Derry, New Hampshire 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

304
FXUS61 KGYX 190039
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
839 PM EDT Thu Jun 18 2026

.WHAT HAS CHANGED...
Convection has moved off and PoPs have been lowered.

&&

.KEY MESSAGES...
1. An unseasonably strong storm will cross the region today,
with increasing chances for severe thunderstorms and localized
flash flooding this afternoon and early evening.

2. Unsettled pattern continues over the weekend into early next
week.

&&

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

Overall expectations remain largely unchanged for the remainder of
this afternoon into early this evening as we are currently watching
a squall line quickly approaching the Connective River from the
west. The environment out ahead of this line is packing plenty
of wind shear with SPC mesoanalysis having 50-60kt of effective
shear across NH and parts of far western ME and 0-1km SRH
exceeding 300 m^2/s^2. This level of speed and directional shear
is more than sufficient for severe weather in areas where there
is enough instability with the primary hazard being damaging
wind gusts along with the threat of tornadoes. The tornado
threat exists both within any discrete cells than can develop
out ahead of the line as well as within the line.

Looping back to the instability, the highest risk for severe storms
will be in NH, especially along the CT River Valley and central
and southern NH where model consensus brings in roughly 500
J/kg of surface-based CAPE. There may also even be instances of
damaging winds within cells with little to no lightning as
motion will be fast. The threat extends east into far western
ME, but SBCAPE then drops off pretty readily with eastward
extent with onshore flow in place. That said, it won`t take much
to mix down the strong winds from aloft, but it`s possible
storms become more elevated in nature as they progress through
western ME (although still enough elevated instability for
thunderstorms and torrential downpours).

Last, but certainly not least, is the threat of flash flooding,
especially in and around the White Mountains extending into parts of
the western ME mountains and foothills. These areas have seen the
most rain over the past week, and torrential downpours combined with
rain from this morning will pose a risk of flash flooding. Some area
river rises are also expected with the Pemi at Woodstock and the
Swift River at Roxbury possibly reaching minor flood stage.

A few showers may stick around through this evening behind the
primary line of convection, but drier air and subsidence moving in
behind the front keep things dry the rest of tonight, with the
exception of a few upslope rain showers in the mountains. Winds
could get a bit breezy overnight with low temps in the 50s.

The upper-level low will slowly move east toward the Canadian
Maritimes during the day Friday. Broad cyclonic flow and embedded
waves stemming from this low will provide enough lift to get showers
and a few storms going, mainly in the afternoon to early evening, as
daytime instability builds. This will be mainly in and around the
mountains, and a few storms may contain small hail.

Otherwise, it will be a breezy and partly sunny day, although there
may be periods of mostly cloudy skies with steep low-level lapse
rates and enough instability aloft. The steep lapse rates and good
mixing will bring down stronger winds from aloft resulting in a
breezy day. Based on forecast soundings, wind gusts of 25-35 mph are
likely with up to 40 mph possible on the higher end. Highs will be
mainly in the 70s to low 80s south of the mountains, except a bit
cooler in the mountains.

Winds likely remain somewhat breezy into Friday evening but are
expected to continue to ease overnight. A few showers remain
possible in the mountains with upslope flow, but whatever activity
develops during the day is expected to fade around sunset. Overnight
lows will be mostly in the 50s, except some upper 40s are possible
in the mountains.

KEY MESSAGE 2 DESCRIPTION...

One upper low will be lifting northeast away from the area Saturday
into Sunday while another upper low approaches from SE Canada late
Sunday into Monday.

Saturday will be breezy with NW winds 15 to 20 mph gusting up to 30
mph. Upslope near blocked flow will lead to plenty of clouds and
showers along and north of the mountains while shower chances
decrease south of the mountains with more in the way of sunshine.
Highs will range from the low 60s north to near 80F near the coast
with the aid of downsloping winds. The gradient will slacken over
the area Sunday allowing for light winds turning onshore in the
afternoon. A surface trough over the area combined with SB CAPE
building to 1000 J/kg will allow for diurnal showers and
thunderstorms to develop across much of the area Sunday. The lack of
shear will likely keep any storms sub-severe while steep mid level
lapse rates and low freezing levels could allow for some small hail.

Short wave ridging briefly builds in Sunday night into Monday
morning ahead of the next upper low for a drying trend. As the upper
low over SE Canada approaches another short wave will be
tracking through the Ohio Valley. Models diverge on whether the
short wave and upper low can phase. A phased solution would lead
to an organized area of low pressure bringing a soaking rain
late Monday into Tuesday morning while an unphased system would
bring more in the way of showers. The latest NBM brings PoPs
into the 60-70 percent range Monday afternoon through Monday
night, which seems reasonable given the time range. While there
will likely be some dry periods through the middle of next week
the overall pattern will support active weather with persistent
troughing near the Great Lakes through the Northeast.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Through 00Z Saturday...IFR to MVFR restrictions remain likely this
afternoon into early evening with ongoing SHRA and TEMPO TSRA that
could also produce strong winds gusts. Conditions likely improve
fairly quickly to VFR from west to east this evening and tonight
behind a cold front with the last sites to improve being RKD and
AUG. However, HIE may see a period of MVFR ceilings. Southwest
winds may also gust 20-25 kt. LLWS is also forecast through
tonight.

For Friday, primarily VFR, but scattered showers and isolated storms
could produce brief MVFR restrictions with the higher potential
will be at HIE. Otherwise westerly wind gusts 25-30 kt are
likely with some gusts possibly exceeding 35 kt.

Outlook:

Friday night: VFR expected. Winds may remain breezy early on but are
expected to ease overnight.


Saturday: Upslope clouds and showers will likely bring at least
MVFR to HIE while mainly VFR prevails elsewhere. NW winds 15 kts
will gust to around 25 kts during the morning into the afternoon
and will diminish Saturday evening.

Sunday: Mainly VFR with light winds turning onshore in the
afternoon. Late morning through afternoon showers and
thunderstorms will bring brief restrictions.

Monday-Tuesday: Low pressure may bring more widespread IFR and
rain showers.

&&

.MARINE...
Gales remain likely this afternoon and early evening over the waters
with potential for even stronger gusts in offshore-moving
thunderstorms, especially from Casco Bay southward. Conditions then
turn to SCA levels tonight and Friday as winds turn west to
southwesterly behind a cold front. Winds are expected to stay below
SCA levels Friday night, but seas may remain above 5 ft for portions
of the waters.

WNW winds will gust to 20 kts Saturday with wave heights running
3-5 feet. Winds and seas diminish Saturday night and Sunday. An
area of low pressure may develop near coastal New England Monday
bringing the potential for at least SCAs.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ150-152.
     Small Craft Advisory until noon EDT Friday for ANZ151-154.

&&

$$

DISCUSSION...Combs/Ekster/Schroeter
AVIATION...
MARINE...