833
FXUS61 KBOX 230240
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1040 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/...
Rather quiet weather continues across southern New England this
evening. Satellite data showed a couple bands of stratus. One
was across the upper and mid Cape, with another extending NE
across Nantucket. Will be monitoring these clouds closely
overnight.

The closest rainfall was across southern NY/northeast PA.
Lightning activity was diminishing as it moved east. Not much
buoyancy or shear across southern New England per the latest SPC
mesoanalysis. Expecting only a few showers at most, should these
showers persist long enough to move into southern New England
overnight.

Otherwise, only minor tweaks to bring the forecast back in line
with observed trends.

Previous Discussion...

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 /03Z THURSDAY THROUGH MONDAY/...
Forecast Confidence Levels:

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

Tonight...High Confidence across the interior. Moderate
confidence for Cape/Islands terminals

LIFR/IFR cigs in stratus/fog possible over southeast MA
terminals, but low confidence in areal extent. VFR elsewhere
with light southwest winds.

Thursday...Low to moderate confidence.

Mainly VFR, with areas MVFR/IFR in heavier showers and
thunderstorms. Best chance of thunderstorms is between 16Z 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. TSRA possible as early as
16Z, but more likely between 18-00Z.

KBDL TAF...High confidence in TAF. TSRA possible as early as
15Z, but more likely between 18-22Z.

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...Belk/BW
SHORT TERM...BW
LONG TERM...RM
AVIATION...BW/RM
MARINE...BW/RM