Londonderry, New Hampshire 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

237
FXUS61 KGYX 231030
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
630 AM EDT Mon Jun 23 2025

.SYNOPSIS...
Dangerous heat and humidity build early this week before
temperatures cool back down towards seasonable levels mid to
late week. Unsettled weather expected at the end of the week and
through the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

630AM Update...The going forecast is on track with only minor
tweaks needed to temps and dewpoints.

Previous...

Surface high pressure is currently overhead, and with light
winds patchy valley fog has developed in some areas but
shouldn`t last beyond 7-8AM. The high will then become centered
south of the region today while the already strong ridge over
the eastern half of the US will further strengthen and expand
toward New England with heights continuing to rise across the
region, bringing a very hot and humid day for most.

850 mb temperatures support highs reaching the 90s across much
of NH and then upper 80s to low 90s across portions of the
western ME mountains and foothills. The combination of these
temperatures and high dewpoints will produce dangerous heat with
headlines remaining in effect and unchanged from the previous
forecast. However, even those not under any heat product could
still see heat indices in the low to mid 90s.

An area where it will be less hot will be generally along and
to the south and east of the I-95 corridor as the weak pressure
gradient will allow the seabreeze to move inland pretty quickly
from late this morning and through the afternoon. Just inland
from the coast could still see heat indices in the upper 80s to
lower 90s.

Skies will be partly-mostly sunny with passing cirrus associated
with convection north of the Great Lakes, and there should be
enough low-level moisture for a cumulus field. The HRRR was
trying to generate a couple of showers later this afternoon or
evening, but with the suppression of the ridge and very dry air
aloft I don`t have any precip forecast.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Cumulus that develops this afternoon will quickly dissipate by
sunset with mostly clear skies overnight. The exception may be
central Somerset county where there may be a shower or storm,
but it may stay just to the north. There won`t be much in the
way of relief from the heat with little southerly flow and high
dewpoints allowing temperatures to only bottom out in the 60s
for much of the area, possibly low 70s in some spots. Similar to
this morning, patchy valley fog may develop in some areas.
There`s also hints at marine fog or stratus lurking near the
coast in some model guidance, but sometimes model guidance can
be too quick in bringing it in...so we`ll see if any shows up on
satellite later today.

Any fog/low stratus that develops overnight will quickly dissipate
early Tuesday morning. The very strong upper ridge remains
fully in place across the East Coast with another day of
dangerous heat and humidity across New England but this time
over a wider area than today. This is due to W/WNW winds
increasing ahead of a frontal boundary approaching from the
north, which will prevent or at the very least delay the
seabreeze. So those that see relief from the seabreeze today
will not get it on Tuesday. 850mb temps of +19C to +22C support
temperatures in the low 90s to upper 90s across much of the
area, with the exception of the far north maybe staying the
upper 80s. Dewpoints are expected to be in the upper 60s to
lower 70s, and with these temperatures, it will feel oppressive
out as heat index values are expected to be above 100F for most
areas south of the mountains and over 105F in some areas. I have
not made any changes to heat headlines with this forecast as
heat indices in the Extreme Heat Watch are right on the cusp of
being a Heat Advisory or Extreme Heat Warning. I`ll let the day
shift take another look at things today as there is potential of
mixing drier air aloft down to the surface lowering
dewpoints/heat indies. Regardless of the eventual product, it
will be a very hot day.

We could see a few showers and storms in the afternoon and early
evening as the front sags southward, but the very dry air aloft is
expected to keep coverage very low and could even be too much to
overcome for anything to develop. However, if something does
develop, the dry air aloft and sufficient DCAPE could make for
strong to marginally severe downburst winds a threat.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
*Unsettled and cooler weather towards the end of the week,
 continuing through the weekend.

Wednesday and onwards:
The ridge boosting our temperatures shifts west as an upper-
level low over the Canadian Maritimes moves southward. This
shift should allow for temperatures to sharply drop off through
the remainder of the week, with upper-80s likely on Wednesday,
and low 70s on tap for the end of the week. The cooler
temperatures will arrive with unsettled weather on Friday,
continuing through the weekend.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
Short Term...Patchy fog, mainly in the valleys, will be around early
this morning, but it`s not expected to last beyond 12Z. VFR the
rest of the day. Patchy fog or low stratus may develop
overnight into early Tuesday with the higher chances at HIE-AUG-
RKD. Mainly VFR Tuesday with a cold front approaching from the
north late in the day, which may produce a couple of showers and
storms.

Long Term...Mainly VFR expected through Wednesday as mostly
clear skies continue. Lower restrictions are possible on
Thursday and Friday as unsettled weather may move through the
area.

&&

.MARINE...
Short Term...High pressure becomes centered south of the waters
bringing a southerly flow across the waters today. A weak
disturbance passes well to the north this evening, and increasing
southerly winds may bring a brief period of gusts approaching 25 kt.
Have opted to not issue an SCA as duration would be very short.
Conditions remain below SCA levels Tuesday with southwesterly winds,
and a cold front approaches late in the day from the north. Marine
fog and stratus may be present later today and tonight, mainly over
the eastern waters and Penobscot Bay.

Long Term...Winds become variable at 7-12kts through late week.
Seas will be 2-4ft through the remainder of the week.

&&

.CLIMATE...
Anomalously high pressure will allow for dangerous heat and
humidity to build into the region. This heat could potentially
be record breaking for today (6/23) and Tuesday (6/24). Below
are the records for our climate sites

  Site             Record June 23rd    Record June 24th

Portland Jetport    91 (1983)              93 (1976)
Augusta             89 (1999)              93 (1963)
Concord             94 (2020)              95 (1980)

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Heat Advisory from 11 AM this morning to 8 PM EDT this evening
     for MEZ012-033.
     Extreme Heat Watch from Tuesday morning through Tuesday
     evening for MEZ012>014-018>028-033.
NH...Heat Advisory from 11 AM this morning to 8 PM EDT this evening
     for NHZ002>006-009-010-013.
     Extreme Heat Watch from Tuesday morning through Tuesday
     evening for NHZ004>006-009-010-013-014.
     Extreme Heat Warning from 11 AM this morning to 8 PM EDT
     Tuesday for NHZ007-008-011-012-015.
MARINE...None.

&&

$$


NEAR TERM...Combs
SHORT TERM...Combs
LONG TERM...Palmer