424
FXUS61 KBOX 222025
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
425 PM EDT Sat Jun 22 2024

.SYNOPSIS...
Warm and muggy this weekend with more rounds of diurnal showers and
thunderstorms with the focus across the interior this afternoon
and again Sunday. Some storms may again become severe and
result in a localized flash flood threat. A few diurnally
driven showers will remain possible Monday with but lowering
humidity. Dry and very warm weather is on tap for Tuesday with
increasing humidity by Wednesday. Another round of showers and
thunderstorms is possible later Wednesday perhaps lingering into
Thursday. Otherwise...drier weather with lower humidity returns
by Friday.

&&

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

Key Points...

* Warm and muggy conditions continue tonight
* Chance for elevated showers and thunderstorms overnight

Ongoing convection across western portions of SNE will weaken
tonight as it moves east into a less favorable environment and
come to an end between 8pm-12pm tonight. After 12pm tonight,
there is considerable uncertainty on how things evolve
overnight. Some of the Hi-res CAMS, mainly the HRRR, show
scattered to widespread elevated convection overnight firing
off a 30-40 knot LLJ and some elevated instability around 500
J/kg. Most of the other Hi-res CAMS keep the region dry or show
only isolated convection. Overnight convection can be tricky as
models generally struggle with it. Capped POPs below 40% across
the region for the overnight period.

Warm and muggy still overnight with lows only in the mid to
upper 60s, to even low 70s. Dewpoints also in the mid to upper
60s will keep conditions saturated allowing for low clouds and
patchy light fog to form again overnight.

&&

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

Key Points...

* Severe thunderstorms expected tomorrow afternoon NW of I-95
* Primary threat is strong damaging winds and heavy rainfall
* Secondary threat is hail and even a few tornadoes
* Hot and humid conditions continue with heat indicies in the
  mid to upper 90s.

A potent Shortwave trough and associated surface low begins to
drop out of the Great Lakes on Sunday. A weaker leading
shortwave associated with the warm front out ahead of the
surface low will cross through the region Sunday afternoon. With
deep SW flow at the surface, we should see cloud cover clear
except near the South Coast and Islands. With plenty of
sunshine tomorrow afternoon, temperatures will quickly rise into
the 90s. With dewpoints in the low 70s tomorrow, heat indices
will once again jump into the low to mid 90s across eastern MA
and RI, and mid to upper 90s across western MA and CT. The heat
advisory will remain in effect tomorrow for the southern CT
river valley.

The heat and humidity will lead to large amounts of instability
tomorrow especially with sunny conditions. HREF mean SBCAPE
around 2000J/kg and 0-6km between 30-50 knots, this will set
the stage for more scattered to numerous severe thunderstorms
tomorrow between 1pm-10pm NW of I-95. The primary threat will be
severe wind gusts and heavy rain, with secondary threats of
hail and even a few tornadoes. Breaking down each of the threats
below.

Strong to damaging winds

The primary threat for severe weather tomorrow will be the
strong to damaging wind gusts. An 850 mb LLJ of will be around
35-45 knots and steep low level lapse rates will already bring
gusty SW winds to much of the region in the afternoon. With
DCAPE values around 900J/kg the already gusty winds will be
enhanced due to thunderstorms. SPC has highlighted much of
western SNE in a 15% wind risk, with 30% in far NW MA for severe
wind gusts.

Heavy rain and flash flooding

Precipitable water values will again be near 2.0 inches
combined with tall skinny cape values and warm cloud depths
approaching 10kft, storms will produce heavy rain. Storms should
be moving rather quickly, but some guidance suggests the
possibility for multiple rounds of convection. Given the
antecedent wet conditions from all the storms the past couple
days, this will elevate the flash flood risk esspically in urban
centers with poor drainage.

Tornadoes

Tomorrow certainly has the greatest risk for tornadoes compared
to the last couple days with a significantly more shear and
better forcing. HREF 0-3km SRH values around 300 m/s, 0-1 km SRH
around 100 m/s, and STP values between 1.0-2.0 certainly
indicates the possibility for tornadoes with severe
thunderstorms. LCL heights under 500m in a deeply moist
environment will also be favorable tornado development. Very
elongated straight line hodographs in the low level could
support a couple discrete splitting supercells, but confidence
on this remains low. With this being said, SPC has high lighted
a 5% TOR risk for much of Western MA and CT, and a 10% TOR risk
in the very far NW MA.

Hail

The hail threat appears to be the most limited threat of the day
with mid level lapse rates only around 5-6 C/km and a freezing
level around 15kft. In order to get hail to the ground, a very
strong and robust supercell will be needed esspically if there
are any left splits. Given the unfavorable conditions SPC only
highlights a 5% risk of hail for western MA, and CT.

Sunday night

The threat for severe thunderstorms will quickly wane between
8pm-10pm with only lingering showers and thunderstorms after
10pm. Generally should see drying conditions by day break with
lows only in the upper 60s to low 70s.


&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Key Points...

* Turning less humid Mon with a few diurnally driven showers
* Dry & Very Warm Tue with tolerable humidity
* Main risk of showers/t-storms Wed PM may linger into Thu
* Dry weather returns Thu or by Fri with lower humidity

Details...

Still some uncertainty for Monday...but the latest guidance
indicates initial surface trough may have already pushed deeper
moisture east of the region. If this does indeed happen we would
still have highs mainly in the 80s...but lowering humidity as
the day progresses. The upper trough/cold pool aloft may still
allow for diurnally driven showers...but if the deeper moisture
is shunted east we may not have to worry about severe weather.
Too early to lock that in for sure...but that is the 12z model
trend.

Mid level ridge axis crosses the region Tue allowing for dry
and warm weather...but not the humidity of what we had much of
this week. Highs will still be in the upper 80s to near 90 in
many locations.

The humidity will again return to the region by Wednesday as the
next shortwave trough approaches from the west. Many locations
away from the south coast will probably see highs in the upper
80s to the lower 90s. Approaching shortwave/cold front will
bring the risk for a round of showers & t-storms Wed PM. Plenty
of uncertainty regarding timing/instability...but appears we may
be dealing with sort of remnant EML. That will need to be
watched for a severe weather risk as we get closer.

Depending on the timing...some activity may linger into Thu if
the shortwave/cold front ends up on the slower side.
Otherwise...drier weather with lowering humidity by Fri.

&&

.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

12z TAF Update...

Rest of Today...High Confidence on trends but lower on specific
timing.

Thunderstorms will affect western terminals this afternoon
through about 20-23z. Further east, storms should weaken into
showers, although embedded thunder is still possible. CIGS
mainly MVFR/VFR this afternoon except across the Cape and
Islands.

Tonight...High Confidence on trends but low confidence on rain.

Daytime showers and thunderstorms should end by 00 to 02z. Some
guidance suggest elevated showers and thunderstorms are
possible overnight from 04z-12z, but confidence in this is low
at this time and thus left out mention in the TAFs, but may be
added in future TAF sets. IFR/LIFR conditions region wide. 30-40
knot LLJ may bring some light wind shear/turbulence towards
daybreak.

Sunday...Moderate Confidence on trends and rain/thunderstorms

Any overnight rain should clear by 14z the latest with rapid
improvements from IFR to VFR in the afternoon. The exception to
this will be the South Coast and Islands will likely remain
MVFR/IFR tomorrow.

KBOS TAF... Moderate Confidence in TAF

MVFR this afternoon with periods of IFR possible. Still
expecting showers with a rumble of thunder possible later this
afternoon between 20z-23z. IFR tonight. Low chance for showers
overnight, but confidence is low at this time. IFR improving to
VFR tomorrow afternoon. Thunderstorms are possible tomorrow
afternoon.

KBDL TAF...Moderate Confidence in TAF.

MVFR/VFR this afternoon with scattered thunderstorms through
about 23z. MVFR/IFR overnight. Low chance for showers and
thunderstorms overnight, but confidence is low at this time. VFR
tomorrow with a moderate chance for thunderstorms in the
afternoon, but timing remains uncertain.

Outlook /Monday through Thursday/...

Monday: Mainly VFR, with areas MVFR possible. Breezy. Slight
chance SHRA.

Monday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA.

Tuesday: VFR.

Tuesday Night: VFR. Breezy.

Wednesday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, chance TSRA.

Wednesday Night: Mainly MVFR, with areas VFR possible. Breezy.
Chance SHRA, chance TSRA.

Thursday: Mainly MVFR, with areas VFR possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

*** SCA in effect for Sunday ***

Saturday night...

Early evening scattered thunderstorms dissipate by late evening.
Then patchy fog reduce vsby. Light and variable winds become SW
toward Sunday morning.

Sunday...

SW winds increase 15 to 25 kt, with some gusts possibly up to 35
kt at times. Low vsby in morning fog will improve during the
afternoon.

Sunday Night...

Scatted showers and thunderstorms move out over the waters in
the late evening. Low clouds and fog fill back in. Winds remain
gusty out of the SW at 15-25 knots. Sea increase to 5-10 feet.

Outlook /Monday through Thursday/...

Monday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers.

Monday Night: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday Night: Winds less than 25 kt.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas up to 5 ft. Slight chance of rain showers,
slight chance of thunderstorms.

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers,
isolated thunderstorms.

Thursday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 7 PM EDT Sunday for CTZ002>004.
MA...Heat Advisory until 7 PM EDT Sunday for MAZ010-011.
RI...None.
MARINE...Small Craft Advisory from noon to 8 PM EDT Sunday for ANZ230-
     250-251.
     Small Craft Advisory from noon Sunday to 8 AM EDT Monday for
     ANZ231.
     Small Craft Advisory from 8 AM Sunday to 3 PM EDT Monday for
     ANZ232>235-237.
     Small Craft Advisory from 10 AM Sunday to 9 AM EDT Monday for
     ANZ236.
     Small Craft Advisory from 8 AM Sunday to 9 PM EDT Monday for
     ANZ254>256.

&&

$$

SYNOPSIS...Frank/KP
NEAR TERM...KP
SHORT TERM...KP
LONG TERM...Frank
AVIATION...Frank/KP
MARINE...Frank/KP