935
FXUS61 KBOX 221638
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1238 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. 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 PM THIS EVENING/...

10am Update...

Much of the region remains under low cloud cover with light fog
this morning with near saturated conditions and onshore flow.
Guidance is not showing much improvements in cloud cover, but
general thinking is that the strong June sun angle should be
able to erode some of the clouds in western MA and CT. This
will set the stage for thunderstorm development later this
afternoon. Latest 12z guidance seems reasonable confining much
of the thunderstorm activity to interior MA and CT.

355 AM Update...

Key Points...

* Showers & T-Storms are expected to develop again Sat PM
* Severe thunderstorms & localized flash flood risk again
* Heat Advisories continued for CT and southwest MA

Warm and muggy morning in progress with temps and dew pts in the 60s
to lower 70s. This is resulting in lots of low clouds and areas of
fog. However, strong late June sun will burn through this low level
moisture and give way to partial sunshine this afternoon, especially
across CT and RI. Thus, another warm and humid day with highs in the
80s to around 90 in CT and southwest MA. These very warm temps will
combine with dew pts in the low to mid 70s, yielding heat indices
into the mid 90s, with hottest conditions across CT into southwest
MA, including the Greater Springfield area. Therefore, the heat
advisory remains in effect for this region.

Not quite as warm or humid across eastern MA, as the backdoor front
from Friday has stalled across Worcester county and extending into
southeast MA. On this relatively cooler side of the boundary,
including metro Boston, note quite as warm and humid with highs 75-
80 and dew pts in the 60s.

As for the risk of showers & thunderstorms today, this stalled
boundary across Worcester county extending into southeast MA will
serve as a focus for convection, providing low level convergence and
differential heating. This combined with weak height falls today
should help erode mid level cap and be sufficient for scattered
convection to fire. Along and west of the stalled boundary is where
the warm sector and greatest instability exist. Hence, this is where
the strongest storms should reside. HREF MU-CAPE on the order of
1500-2000 j/kg and combined with steep low level lapse rates, storms
will be capable of producing strong to damaging winds. In addition,
despite high freezing and wet bulb zero heights, there may be
sufficient instability for a low risk of large hail. Also, can`t
rule of a low prob of an isolated tornado given low level helocity
in vicinity of the stalled boundary, coupled with strong
destabilization and modest deep layer shear of 25-30 kt. This is
supported by much of the Machine Learning guidance as well as the
HREF Updraft Helicity Swaths.

Finally, very moist airmass remains in place, with dew pts of 70-75
this afternoon, PWATs up to 2 inches and deep unidirectional flow
supporting risk of training cells and back building WNW into warm
sector, will yield another day with the risk of flash flooding. 00z
HREF has up to a 30% chance of rainfall exceeding 3 inches in 3
hours this afternoon and unfortunately in the same area as Friday`s
heavy rains, eastern CT into western RI. Much of this forecast will
hinge on how much morning clouds erode and gives way to partial
afternoon sunshine and resulting strong destabilization. Some of the
guidance keeps mostly cloudy conditions across the area, yielding
less instability and hence lower probabilities of strong storms and
severe weather, including flash flood threat. Something we will need
to watch as the morning unfolds.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/...

355 AM update...

Key Points...

* Even warmer and more humid Sunday
* Another round of PM thunderstorms, possibly strong with heavy rain
  too

Tonight...

Given stronger deep layer shear (up to 30-35 kt), convection may
linger a few hours after sunset. Otherwise, drying trends overnight
with fog reforming.  Another warm night, with lows in the 60s and
remaining humid as dew pts remain in the 60s.

Sunday...

Lead short wave trough approaches from the west and induces a
prefrontal trough. Strong low level WAA ahead of this feature, with
925 mb temps warming to +24C to +26C across SNE. This will easily
support highs in the upper 80s to lower 90s away from the south
coast. This combined with dew pts in the low 70s will yield hot and
humid conditions with heat indices 95-100 across CT and southwest
MA. Hence, heat advisory remains in effect. Although, gusty SW winds
15-20 mph with gusts up to 30 mph at times across RI and eastern MA
will help take some of the edge off the heat and humidity.

These hot temps and high dew pt air will yield 1500-2000 j/kg of
CAPE and deep layer shear increasing up to 35 kt with approaching
short wave trough. Also, RRQ of upper level jet streak will enhance
QG forcing for ascent.  Thus, sufficient synoptic scale lift
combined with CAPE/Shear overlapping to support at least a few
strong to severe storms.  Heavy rainers too with PWATs up to 2
inches.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...

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 /16Z SATURDAY THROUGH WEDNESDAY/...

Forecast Confidence Levels:

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

12z TAF Update...

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

Mainly IFR-LIFR conditions in low clouds and fog should improve
to VFR across the distant interior by early afternoon. Along
the coastal plain IFR/LIFR may briefly improve to some low end
MVFR levels by midday...but if this happens it will be short-
lived. Otherwise...the main concern will revolve around another
round of showers & t-storms impacting the region this afternoon
and early evening. A few of these will be capable of producing
locally strong wind gusts and torrential rainfall. Greatest risk
for these storms will be southwest of a Boston to Fitchburg
line. Winds generally less than 10 knots.

Tonight...High Confidence on trends but lower on specific
timing.

The bulk of the showers & t-storms will wind down after
sunset...but can not rule out at least the low risk for a few
showers and perhaps a t-storm or two overnight. Otherwise...
high probability for widespread IFR/LIFR, just some uncertainty
how quickly this spreads westward from eastern MA into the
distant interior. Light and variable winds shifting to the south
by daybreak Sunday.

Sunday...High Confidence on trends, lower on exact timing and
details.

Widespread IFR/LIFR in low clouds and fog in the morning. Then
gradually improving to VFR in the afternoon, except MVFR/IFR
along the south coast. Thunderstorms, possibly strong to severe
is possible late in the day across CT into western-central MA.

KBOS TAF... Moderate Confidence in TAF

Mainly IFR to low end MVFR today. Low chance for a shower or
thunderstorm in the afternoon between 20z and 00z, but
confidence is low at this time. Bulk of thunderstorm activity
will be across western- central MA into CT and RI.

KBDL TAF...Moderate Confidence in TAF.

IFR this morning improving to MVFR/VFR this afternoon. Chance
for thunderstorms again this afternoon anytime between 18z-00z,
but higher confidence between 20z-22z.

Outlook /Monday through Wednesday/...

Sunday Night: Mainly VFR, with local IFR possible. Windy with
areas of gusts up to 30 kt. SHRA likely, isolated TSRA.

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.

&&

.MARINE...
Forecaster Confidence Levels:

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

355 AM update...

Through Sunday...High Confidence.

*** SCA in effect for Sunday ***

Saturday...

Light/variable winds Saturday with low clouds and fog burning off
this afternoon. Scattered thunderstorms expected this afternoon
especially across the RI waters eastward to Cape Cod and Nantucket.
A few strong storms possible.

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 30 kt
at times. Low vsby in morning fog will improve during the afternoon.

Outlook /Monday through Wednesday/...

Sunday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Rain showers likely,
isolated thunderstorms.

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.

&&

.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-
     231-250-251.
     Small Craft Advisory from 8 AM Sunday to 8 AM EDT Monday for
     ANZ232>235-237-254>256.
     Small Craft Advisory from 10 AM to 8 PM EDT Sunday for ANZ236.

&&

$$

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