Portsmouth, New Hampshire 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

001
FXUS61 KGYX 301046
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
646 AM EDT Mon Jun 30 2025

.SYNOPSIS...
High pressure moves east of New England today, bringing mostly
dry conditions and a warming trend, as well as increased
humidity. Low pressure tracking through Quebec will drag a cold
front across the area Tuesday bringing chances for
thunderstorms, a few of which may be strong to severe. Outside
of some mountain showers, the middle to second half of the week
looks mostly dry and seasonably warm.

&&

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

6:45am Update... Fog burns off through mid morning, with mainly
sunny skies for today and quickly warming temps through the
morning. No notable changes with this update as the forecast
remains on track.

Previous...

High pressure moves south and east of New England through the
day, bringing an increasing southwest flow as ridging builds
across the Northeast. This brings a notable warm up to the
region today, with highs warming into the 80s to low 90s in most
spots. The warmest readings will be found across southern New
Hampshire. The coastline is likely to see the coolest highs
today with a developing sea breeze keeping highs in the 70s to
low 80s. Mainly sunny skies are expected.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Fog likely moves onshore during the evening hours tonight as
the airmass becomes more moist on a southerly flow. Valley fog
is also expected to develop overnight again across the interior.
This moist onshore flow keeps lows in the 60s tonight.

Driving this moisture increase, a cold front approaches from
the west through the overnight hours. The high res guidance is
increasingly suggesting that leftover convection from the
daytime today is likely to move into the region after midnight
tonight. Scattered showers and thunderstorms reach western areas
first after midnight, and likely reach the coast towards
daybreak on Tuesday.

Lingering showers and clouds are likely to hamper Tuesday`s
high temperatures. Originally looking like the hottest day of
the week, Tuesday is looking more and more like a warm and
sticky day with showers and storms in the morning. Following a
break midday with some sunshine, more thunderstorms are likely
to develop during the afternoon hours as the cold front moves
through. Strong to severe storms remain possible during the
afternoon hours, with gusty winds and heavy downpours the
primary concerns.

Forecast highs were scaled back a bit because of the cloud
cover, with highs forecast from the mid 70s across parts of
central and northern Maine, to the mid 80s across southern New
Hampshire. Dew points are expected to climb into the low to mid
70s during the day, making for a sticky feel and possibly
pushing heat indices into the mid 90s across southern New
Hampshire Tuesday afternoon.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Pattern Overview: The pattern remains unsettled as broad
troughing dominates at the 500 mb level, but periods of more
westerly flow aloft will introduce drier air and keep
precipitation minimal through late week despite a sharper trough
passage (or even a closed low). Deep layer ridging may get a
chance to build back in toward the end of the weekend.


Impacts and Key Messages:
* Tuesday remains a day to watch as warm moist conditions ahead
  of a cold frontal passage may lead to the development of a few
  strong to severe thunderstorms and heavy rain.

Details: Tuesday: Dewpoints increase to the upper 60s and low
70s on southwesterly flow Tuesday morning. However, 850 mb
temperatures will be decreasing, so while the building humidity
will make it feel warmer actual surface temperatures are going
to be a few degrees cooler than Monday. In fact, parts of
southeastern New Hampshire may approach Heat Advisory criteria
as apparent temperatures may feel close to the mid 90s.
Elsewhere, expect it to feel more in the upper 70s north of the
mountains and upper 80s to the south. Low pressure moving out of
the Great Lakes Region looks to drag a cold front through later
on Tuesday and with plenty of moisture and instability in
place, this will be the forcing for afternoon thunderstorms to
develop, a few of which may be strong to severe. The latest LREF
has trended a bit farther south with the higher values of
MUCAPE, but we are still left with 1000-1800 J/kg and deep layer
shear 30-35 kts. This suggests more organized convection, but
the limiting factor looks to be the lapse rates. Now that we are
in the window of the NAMNest, the 12Z run suggests poor lapse
around 5.5 to 6 C/km which may keep things sub-severe. Other
failure modes like timing of the front and cloud cover will have
to be watched as well. SPC has introduced a Day 3 Marginal
Risk. Forecast soundings also show the potential for these
storms (severe or not) to produce very efficient rainfall with
warm cloud depths 12kft+ and feeding on a very moist airmass
with PWATs around 2". Fast storm motions should limit instances
of flooding, but it can`t be ruled out completely. WPC maintains
a Day 3 Marginal Risk for excessive rainfall.

Wednesday and Thursday: Seasonable weather will return as
dewpoints come down behind the front. Wednesday looks to be
mostly dry outside of the mountains, where westerly surface flow
may help sustain a few showers, with full sun helping
temperatures climb into the 80s areawide. Skies remain clear
overnight, with temperatures dropping into the upper 50s and low
60s. Thursday could feature some scattered showers and some
rumbles of thunder as models continue to suggest a sharper
trough digging in from the Great Lakes Region along with an
advancing surface cold front. High temperatures will be similar
to Wednesday with slightly cooler low temperatures Thursday
night.

Friday-Sunday: The July 4th holiday continues to look mostly
dry, but some uncertainty remains with respect to the location
of the trough and an upper low, which the Euro, GFS, and
Canadian now all have. Being in the proximity of an upper low
may equate to more cloud cover and/or a better chance for
periods of showers. This trend will have to be watched, but for
now long range ensembles and the NBM remain on the optimistic
side so I will too.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
Short Term...Valley fog clears by mid morning, with VFR
prevailing through the day today. Valley fog likely develops
again tonight at LEB, HIE, and CON. Marine fog likely moves
onshore tonight, bringing IFR conditions to PSM, PWM, RKD, and
AUG. This marine fog likely slowly clears through mid morning on
Tuesday. Additionally, scattered showers and thunderstorms are
likely after midnight across western terminals, and reach
coastal terminals around daybreak on Tuesday. Scattered showers
and storms likely brings periodic restrictions on Tuesday, with
mainly MVFR to VFR ceilings otherwise.

Long Term...Thunderstorms look to cross the area on Tuesday
which may bring about brief restrictions to area terminals.
Terminals that see rain during the day may also see some patchy
fog develop briefly on Tuesday night. A frontal passage will
quickly bring conditions back to VFR Wedensday, with that being
the prevailing condition through Friday.

&&

.MARINE...
Short Term...High pressure moves east of the waters through the
day today. A cold front approaches from the west overnight, and
crosses the waters late Tuesday afternoon. Marginal SCA
conditions are possible across the eastern waters in
southwesterly flow tomorrow afternoon.

Long Term...A brief period of SCA winds and seas are possible
with a frontal passage Tuesday night. This front may also push
thunderstorms and heavy rain across the waters Tuesday evening.
The waters then are on the periphery of high pressure through
Friday with sub-SCA conditions expected. Winds will be generally
southwesterly, with seabreezes likely to develop each
afternoon.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.

&&

$$


NEAR TERM...Clair
SHORT TERM...Clair
LONG TERM...Baron