thusly the warming
trend, lasts into midweek with high temperatures back into the
low 80s by Tuesday. Models are beginning to hint at a
disturbance around the midweek timeframe that may bring some
precipitation to the region, but as expected at this time range,
they all have wildly different depictions of it and the
ensemble spread on QPF is large. Due to the high uncertainty
have left the low chance PoPs from the NBM in the forecast. The
main takeaway is once again going to have to be that little to
no rain through the extended forecast period is going to lead to
worsening drought conditions.
&&
.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
Short Term...Valley fog brings LIFR conditions again tonight at
HIE and LEB. Patchy valley fog is possible at times elsewhere,
and is more likely at CON, AUG, and RKD. VFR returns on
Thursday. Some valley fog is possible late Thursday night at HIE
and LEB, but is less likely than the last few nights.
Long Term...Other than the potential for fog at night. VFR
should be the prevailing condition through the forecast period
as high pressure dominates the weather pattern. Friday may see
northwesterly gusts 15-20kts.
&&
.MARINE...
Short Term...Conditions remain below SCA levels as high
pressure remains across the waters through Thursday. A cold
front crosses the waters late Thursday night and early Friday.
Long Term...Wind gusts and seas are expected to remain below
SCA criteria through midweek as high pressure moves over the
Gulf of Maine. Winds start North-northwesterly on Friday, but
shift around to southerly on Sunday. Gusts around 20kts are
possible Friday night as a front crosses the waters during the
day.
&&
.FIRE WEATHER...
Post frontal northwesterly winds look to be gusting around 15
to 20mph with gusts up to 25mph possible at times on Friday.
This combined with relative humidity values dropping into the 35
to 40 percent range will make for elevated fire weather
concerns. Saturday will be less breezy, but much drier as an
arctic originating airmass builds into the region. Relative
humidity values are expected to drop into the 25-30 percent
range which, despite light winds, will continue elevated fire
weather concerns, especially if fuels are readily available.
&&
.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.
&&
$$
NEAR/SHORT TERM...Clair
LONG TERM/FIRE WEATHER...Baron

NWS GYX Office Area Forecast Discussion