Londonderry, New Hampshire 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

853
FXUS61 KGYX 300000
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
800 PM EDT Mon Jun 29 2026

.WHAT HAS CHANGED...
Minor update to the Aviation section for the 00Z TAFs.

&&

.KEY MESSAGES...
1. Heat builds into the region by Wednesday and continues
through at least Friday. Saturday is also trending hotter as
well. High dew points combine with the heat to bring hazardous
conditions from Wednesday through at least Friday. To be
completed ahead of Wednesday`s heat, all AC installations and
preparations should be completed by Tuesday.

2. Showers and thunderstorms will be possible most days through
the holiday weekend. Given the ample heat and humidity any of
these thunderstorms days could also see strong to severe storms.

&&

.DISCUSSION...
KEY MESSAGE 1 DESCRIPTION...

Temperatures climb into the upper 80s to low 90s back from the
coastline on Tuesday. With dew points still in the low 60s, heat
indices remain close to the actual air temperatures, and remain
below advisory levels.

The significant heat arrives on Wednesday, at least for New
Hampshire and western Maine. Highs climb into the mid to upper
90s here, with mid 80s to low 90s elsewhere. Wednesday likely
still features a sea breeze along the immediate coastline,
helping to keep highs in the 80s along the Maine coast. Dew
points climb into the low to mid 70s, pushing heat indices into
the 105 to 110 degree range for much of New Hampshire and
western Maine. Dew points likely drop a little more than the NBM
is suggesting through the Merrimack River Valley with afternoon
mixing, so heat indices have been tapered down by a few degrees
from the NBM. However, this would still be warning level heat.
We are holding with the watch for now with uncertainty around
possible convection, which is discussed in more detail in Key
Message 2.

Thursday continues to look to be the peak of the heat. With
850mb temps around 22C, high temperatures either side of 100
degrees looks likely for much of the interior, and especially
for southern New Hampshire. There remains some question as to
whether the coast will see a sea breeze develop of not, but this
will also be influenced by the possible convection. Dew points
look to push into the low to mid 70s again, with any rain
Wednesday night only helping to push dew points higher. This
pushes heat indices to near 110 degrees anywhere the air temps
reach the upper 90s to low 100s.

The story remains much the same for Friday, with temperatures
likely within a couple degrees of Thursday`s highs. Widespread
upper 90s to near 100 degrees looks likely again. Dew points may
start to come down slightly, but at this point still look to
run in the upper 60s to low 70s.

Saturday increasing looks hot, with heat likely continuing for
at least New Hampshire. Model trends over the last couple of
days have trended toward the ridge holding on for at least one
more day. With this still being five days out, and daily
convection playing a compounding roll over time on the airmass
evolution, it`s still too early to say how hot it will be and
where for the 4th of July. However, it`s likely a good idea to
plan for another day of highs in the mid to upper 90s across
much of the interior.

Sunday looks likely to offer some relief from the heat, but
doesn`t look to be a major cool down at this point. Right now
80s to low 90s looks most likely, but will depend on how the
preceding days evolve.

Nighttime low temperature forecasts will be tricky Tuesday
night through Friday night. Without storms, lows only making it
down into the mid to upper 70s is likely across southern and
central locations. However, with any thunderstorms temperatures
would quickly drop into the upper 60s to low 70s. So the low
temperature forecast tries to split the difference between these
two outcomes, but acknowledges that there will be variability
across the area. Without storms, lows around 80 degrees are
likely through the Merrimack River Valley Wednesday and Thursday
nights.

The temperatures discussed each day are for outside of any
convection or thunderstorms. There remains uncertainty on the
extent and timing of any storms, with any daytime convection
limiting the heat potential. This is discussed further in Key
Message 2.



KEY MESSAGE 2 DESCRIPTION...

It will be a difficult convective forecast thru the extended.
The long story short is that given the high temps and climbing
dewpoints, CAPE will not be a problem. Any storms will have
plenty of fuel. The 29.12z GFS forecast also shows a good
example of how shear is rarely an issue around these parts too.
Thru its forecast for the next week bulk shear remains at least
25 kt, which is enough to keep storms semi-organized. Looking at
guidance from various sources, including the ECMWF EFI, the
CAPE/shear combo remains seasonably strong into the weekend. So
we have the shear, the instability, and the moisture. The
remaining question is lift - we will need a trigger to produce
thunderstorms.

There are a few potential options for triggering convection
thru the period. To get more widespread thunderstorms and severe
weather would be to get a synoptic shortwave to swing thru the
region. But model guidance does not really want to lower heights
until Fri at the earliest. That means any height falls aloft
will have to come from convectively induced shortwaves. Those
will be difficult to predict with much more than 24 hour lead
time. Finally local influences could also trigger storms,
including differential heating the mtns and along any sea breeze
circulation. So my general thought is that most days at least
isolated severe thunderstorms will be possible, and if a well
defined convective shortwave is approaching or the late week
height falls materialize the threat could become more widespread
in coverage. Looking at forecast soundings and taking
conceptual models into account, the primary hazard from any
storms will be first damaging wind. Large hail is possible but
given how warm it will be that threat is somewhat lower than
usual. Likewise without a strong synoptic influence the tornado
threat is lower than usual. And as always any storm will be
capable of torrential rain and frequent lightning.

At least during the work week there will be the threat for both
afternoon and nocturnal convection. CAM guidance continues to
hint at ridge rolling MCSs approaching and decaying thru the
region Tue and Wed. Thru the week that threat for storms should
gradually shift to the south.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
Through Wednesday... Mainly VFR through Tuesday. Winds turn
southwest tonight and that should keep marine fog at bay for the
next 24 hours anyway. Some guidance does hint at fog for LEB
and HIE tonight, but given it did not happen in any widespread
fashion last night I have held off from including in the TAF.
Showers and storms become possible late Tuesday through Tuesday
night. These could bring localized restrictions. Increasing low
level moisture will also bring potential restrictions from low
cigs and/or fog. Mainly VFR Wednesday outside of isolated
thunderstorms.

Outlook:

Wednesday Night: Conditions may begin to deteriorate as marine
fog/stratus becomes more likely. Confidence is low at this time
in MVFR or lower conditions. Local MVFR or lower also possible
in showers/thunderstorms.

Thursday - Saturday: Local MVFR or lower possible in
showers/thunderstorms. Areas of IFR or lower possible at night
with marine fog/stratus.

Saturday Night - Sunday: Some clearing possible from the north
as the heat and humidity gets pushed south.

&&

.MARINE...
Winds and seas expected to remain below SCA thresholds thru the
holiday weekend. The main hazards to watch for will be
developing marine fog, especially as winds become onshore ahead
of any low pressure approaching, and then thunderstorms which
will be possible most days into the weekend.

&&

.CLIMATE...
High temperature records at long term climate sites...

 July     1st       2nd       3rd

 AUG   92 (1971) 93 (1963)  94 (2002)

 PWM   93 (1971) 98 (1941)  95 (2002)

 CON   99 (1913) 98 (1966) 102 (1966)

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Extreme Heat Watch from Wednesday afternoon through Friday
     evening for MEZ012-013-018>021-023>026-033.
NH...Extreme Heat Watch from Wednesday afternoon through Friday
     evening for NHZ004>015.
MARINE...None.

&&

$$

DISCUSSION...Clair/Legro
AVIATION...Schroeter
MARINE...Legro