Portsmouth, New Hampshire 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

267
FXUS61 KGYX 071420
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1020 AM EDT Wed May 7 2025

.SYNOPSIS...
A cut-off low continues northward this evening, bringing
showers and thunderstorms. Fog sticks around through the day tomorrow
along the immediate coast. Relatively warmer and drier
conditions return, but showers and even an isolated thunderstorm
are likely today, with additional shower chances Thursday. Rain
and cooler conditions return with the next low pressure system
for Friday into Saturday. Warmer and drier conditions return
with High pressure for Sunday and early next week.

&&

.NEAR TERM /UNTIL TONIGHT/...

1000 AM Update...Forecast remains on track, with fog starting to
mix out from west to east. Guidance has been showing some more
clouds likely clearing through the mid-day, so opted to
increase high temperatures a little more. The environment
destabilizes as temperatures warm. A frontal boundary develops
and moves across northern New England this afternoon.

Forecast soundings show very low freezing levels, combined with
500-1000J of CAPE. This combination of variables typically
presents a small hail threat, but upper-level troughing is
allowing for lower tropopause heights, keeping convection
shallow. Relatively calm winds through the entire sounding also
suggest low amounts of shear this afternoon, preventing storms
from organizing. Storms arrive in New Hampshire early this
aftenoon, with most shower and storm activity out of the area
this evening.


620 AM Update...Minor changes to reflect latest observations as
the forecast remains on track. Fog remains thick early this
morning, but should start to see it dissipate over the next
couple of hours as daytime mixing begins.

Previous Discussion...
Impacts and Key Messages:
* No significant weather impacts expected

At the time of this writing the last of showers are exiting the
CWA. While rates were impressive at times, they were moving
quite quickly which thankfully helped to curb the flooding
threat overnight. In the wake of the showers fog has developed
in many places. It is particularly dense along the coast and
just inland similar to last night, with locations fluctuating
between 1/4 and 1/2 mile visibilities. Will continue to monitor
this for the potential of needing a dense fog advisory if 1/4
mile visibilities become more widespread.

Latest RAP13 analysis shows the center of the upper low over
northern New York. It will continue to slide north and east,
moving over the area. As this happens, drier air will begin to
work its way into the region as flow aloft becomes
northwesterly. The result will be some breaks in the clouds that
help high temperatures climb into the upper 60s. Some locations
may even make a run for the low 70s. The exception will be the
coast, thanks to a seabreeze, and the mountains where cloud
cover may be more persistent. This sunshine will help to develop
a couple hundred joules of MLCAPE according to the latest CAMs,
and with a shortwave rotating through this afternoon, would
expect some convective shower activity to PoP up in the
afternoon. Latest hi-res models suggest PWATs over the region
are still in the 0.75-1 inch range so at the very least there
could be some downpours, but an isolated thunderstorm or two is
not out of the question.

&&

.SHORT TERM /TONIGHT THROUGH 6 PM THURSDAY/...
Impacts and Key Messages:
* No significant weather impacts expected

Tonight: Some light scattered showers may linger into the early
evening, but things will be quickly coming to an end as the
cutoff low becomes more of an open wave and departs to the east,
with a frontal boundary moving in from Canada. This
northwesterly flow behind the front may help to sustain some
isolated mountain showers with a trough slowly starting to pivot
towards the region. Otherwise, I think most of the area will be
precipitation free through night. However, it won`t necessarily
be dry as I expect fog to develop once again, especially in
areas that do get showers. The fog will keep low temperatures in
the mid-40s to low 50s.

Thursday: Northwesterly flow behind the frontal boundary will
be bringing in drier air at the surface, but southwesterly flow
aloft will bring in more moist air. For most of the area this
will equate to just more in the way of cloud cover, but there is
some suggestion that the frontal boundary gets hung up and
dosen`t quite clear the area. Therefore included some areas of
slight chance PoPs for some isolated showers in areas that see
more sun during the morning. Otherwise, the better forcing does
not arrive until later in the night. Areas south of the
mountains have the best chance of seeing the most sun, so high
temperatures likely climb into the upper 60s, with southeastern
New Hampshire making a run at the low 70s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
1040 PM Update...No significant changes noted in the latest NBM.
Another widespread soaking rain is likely Friday into Friday
night as slow moving low pressure crosses the region. The NBM
favors a warming and drying trend over the weekend into early
next week.

Overview...

The slow moving upper level low currently across the Northeast
moves into the Canadian Maritimes on Thursday. Another upper
level low deepens across the eastern Great Lakes by late
Thursday, and slowly moves through the Northeast on Friday and
Saturday. High pressure begins to build in on Sunday, and
persists into early next week with a warming trend.

Details...

By Friday, the next upper level low approaches from the west,
with a surface low likely developing and bringing another batch
of showers into the region for Friday. Cool, cloudy, and raw
conditions are expected Friday as highs remain in the 50s.
Depending on just how organized the surface low can become,
there is the potential for some heavier rainfall amounts.
Similar to this past system, this would likely be more localized
and convective in nature, so we`ll have to wait a little longer
to get into the high res model guidance window to try to pin
down these areas.

Saturday features slowly improving conditions as the low slowly
transits through New England. Scattered showers are still
likely, but more breaks of sun should allow temperatures to
reach into the 60s by Saturday afternoon.

Sunday marks the start of a warmer and drier pattern as high
pressure builds into the Northeast. Another cut off low likely
develops, but this time across the southern US, with ridging
building north of the low. This directs a warmer airmass north
of the low and into New England on a westerly flow. Temps warm
into the 60s to low 70s on Sunday, with more widespread 70s on
Monday.

At this point, Tuesday looks even warmer as highs warm into the
mid to upper 70s across much of the area. However, it`s wise to
be suspicious from this far out at this time of year as a back
door could easily keep temps cooler as high pressure moves
offshore. So we`ll continue to monitor the progression of this
warm up over the next few days.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
Short Term...Widespread LIFR conditions are ongoing this
morning and expected to persist through the early morning. The
difference today is that we will actually see some improvement
later in the morning as fog mixes out and most terminals see VFR
ceilings by late morning/early afternoon. Isolated lightning is
possible this afternoon mainly at New Hampshire terminals.
Confidence is too low to include in the TAFs at this time. Fog
returns tonight with local IFR/LIFR restrictions possible. VFR
will prevail again Thursday.

Long Term...Conditions lower to MVFR and IFR Thursday night and
Friday with periods of a rain and fog. Conditions gradually
improve on Saturday, with VFR returning by Sunday and persisting
into early next week.

&&

.MARINE...
Short Term...Wave heights and wind gusts will remain below SCA
criteria through Thursday. Areas of fog will persist through
tonight as well. Warmer temperatures today and Thursday will
allow for seabreezes to develop each day.

Long Term...A broad area of low pressure slowly approaches the
waters on Thursday and Friday, and moves east of the waters
Saturday night and Sunday. Some marginal SCA conditions are
possible during the Friday and Saturday timeframe from this
system. High pressure builds in for late Sunday and early next
week.

&&

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

&&

$$


NEAR TERM...Baron/Palmer
SHORT TERM...Baron
LONG TERM...Clair/Schroeter