-- Discussion containing changed information from previous version are highlighted. --
442
FXUS61 KBOX 220621
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
221 AM EDT Sat Jun 22 2024

.SYNOPSIS...
-- Changed Discussion --
Warm and muggy this weekend with more rounds of diurnal showers
and thunderstorms. Some of those may again become severe and
result in a localized flash flood threat. A cold front brings a
short window of reprieve early next week, though increasing heat
and shower chances by midweek.
-- End Changed Discussion --

&&

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

-- Changed Discussion --
222 AM Update...

Quiet overnight in progress with dry weather, but warm and humid
along with patchy fog, with dew pts mainly between 65-70.
-- End Changed Discussion --

&&

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

Key Points...

* Showers & T-Storms are expected to develop again Sat PM
* Severe Weather & Localized Flash Flood Risk Again
* Heat Advisories Continued for CT/SW MA

Details...

Saturday and Saturday night...

The backdoor front that washes out tonight...seems to want to setup
across northeast MA on Saturday. Therefore...a quite warm and humid
day is on tap with highs 85 to 90 in most locations with the hottest
readings around 90 in the Lower CT River Valley. Given Heat Indices
are expected to be in the mid to upper 90s...we have continued the
Heat Advisory across southwest MA/northern CT through the weekend.
It will be cooler though in northeast MA especially by Cape Ann
where highs will likely remain below 80.

The main concern Saturday afternoon and early evening will be for
another round of showers & t-storms. A shortwave working across the
region in west northwest flow will combined with diurnal heating and
a remnant boundary. We expect Capes on the order of 1500 to 2500
J/KG to develop and not much of a mid-level Cap. This should trigger
another round of showers & t-storms. While mid level lapse rates are
poor...effective shear will be stronger than the past few days on
the order of 30-40 knots. This should be enough for the potential of
scattered severe thunderstorms...which is supported by much of the
Machine Learning guidance as well as the HREF Updraft Helicity
Swaths.

Also...Pwats remain on the order of 2+ inches. Therefore...these
storms will be capable of producing torrential rainfall and
localized flash flooding. The CSU Machine Learning probs as well as
the HREF indicated low probs of 3"+ of rain inside 3 hours highlight
this concern. It is tough to pinpoint the area of greatest risk
given the mesoscale nature of these potential events. That being
said...the guidance is tending to indicate a higher risk in areas
from northern CT into western/central northeast MA closer to the low
level convergent zone.

A few storms may linger a bit longer Sat night...but will have to
wait and see how things unfold. Otherwise...more low clouds and fog
are expected to develop Sat night with the cooling boundary layer.
Overnight lows should drop into the 60s to near 70.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...

Highlights:

* Hot, humid, and potential for strong to severe storms Sunday
  afternoon.

* Cold front moves across the region Monday, bringing brief relief
  from hot and humid conditions. But signal for a return of the
  summer heat and humidity by mid next week.

Sunday: Hot and humid conditions fuel storms during the afternoon,
which could become locally strong to severe. A broad mid-level
trough moves across the northeast with 850mb shortwave energy moving
across the region with an associated warm front. WAA at 850mb
continues to indicate temperatures 20C to 22C, thus expecting highs
in the middle and upper 80s, 90F for Connecticut River Valley. With
dew points expected to be in the low 70s the `Heat Index` to reach
the middle and upper 90s at northern Connecticut and the southern
Pioneer Valley of Massachusetts. In collaboration with neighboring
WFOs we`ve extended the `Heat Advisory` through Sunday, for those
areas mentioned. All this fuels the potential for convection during
the afternoon as there`s sufficient CAPE, modest effective shear
between 30 and 40 knots, and helicity greater than 150 m2/s2. While
the greatest threat appears to be north, a few stronger to severe
thunderstorms remain possible, even a low risk of a brief tornado.
CSU machine learning show a chunk of southern New England between
15% and 30% probabilities for wind, and widespread 2% prob for a
tornado across most of New England, with a 5% to 10% across southern
Vermont to southern New Hampshire and along the northern
Massachusetts border. Don`t get hung up on the placement, more of
less gives us greater confidence in severe weather occurring. Lastly,
in addition to any severe weather there is the threat for heavy
rainfall as well given PWATs are in the ballpark of two inches.

Next Week: Mid-level trough moves east with a cold front on Monday
with another round of showers and storms, could be lingering diurnal
showers on Tuesday with a mid-level low over the Gulf of Maine,
though guidance is still split. Maintained `slight chance` POPs
across northern MA. Beyond, midweek it looks unsettled, forecast
confidence is low due to model uncertainty with the timing of any
showers and/or storms. Have greater confidence with temperatures, a
warmer than normal week is likely as ensemble situational awareness
tables indicate surface temperatures are between the 90th and 99th
percentile. Highs returning to the middle and upper 80s with near
90F in northern Connecticut and southwest Massachusetts. And nightly
lows in the 60s.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
Forecast Confidence Levels:

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

06z TAF Update...

Thru 12z...Moderate to Low Confidence.

High confidence in mainly dry runways through 12z but some
uncertainty on areal coverage of IFR/LIFR conditions in low
clouds and fog.

After 12z...moderate to low confidence.

IFR-LIFR conditions should improve to VFR across the interior
and mainly MVFR near and along the coast by early afternoon.
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.

The shower & t-storm activity should dissipate again not too
long after sunset Saturday night. Low end MVFR-IFR conditions
should re-develop with the cooling boundary layer with some LIFR
thresholds being met as well.

Saturday night...high confidence on trends but lower on exact
timing and specifics.

High probability for widespread IFR/LIFR, just some uncertainty
how quickly this spreads westward from eastern MA into RI/CT and
western-central MA. Any leftover scattered showers and
thunderstorms at 00z will quickly dissipate shortly thereafter.

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 to low Confidence in TAF.

Mainly IFR/MVFR today. Low chance for a shower or thunderstorm
in the afternoon between 18z 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.

MVFR/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 /Sunday Night through Wednesday/...

Sunday Night: Mainly MVFR, with areas VFR possible. Breezy.
SHRA likely, slight chance TSRA.

Monday: Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA, slight chance TSRA.

Monday Night: Mainly VFR, with areas MVFR possible. Chance
SHRA, isolated TSRA.

Tuesday through Tuesday Night: VFR.

Wednesday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Forecaster Confidence Levels:

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

Tonight through Saturday night...High Confidence.

The pressure gradient remains relatively weak as the backdoor cold
front of today...gradually washes out and lifts northeastward
through Sat night. Long continues southwest fetch may eventually
build seas to 4-5 feet toward daybreak Sun across our southern most
outer-waters...but otherwise winds/seas will remain below small
craft advisory thresholds. Otherwise...the main concern will be
areas of overnight and early morning fog. In addition...some
afternoon and evening t-storms may impact some of our mainly
nearshore waters at times.

Outlook /Sunday Night through Wednesday/...

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,
slight chance of thunderstorms.

Monday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers, slight chance of 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.

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

Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft.
-- End Changed Discussion --

&&

.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...None.

&&

$$

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