-- Discussion containing changed information from previous version are highlighted. --
669
FXUS61 KBOX 211338
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
938 AM EDT Fri Jun 21 2024

.SYNOPSIS...
A backdoor cold front moves in from northeast to southwest
Friday...bringing another round of showers and thunderstorms but
also breaking the dangerous heat and humidity for much of the
region. Thunderstorms will be focused across western-central
Massachusetts and Connecticut...where some of the storms may become
severe and also result in a localized flash flooding. Warm and muggy
this weekend with rain and thunderstorm chances both Saturday and
Sunday. A cold front sweeps across the region Monday with additional
showers and storms, then a drying trend and less humid Tue before
heat and humidity return midweek.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

-- Changed Discussion --
935 AM Update...

* Another round of showers & t-storms this afternoon/early eve
* Focus will be across interior MA, CT, western RI
* Severe Weather/Flash Flood Risk...interior MA/CT/Western RI

Many locations have already seen temperatures reach into the
80s. However...a backdoor cold front will be pushing westward
towards the CT River Valley by early afternoon. As a
result...temps along the immediate eastern MA coast will drop
into the 70s but remain in the 80s not too far inland from the
coast. In fact...the flow behind the front is relatively weak
so expect highs to still reach the lower to middle 90s in the
lower CT River Valley where Heat Advisories continue through the
afternoon.

The biggest concern will be for another round of showers &
t-storms as shortwave energy coupled with diurnal instability
combined with the backdoor cold front. Activity should develop
by early-mid afternoon and while all areas are subject to be
impacted...the bulk of the storms today will focus across
interior MA/CT and western RI. This is where MLCapes may reach
between 1500 and 2000 J/KG and also have some forcing along the
backdoor cold front. While instability is not as high as
yesterday...0-6 KM shear will be on the order of 20 to 30 knots
somewhat better and low level lapse rates will be steep. This
will support the risk for a few severe t-storms with the focus
for them across interior MA/CT and western RI. The various
Machine learning guidance also highlights this region.

In addition...these backdoor cold fronts always need to be
watched for a localized flash threat. Pwats of 2+ inches will
support torrential rainfall with any storms and there is the
potential for some training along the backdoor front. The CSU
Machine learning probs do show the potential for excessive
rainfall across interior MA/CT and western RI. In fact...the
HREF indicates some low probs for the 6 hour QPF to exceed a 100
year rainfall event. That certainly does not mean that will
happen...but is at least a signal for potential localized flash
flooding. This is particularly true if the activity would train
over a particularly vulnerable urban center.
-- End Changed Discussion --

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
220 AM update...

Much like last night, instability will drop off quickly with sunset
as an elevated mixed layer develops above the cap. Thus, don`t
expect the storm/low severe threat to persist much past 0

Will need to watch for stratus/fog formation tonight given onshore
flow and a lot of residual moisture post storms. Overnight lows will
again remain quite mild across the interior, but lower dewpoints in
the low 60s across eastern MA will provide somewhat of a reprieve
there.

It`s almost a "copy paste" type forecast for Saturday with the
quasistationary frontal boundary still draped over our region.
Onshore flow should advect "cooler", well in this case "seasonable"
air well inland, with high temperatures ranging from the low 70s in
northeastern MA to the upper 80s to perhaps 90F across central
Connecticut.

Will need to monitor another wave of convection Saturday afternoon,
though hi-resolution guidance remains widely varied in the coverage
and intensity of storms, with the NAM3k showing a more conservative
solution of isolated cells and other guidance like the FV3 diagnosing
an early evening convective line. The strongest cells are again
expected along the frontal boundary, dissipating quickly after 01-
02Z tomorrow evening as the elevated mixed layer redevelops.

Hazards on Saturday are generally consistent with what we will see
on Friday given a near identical synoptic setup. Damaging wind and
urban/localized flash flooding are again possible.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
220 AM update...

Highlights:

* Heat and humidity likely return Sunday with PM T-storms possible,
  especially CT into western-central MA

* Another round of T-storms possible Mon, followed by a drying trend
  Tue and less humid, before heat and humidity return mid week

Saturday night...

Any early evening convection along the stalled boundary should
diminish with sunset or shortly thereafter, as short wave ridging
begins to advect in from the west. This will also lift the stalled
boundary northward as a warm front. Thus, warm sector airmass will
yield a mild night along with patchy fog given increasing dew pts.
Low temps will only fall into the 60s.

Sunday...

Strong WAA with ensembles advecting +20C to +21C 850 mb air across
SNE! Deterministic guidance has +25C air at 925 mb streaming into
the region from SW to NE, with hottest air over CT. However,
guidance is not as hot given such warm temps aloft. Much of the
guidance offering highs 85-90 Sunday afternoon, with hottest numbers
across CT. Models do have a lot of cloud cover around Sunday and
some of the guidance suggest the stalled boundary from Saturday
doesn`t completely lift north and exit the region, supporting the
idea of cooler temps especially across northeast MA. For now, stuck
close to guidance but if true warm sector airmass overspreads the
region, highs could soar well into the 90s and combined with dew pts
of 70-75, heat indices of 95 to 104 would be possible. Hence,
dangerous heat and humidity. Scattered thunderstorms are possible in
the afternoon/early evening across CT and western-central MA,
pending the amplitude of a lead short wave which may induce a
prefrontal trough to focus the convection. Greatest risk for PM
storms will be across CT into western-central MA.

Monday...

Another round of thunderstorms expected Monday, as potent short
wave/closed mid level low and attending cold front approach the
region. Not as hot as Sunday, but remaining very warm and humid,
with highs 80-85 and dew pts in the upper 60s to lower 70s.

Tuesday...

A brief break from the heat, humidity and drier weather, as closed
low exits and moves east of our longitude, with drier WNW flow. Dew
pts fall into the low and mid 60s, possibly some upper 50s western
MA. Pleasantly warm with highs in the low to mid 80s, but noticeably
lower humidity.

Wednesday/Thursday...upper air pattern is progressive so return flow
develops mid week, with warm and more humid weather advecting into
SNE. Highs 85-90 Wed, then likely not as hot Thu given increasing
risk for showers/thunderstorms with highs in the upper 70s and lower
80s.

&&

.AVIATION /13Z FRIDAY THROUGH TUESDAY/...
Forecast Confidence Levels:

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

12z TAF Update...no change from previous TAF thinking. Scattered
thunderstorms from 16z-22z approximately today, with coverage
most numerous across CT/RI and into western-central MA. SW winds
become ENE across eastern MA, as backdoor front comes onshore.
Earlier discussion below.

=====================================================================

Friday...Moderate Confidence.

More showers and thunderstorms this afternoon with more north to
south coverage compared to Thursday when storms stayed north of
the MA Turnpike. Anticipating VFR conditions, with the exception
of patchy IFR due to fog, through 15Z after which time storms
will bubble up. VFR/high end MVFR with cigs around 3500ft
outside of storms. MVFR to IFR in storms with potentially
torrential downpours and vsby reductions. Will be difficult to
pinpoint exact storm location but terminals at greatest risk are
BAF, BDL, ORH, PVD. Winds variable today, from the ENE from ORH,
east, and from the NW/W across the CT River Valley as cold
front pushes west towards interior southern New England.

Friday night...Moderate Confidence.

Bulk of storms wind down by 00Z tonight, but residual moisture
and onshore ENE flow will allow pockets of fog to develop.
Generally MVFR to IFR due to cigs.

Saturday... moderate confidence.

Another afternoon of scattered thunderstorms, though confidence
on intensity and aerial coverage is low at this time. Could
impact all terminals, or none at all with the best chance for
tstorms coming after 17Z. Conditions improve from IFR/MVFR to
VFR with perhaps the exception of east coastal MA where MVFR
will persist. Locally lower flight categories likely again in
storms.

KBOS TAF... Moderate confidence.

Expect thunderstorm activity to stay inland of Boston this
afternoon, but can`t rule out a stray storm. Generally VFR this
morning becoming MVFR/IFR overnight due to low stratus/fog.
Winds quickly shift to the ENE this morning and will persist.

KBDL TAF...Moderate Confidence in TAF.

Another round of thunderstorms this afternoon with the best
chance to impact BDL between 17-22Z. Storms wane quickly after
sunset. VFR outside of storms, MVFR in storms, MVFR to IFR
developing tonight due to fog. Winds from the West for much of
the period but depending on the position of the back door cold
front, could go N or ENE late in the period.

Outlook /Saturday Night through Tuesday/...

Saturday Night through Sunday: Mainly VFR, with local MVFR
possible. Breezy. Chance SHRA, isolated TSRA.

Sunday Night through Monday: Mainly MVFR, with areas VFR
possible. Breezy. SHRA likely, isolated TSRA.

Monday Night: VFR. Breezy. Chance SHRA, isolated TSRA.

Tuesday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

220 AM update...

Through Saturday...High Confidence.

Back door cold front will move across the northeastern waters on
Friday morning but likely stalls somewhere across the southern
waters, which will lead to converging winds; northeast vs
westerly.

Afternoon showers and thunderstorms across the mainland may
impact the waters very late both today and Saturday, but
confidence is low as storms are expected to lose their strength
after sunset. Winds and seas should stay below SCA criteria.

Fog and stratus development likely tonight and will linger into
the day on Saturday. Reduced vsbys should be expected.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, isolated
thunderstorms.

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Local rough seas. Chance of rain showers, isolated
thunderstorms.

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. Local rough seas. Rain showers
likely, isolated thunderstorms.

Monday Night: Winds less than 25 kt. Local rough seas. Chance
of rain showers, isolated thunderstorms.

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

&&

.CLIMATE...
All Time June High Temperature Record (Daily)

BOS - 100 F (06/06/1925, 06/26/1952 and 06/30/2021)
BDL - 100 F (06/26/1952, 06/30/1964)
PVD - 98 F  (06/25/1943, 06/15/1945 and 06/30/1945)
ORH - 98 F  (06/26/1952)

Most recent day of 100 F (or greater)

BOS - 100 F (07/24/2022)
BDL - 100 F (07/21/2019)
PVD - 100 F (07/28/2020)
ORH - 102 F (07/04/1911)*

* For ORH this is the only 100+ degree day in its period of record.

Daily High Temperature Records...

June 21st

BOS - 96 F (2012)
BDL - 96 F (1953 and 2012)
PVD - 96 F (1941)
ORH - 91 F (Multi-Years)

Daily Warmest Low Temperatures Records...

June 21st

BOS - 80 F (2012)
BDL - 73 F (1923 and 2012)
PVD - 75 F (2012)
ORH - 74 F (1923 and 2012)

&&

.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 MAZ008>011.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...Nocera/KS
NEAR TERM...Frank
SHORT TERM...KS
LONG TERM...Nocera
AVIATION...Nocera/KS
MARINE...Nocera/KS
CLIMATE...BL