836
FXUS61 KBOX 221909
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
309 PM EDT Wed May 22 2024

.SYNOPSIS...

A high pressure remains just southeast of New England, setting the
stage for summerlike warmth through Thursday away from the immediate
south coast. Mostly dry weather is expected today, then an
approaching cold front will bring a round of showers and
thunderstorms Thursday afternoon through at least the evening. A few
strong to severe storms are possible. Mainly dry on Friday with it
remaining mild. Unsettled through this weekend with hit or miss
showers and thunderstorms. Turning more seasonable early next week
with a better opportunity for widespread rains.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...

Hot temperatures and clear skies (save for some diurnal cumulus over
western MA) are the story of the day this afternoon as we remain
under control of the same high pressure to our south that has been
in place for a few days. Locations in east/northeast MA are
currently sitting in the low 90s (with a few spots in the CT valley
as well). We should see a few more locations join the 90 degree club
before things start cooling this evening. The exception to the rule
is south coastal RI and MA where the cool onshore flow is keeping
temps in the 60s and 70s. Can`t rule out a few isolated
thunderstorms to make it into western MA or CT this afternoon. CAPE
values are over 2,000 J/kg but soundings show a strong capping
inversion keeping instability elevated and storms from initiating
lacking extra forcing besides diurnal heating. Any storms that do
form are not expected to last long or become severe as lack of bulk
shear (less than 10 kts) and meager mid level lapse rates keep any
updrafts from persisting.

Tonight high level clouds increase ahead of of the incoming mid
level trough as heights begin to fall and low pressure approaches.
Continued warm southwesterly flow keeps lows temperatures mild, in
the mid 50s (south) to mid 60s (north). This also will bring the
return of fog and low stratus clouds to the south coast between
midnight and 8-10 am, though confidence in the areal extent is only
moderate as guidance has struggled with this as of recent.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...

Thursday will be the most active weather day of the week with
convection and even a few severe thunderstorms expected during the
afternoon and evening. The driving force of these storms is a mid
level trough and surface cold front that will move east across
southern New England through the day on Thursday, potentially
slowing/stalling near the south coast overnight. In the warm sector
dewpoints climb to the low to mid 60s. This, with temperatures in
the 80s (a bit cooler than today due to increased cloudcover) and
steep lapse rates will lead to CAPE values over 2,000 J/kg for all
except southeast MA. The shear environment will be relatively
supportive of prolonged updrafts, with 0-6 KM bulk shear values in
the 30-35 kt range. While flow in the low levels will be weak,
stronger mid/upper level flow will support potential for scattered
severe thunderstorms with the greatest threat being damaging winds
and hail. The window for greatest thunderstorm threat will be 12pm-
8pm. Confidence is only moderate as to where exactly any severe
storms occur, but generally a line from PVD to BOS and west will
stand the best chance. As the front sags south it slows around or
just offshore and this boundary may serve as a forcing mechanism for
downpours and thunderstorms to continue into the overnight hours for
the immediate south coast or even just the islands. If this does
occur, PWATs are quite elevated, near 1.5 inches, so some heavy
downpours are possible.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...

Highlights

* Warm and dry Friday and Saturday

* Scattered thunderstorms possible Sunday afternoon

* Unsettled early next week with substantial precipitation possible
  Monday into Tuesday

Friday and Saturday

Deep northwest flow behind a surface cold front will advect a drier
air mass over southern New England to end the week. This will bring
an end to the muggy conditions expected Thursday as dewpoints drop
from the mid to upper 60s to the mid to upper 50s by Friday morning.
Air mass will remain on the warm side with 925 hPa temps continuing
to hover between 17 and 20 Celsius. Thus we should have another
afternoon with high temps in the upper 70s to low 80s, though this
will be a drier/more comfortable warm than what is expected on
Thursday. Warm/dry weather persists into Saturday with very little
change in the forecast, Saturday should feature plenty of sunshine
with highs again in the upper 70s to low 80s. There may be some
increasing cloudiness Saturday afternoon as return flow from the
south begins to advect higher dewpoints back into the region for the
second half of the weekend.

Sunday

Southerly return flow allows dewpoints to climb into the upper 50s
to near 60 across much of southern New England for Sunday. This will
support at least partly cloudy skies for Sunday afternoon with temps
continuing to peak in the low to mid 80s across interior southern
New England. Latest guidance suggests a synoptic setup that would
support background winds weak enough for sea-breezes to develop
along the coastline. Therefore we`d expect slightly cooler
temperatures in the upper 60s/low 70s for the coastal locations.

There does appear to be decent lapse rates and instability in the
atmosphere on Sunday, so there may be a risk for isolated to
scattered thunderstorms during the afternoon. Synoptic forcing for
ascent looks to be on the weak side however, with possible weak
height rises in the mix, so unless guidance trends towards a
possible short-wave aloft to enhance forcing for ascent, would
currently expect convection to be on the isolated side.

Monday and Tuesday

An upper-level low digs over the eastern US early next week. This
will support unsettled weather with a chance for substantial
precipitation Monday into Tuesday. Details are vague at this time
range, but ensembles are suggesting a 50 to 60 percent chance of
rainfall accumulations of 0.5 inches or higher between Monday and
Tuesday morning. Check back for more details this weekend.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
Forecast Confidence Levels:

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

This afternoon...High confidence.

VFR. SW winds 5-10 kts.

Tonight...Moderate confidence.

Light SW winds continue 5-10 kts. IFR/MVFR cigs in stratus/fog
possible over southeast MA terminals, but low confidence in
areal extent. Could also see some status in the lower CT River
Valley. Areas MVFR in weakening showers and thunderstorms,
mainly across western and central MA.

Thursday...Low to moderate confidence.

Mainly VFR, with areas MVFR/IFR in heavier showers and
thunderstorms. Best chance of thunderstorms is between 15Z and
00Z.

Thursday night...Low to moderate confidence.

Mainly VFR, but areas of MVFR/IFR in any lingering showers and
thunderstorms which may stick around for southeastern terminals
through as late as 06z, and even later for ACK/FMH/HYA.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Friday through Monday/...

Friday through Saturday: VFR.

Saturday Night: VFR. Chance SHRA.

Sunday: VFR. Slight chance SHRA, isolated TSRA.

Sunday Night: VFR. Slight chance SHRA.

Memorial Day: Mainly VFR, with local MVFR possible. Chance
SHRA, isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels:

Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.

Through Thursday night...High Confidence.

High pressure south of the waters maintains relatively light
winds and seas through Thursday night. The main concern will be
poor visibility in areas of fog reducing visibility to less
than 1 mile at times tonight.

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Slight chance of rain showers.

Friday Night through Saturday: Winds less than 25 kt.

Saturday Night: Winds less than 25 kt. Chance of rain showers.

Sunday: Winds less than 25 kt. Slight chance of rain showers.

Sunday Night: Winds less than 25 kt.

Memorial Day: Winds less than 25 kt. Slight chance of rain
showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ004>007-
     010>016-026.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...BW/RM
NEAR TERM...BW
SHORT TERM...BW
LONG TERM...RM
AVIATION...BW/RM
MARINE...BW/RM