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883
FXUS61 KBOX 192009
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
409 PM EDT Wed Jun 19 2024

.SYNOPSIS...
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Hot and humid weather continues into Thursday. Isolated
thunderstorms this afternoon weaken shortly after sundown.
Greater thunderstorm coverage is expected Thursday near and
north of the Mass Pike, and some could become strong to locally
severe. Backdoor cold front moves in from northeast to
southwest Friday, though there remains uncertainty how far
inland the front goes. Showers and thunderstorms along the front
Friday afternoon. Unsettled and muggy this weekend, while not
expecting a washout, there will be periods of rain and
thunderstorm activity. A cold front moves across the region on
Monday with drying and warming conditions into next week.
-- End Changed Discussion --

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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400 PM Update:

Hot and muggy with temperatures in the upper 80s to the mid 90s and
dewpoints in the lower 70s away from the south coast. Continue to
practice appropriate heat-related precautions. Radar and satellite
imagery are showing very slow-moving strong t-storms in isolated
coverage in northern MA with a couple of cells west of Greenfield MA
that have produced a copious amt of rainfall in a short period of
time. Strong instability is present with mixed-layer CAPE values
ranging from 2000-3000 J/kg per SPC`s mesoanalysis, although wind
shear is extremely poor and storm motions are really a modest drift
southeastward. Ongoing activity both in northwest MA and over
southwestern should slowly drift southeast and be capable of
torrential rains, frequent lightning and localized wind gusts up to
50 mph, with weakening trends once we lose the heat of the day after
sundown. For the rest of the evening, we should see decreasing cloud
cover although it will still be quite muggy with dewpoints in the
upper 60s to lower to mid 70s leading to little significant relief.
The high humidity levels should also favor potential for marine fog
on the southern waters, and some of that fog could make it landward
into the south coasts of RI and MA, Cape Cod and the Islands. Lows
only in the upper 60s to the lower 70s.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
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400 PM Update:

Key Points:

* Another day of heat and humidity with high heat indices, excessive
  to those with prolonged exposure.

* Scattered strong storms with localized strong/damaging winds,
  heavy rain capable of street flooding and frequent lightning
  potentially as soon as early afternoon, decreasing in coverage and
  strength into early evening. Greatest risk from the Mass Pike
  northward, with isolated storm coverage to the Hartford-Providence
  corridor.

The sprawling upper level ridge from the mid-Atlantic into coastal
Southern New England continues to shift ever so slowly southward Thu
and Thurs night. Given the current coverage of t-storms Wed aftn
which lies on the northern periphery of the ridge axis, it would
seem reasonable to expect a little greater t-storm coverage Thurs
and possibly into Thurs night with the upper ridge axis shifting
southward. While vertical wind shear is a touch stronger Thurs than
today, it still is a pretty weakly-forced large-scale setting for t-
storms, one which higher resolution guidance often struggles with.
Though it still should be hot and humid Thurs, the coverage and
if/when these storms develop now casts a level of uncertainty on how
warm we might get; in particular for areas along and north of the
Mass Pike.

Regarding the heat/humidity, it should again be hot and humid and
left heat headlines and heat index values more or less little
changed to keep the message simple. As mentioned though, we`ll also
be destabilizing as the heat and humidity builds on Thurs and we
expect storms to develop near the MA/VT/NH border and sag southward
at some point during the afternoon hours. This looks closer to the
GFS depiction, although some of the CAMs are faster with
development, potentially as soon as early afternoon, while others
are later by late-afternoon. So there is a possible outcome where
storms develop early in the day and could keep us from reaching
Advisory criteria north of the Mass Pike, but for now will leave
these heat headlines as is. Thus highs in the 90s, with heat indices
into the low 100s away from the south coast. Near the south coast,
highs in the upper 70s to lower 80s with heat indices in the 80s.

On the t-storm threat for Thurs, it looks as though we`ll see a
greater extent of storms toward scattered, with PoPs in the 30-50%
range. Moderate to strong instability looks progged with the degree
of heat and humidity around, with CAPE values in the 2500-3000 J/kg
range, decreasing significantly with southern extent mainly south of
the Hartford-Providence corridor. While vertical wind shear is still
weak and supporting pulse to multicellular storm clusters, downdraft
CAPE should be pretty large by the afternoon. A few storms could
pulse to severe limits with localized straight line wind damage
being the main severe weather risk. There are signals for re-
developing storms/backbuilding potential as we move through the
afternoon, which could shift the threat from isolated severe weather
to more of heavy rain/localized hydro problems, with less-strong
instability as individual storms rain themselves out. After consult
with SPC, we think the severe weather potential seems isolated.
Phrased as scattered storms with enhanced wording for gusty winds,
heavy rainfall and frequent lightning near and north of the Mass
Pike, with more isolated coverage to the Hartford-Providence
corridor.

The forecast for Thursday night is low-confidence and will hinge on
how daytime t-storms develop and evolve. Upper level ridge axis
continues to shift slowly southward, with a surface frontal boundary
trying to slip southward from NH/VT. Residual showers/storms from
the daytime hours should continue to progress southeastward. NAM/GFS
each indicate a windshift to N/NNE moving into far northern MA by
early Fri AM, which could bring a somewhat cooler air mass to
northern and northeast MA. There could be continued clouds and
showers around in interior MA/CT/RI into the daytime hrs. Expect
there to be adjustments to the forecast moving forward, but there is
too much uncertainty to lock into any solution or outcome at this
time. It still looks pretty muggy outside of northern MA later in
the overnight with lows in the 60s to lower 70s.
-- End Changed Discussion --

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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Highlights:

* A backdoor cold front will move northeast to southwest on Friday
  with cooler temperatures returning, but uncertain how far inland
  the front will go. In addition, showers and thunderstorms may
  develop along the backdoor cold front.

* Warm and muggy this weekend with rain chances both Saturday and
  Sunday.

* Cold front sweeps across the region on Monday, drier and returning
  heat into mid next week.

Friday:

Shift in mid-level pattern, sub-tropical ridge of high pressure
"breaking down" and drifting south. As previously mentioned there is
a backdoor cold front should move across the region from the
northeast to the southwest. That said, there is a bit of uncertainty
with the magnitude of the front, and how far inland the cold front
will penetrate. Combination of the front and convection will play a
significant role in how warm it gets.

The Dynamic Ensemble-Based Scenarios for IDSS, or DESI for short,
suggests highs greater than 85F are most likely areas west of I-495,
with probabilities of 85+ percent for all of the Connecticut River
Valley. When increased to greater than 90F, found probabilities
between 55% and 70%. Conversely, highs less than 80F, there are
probabilities between 20% to 30% along east coast of Massachusetts,
with probabilities between 40% to 60% for high temperatures less than
80 degrees across eastern Essex County!

Given these stats, opt`d to keep a `Heat Advisory` for northern
Connecticut and southern Pioneer Valley of Massachusetts.

The aforementioned front provides the necessary lift for development
of pop-up thunderstorms, given SBCAPE are between 800 and 1200 J/kg.
Don`t expect organized convection due to the lack of shear, though
heavy downpours are possible as PWATs are around two inches. WPC
kept most of the region highlighted in its day three ERO, the CWA is
highlighted within a marginal risk (at least 5%) for excessive
rainfall.

NAM 3KM now includes Friday into Friday afternoon and suggests areas
further inland, where there SBCAPE is greatest and the atmosphere is
most unstable. Current thinking, area with greatest risk are central
to western Massachusetts, and northern Connecticut. Once again, this
will heavily depend on the exact placement of the front.

Saturday & Sunday:

Mid-level ridge continues to deamplify over the weekend with shower
and/or thunderstorm chances both days. Saturday, the aforementioned
backdoor cold front becomes quasi-stationary with weak low pressure
forming along it, mainly across New York, this provides the lift for
showers/thunderstorms. Do think the better chances for any stronger
storms will be just west of southern New England. The front is on
the move on Sunday, as a warm front with showers/thunderstorms once
again. At this point, hesitate to call this upcoming weekend a total
washout. But with anomalously high PWATs, heavier downpours are
possible within any shower and/or thunderstorm.

While it is cooler this weekend, it is still warm and muggy for late
June. Saturday likely cooler of the two days, highs lower to middle
80s, but dewpoints are in the middle to upper 60s. On Sunday,  highs
are a bit warmer, given placement in the warm sector, highs are
between the middle and upper 80s, dewpoints in the upper 60s and low
70s.

Early Next Week:

Mid-level shortwave and cold front sweeps across the region Monday
with showers and thunderstorms. The cold front brings a slight
reprieve from the humidity Tuesday, but signs for another warmer
than normal week continues into mid next week.
-- End Changed Discussion --

&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
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Forecast Confidence Levels:

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

18z TAF Update:

Through 00z Thursday: High confidence.

Mainly VFR. Remains a low (< 20%) chance of a slow-moving TSRA
at BED, ORH and BAF thru 00z; confidence too low to include a
TAF mention but will TEMPO for a more imminent risk. Any t-storm
in and near these TAFs likely to become strong, and producing
frequent lightning, localized downpours bringing brief sub-IFR
visby and localized gusts to 40 kt. SW winds around 10-12 kt.

Tonight: High confidence.

VFR. Potential for IFR-LIFR visby fog after 08z for the Cape
airports and Nantucket. SW winds around 5-9 kt.

Thursday: High confidence in TAF/trends, lower to moderate on
t-storm timing.

VFR at least into the morning hours. While the exact timing is
still uncertain, better chance (~ 25-40%) at ISO/SCT TSRA than
compared to today from the Mass Pike northward. Storms could
develop as soon as 18z but seem more likely around and after
20z. Storms would be capable of frequent lightning, heavy
downpours and localized strong gusts. SW winds around 10-15 kt.

Thursday Night: Low confidence.

Low-confidence aviation forecast Thu night, which will hinge on
how Thu daytime SHRA/TS evolve. These SHRA/TS should weaken and
shift SE Thu night. Will message VFR but there is continued
potential for sub-VFR. Easing SW winds early Thu night could
shift to N/NE under 10 kt by daybreak.

KBOS TAF... High confidence in TAF. VFR with SW winds around
10-12 kt. VFR for the morning hrs Thurs; better chance at TSRA
(~25-40%) on Thurs aftn and messaged as PROB30. Timing
uncertain, could begin as soon as 18z but more likely after 20z.
SW winds on Thurs continue around 10 kt with nil chance at sea
breeze.

KBDL TAF...High confidence in TAF. VFR with SW winds around 8-12
kt, becoming light southerly tonight. Better chance at TSRA
Thurs, although timing uncertain. Could begin as soon as 18z but
more likely after 20z. Messaged as PROB30 for now.

Outlook /Friday through Monday/...

Thursday: VFR. Breezy. Isolated SHRA.

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

Friday: Mainly VFR, with areas MVFR possible. Chance SHRA,
chance TSRA.

Friday Night: Mainly VFR, with local MVFR possible. Chance
SHRA, isolated TSRA.

Saturday: Mainly VFR, with local MVFR possible. Chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.

Sunday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, isolated TSRA.
-- End Changed Discussion --

&&

.MARINE...
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Forecaster Confidence Levels:

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

410 PM update...

Through Thursday Night: High confidence.

High pressure just offshore provides fairly tranquil boating weather
this period. An isolated t-storm could develop over the northern
MA waters tonight, but better chances for thunderstorms for
Thursday. Otherwise, patchy fog may lower vsby 1-3 miles
tonight along the southern coastal waters. Winds mainly SW
around 15-20 kt through the period, with seas 4 ft or less all
waters.

Outlook /Friday through Monday/...

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Patchy fog.

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, isolated thunderstorms.

Friday through Saturday: Winds less than 25 kt. Chance of rain
showers.

Saturday Night: Winds less than 25 kt. Chance of rain showers,
isolated thunderstorms.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Chance of rain showers, isolated
thunderstorms.
-- End Changed Discussion --

&&

.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 18th

BOS - 94 F (1907 and 1929)
BDL - 95 F (1957 and 1994)
PVD - 94 F (1929)
ORH - 93 F (1907 and 1929)

June 19th

BOS - 96 F (1923)
BDL - 95 F (1995)
PVD - 94 F (1923)
ORH - 93 F (1923)

June 20th

BOS - 98 F (1953)
BDL - 97 F (2012)
PVD - 95 F (1941)
ORH - 93 F (1953)

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 18th

BOS - 72 F (1929)
BDL - 70 F (1905)
PVD - 72 F (1929)
ORH - 69 F (1905 and 1994)

June 19th

BOS - 73 F (1995)
BDL - 72 F (1929)
PVD - 70 F (1995)
ORH - 72 F (1929)

June 20th

BOS - 78 F (1931)
BDL - 74 F (1931)
PVD - 74 F (1931)
ORH - 72 F (1931)

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...
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CT...Air Quality Alert until 11 PM EDT this evening for CTZ002-003.
     Heat Advisory until 7 PM EDT Friday for CTZ002>004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ005>007-
     010>015.
     Heat Advisory until 7 PM EDT Friday for MAZ008>011.
     Heat Advisory until 7 PM EDT Thursday for MAZ002>007-012>019-
     026.
     Air Quality Alert from 11 AM to 11 PM EDT Thursday for MAZ007-
     010>021.
RI...Heat Advisory until 7 PM EDT Thursday for RIZ001>005.
     Air Quality Alert from 11 AM to 11 PM EDT Thursday for
     RIZ001>005.
MARINE...None.
-- End Changed Discussion --

&&

$$

SYNOPSIS...Loconto/Dooley
NEAR TERM...Loconto
SHORT TERM...Loconto
LONG TERM...Dooley
AVIATION...Loconto/Dooley
MARINE...Loconto/Dooley
CLIMATE...BL