Charlestown, New Hampshire Weather Forecast Discussion

506
FXUS61 KGYX 211856
AFDGYX
Area Forecast Discussion
National Weather Service Gray ME
256 PM EDT Sun Sep 21 2025
.SYNOPSIS...
Surface high pressure will remain in control Monday as it
gradually shifts into the Gulf of Maine. A warming trend will
continue through Tuesday, with an increasing chance for showers
and isolated thunderstorms ahead of a cold front on Wednesday.
More seasonable conditions return for the rest of the week.
&&
.NEAR TERM /THROUGH TONIGHT/...
High pressure has moved into the Gulf of Maine today, and will
continue to influence anticyclonic flow over the forecast area
tonight into Monday. The addition of some background winds will
keep some of the lower levels more mixed tonight, thus
radiational cooling won`t be as prevalent. Thus expect a more
mild night, but not by much.
Some uncertainty on development of a stratus deck overnight and
along with it, fog. Only a handful of HREF members support this,
but its enough to introduce fog to areas outside of just valley
spots overnight. This mainly encompasses the coastal plain, and
later into southern NH. This will be dependent on the
onshore/southerly flow advecting enough low level moisture after
sunset. With a faint cumulus field traversing the area this
afternoon, this may be a good tracer of that occuring. However
need to overcome entrenched dry air, and surface.
&&
.SHORT TERM /MONDAY THROUGH MONDAY NIGHT/...
Southerly flow continues Monday with RH values recovering
further from a dry weekend. Clouds increase as do temperatures
as warm air advection intensifies on a steady breeze.
Unsettled conditions will be advancing from the west, and will
bring the chance for showers to the mountains Monday evening.
Did delay these given the broad forcing and present dry air.
Forecast QPF is light, and would only be looking at a couple
spots seeing a tenth of an inch of rain by early Tuesday
morning.
&&
.LONG TERM /TUESDAY THROUGH SUNDAY/...
A rather broad upper level trough will be setting up across the
Great Lakes region, with an elongated axis settling across NY
Monday Night into Tuesday. This will make for a summer-like day
as our local area of surface high pressure slides offshore,
opening the door to warm air advection. This will usher in
dewpoints into the upper 50s to low 60s and highs into 70s to
low 80s. Meanwhile, an elongated cold front associated with the
upper trough to our west will shift SE during the day Tuesday
and Tuesday night, though models continue to exhibit a fair
amount of variance on the timing of this boundary. Looking at
widely scattered showers and even a few thunderstorms to develop
as some instability drifts east by Tuesday afternoon,
increasing in coverage through the evening in association with
the shortwave. Past impulses have fizzled due to antecedent dry
conditions over the area, but this shortwave will be interacting
with increasing moisture from the gulf with PWATs climbing
upwards of 1.5"! This is a favorable trend for widespread
measurable rainfall chances, with about a 50% chance of
receiving 0.25" for much of the area between Tue/Wed with that
first shortwave.
Hopefully we can capitalize on this rainfall because the
forecast uncertainty increases thereafter. Once the main part of
the trough moves eastward into the Canadian maritimes, phasing
differs greatly in the model solutions. Current forecast favors
a blend which is more climatology, highs in the 70s and lows in
the 50s. Some 30 pops possible Saturday with another frontal
passage, but there is strong divergence with this solution so
low chance on amounts even if hit/miss shower activity occurs.
&&
.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Short Term...Generally VFR with just cirrus this evening. Medium
confidence on the development of a stratus deck inland this
evening and overnight. Should this deck form, build down fog
develops as the night continues, affecting terminals from RKD
and AUG into southern NH. VCFG possible for PWM and PSM. If a
deck fails to develop, fog will be more restricted to area
valleys, dissipating shortly after sunrise Monday. Some stratus
may linger towards the Midcoast Monday, otherwise VFR.
Long Term...VFR most of the time. Scattered showers and a few
tstms (Tue) could bring lower CIGs/VSBYs however.
&&
.MARINE...
Short Term...Southerly flow continues over the waters as high
pressure lingers on/over the coast. The general east movement of
the high will promote this wind direction through Monday. Waves
gradually build and increase Monday night, with waves 4 to 6 ft
possible.
Long Term...Small craft conditions are likely with a low
pressure system moving along a frontal boundary late Tuesday
into early Wednesday, with seas building to 5 feet. Seas will
recede later in the forecast, but uncertainty in the development
of additional embedded shortwaves thereafter.
&&
.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.
&&
$$
SYNOPSIS...Jamison
NEAR TERM...Cornwell
SHORT TERM...Cornwell
LONG TERM...Jamison
AVIATION...Jamison

NWS GYX Office Area Forecast Discussion