474
FXUS61 KBOX 230624
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
224 AM EDT Sun Jun 23 2024

.SYNOPSIS...
Warm and muggy Sunday with another round of afternoon/early
evening showers and thunderstorms, with the strongest storms
across western and central MA Sunday afternoon. Some storms may
become severe and result in damaging winds, large hail and a
localized flash flood threat. A few diurnally driven showers
will remain possible Monday, but with lowering humidity. Dry
and very warm weather is on tap Tuesday but with lower
humidity. Hot and humid weather returns later Wednesday. Another
round of showers and thunderstorms is possible late Wednesday,
perhaps lingering into Thursday. Otherwise...drier weather with
lower humidity returns by Friday.

&&

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

2 AM update...

Cluster of thunderstorms across CT continue to weaken given
warming cloud tops and convection is becoming elevated, as it
traverses up and over the shallow, cool and stable low level
airmass across the region. Expecting this trend to continue as
the convection moves into RI and southeast MA. Although the
southern flank of the line may remain somewhat more robust, as
there is some elevated instability across LI to BID lapse rates
on the order of 7C/KM in the 2-6 km layer, per SPC
mesoanalysis. Otherwise, abundant low level moisture trapped
beneath inversion will yield lots of low clouds and areas of fog
early this morning, locally dense in some areas. Seasonable
with lows in the 60s.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/...

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 NIGHT 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 /06Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

06z update...

Thru this morning...high confidence on trends, lower on exact
timing and details.

Showers with embedded thunder moving across CT/RI and eventually
southeast MA early this morning. Brief heavy downpours and
reduced vsby the main issue. Otherwise, widespread IFR/LIFR in
low clouds and fog this morning. Light NE winds trending
southeast through the morning. Cigs and vsbys will be trendign
upward to IFR/MVFR 14z-16z. Most improvement inland and slowest
improvement along the south coast.

This afternoon...high confidence on trends, lower on exact
timing and details.

Cigs and vsbys improving to VFR, except along the south coast,
including Cape Cod and the Islands, only to IFR/MVFR in low
clouds and areas of fog. Increasing low level southwest jet
increases to about 50 kt at 2 kft over Cape Cod and Islands by
00z. This will yield LLWS, less inland with better low level
mixing and surface winds SW 15-20G30 kt over RI and eastern MA.
A line of strong thunderstorms is likely 19z-01z northwest of
I-95. Strong winds, large hail and very heavy rain with reduced
vsby are the main concerns.

Sunday night...Moderate to high Confidence.

Lingering thunderstorms at 00z across eastern MA will be
weakening and moving offshore. However, cigs and vsbys lower to
IFR/LIFR in low clouds and fog. Southwest winds remain robust at
15-25 knots. LLWS lingers over southeast MA with low level jet
up to 50 kt at 2 kft at 00z, but slowly moving offshore by
morning.

Monday...high confidence.

IFR/LIFR in the morning lifts and improves to VFR all terminals.
Mist/drizzle possible early, but dry runways late morning into
the afternoon. Low risk of a few spot showers inland in the
afternoon.

KBOS TAF...high confidence in TAF trends. Strongest storms this
afternoon should remain west of the terinal, with weakening
storms impacting the terminal 22z-02z. Frequent lightning, gusty
winds and heavy rain are the main impacts.

KBDL TAF...high confidence in TAF trends. Strong thunderstorms
may impact the terminal 20z-00z. Strong winds, large hail and
heavy rain are the main concerns.

Outlook /Monday Night through Thursday/...

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 Night 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 this evening for CTZ002>004.
MA...Heat Advisory until 7 PM EDT this evening for MAZ010-011.
RI...None.
MARINE...Small Craft Advisory from noon today to 8 PM EDT this evening
     for ANZ230-250-251.
     Small Craft Advisory from noon today to 8 AM EDT Monday for
     ANZ231.
     Small Craft Advisory from 8 AM this morning to 3 PM EDT Monday
     for ANZ232>235-237.
     Small Craft Advisory from 10 AM this morning to 9 AM EDT
     Monday for ANZ236.
     Small Craft Advisory from 8 AM this morning to 9 PM EDT Monday
     for ANZ254>256.

&&

$$

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