-- Discussion containing changed information from previous version are highlighted. --
123
FXUS61 KBOX 191803
AFDBOX

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

.SYNOPSIS...
Dangerous heat and humidity continues through Thursday, although not
as hot along the south coast, where winds off the cooler ocean will
provide some relief. The heat peaks on Thursday when record highs
are possible, before more seasonable temperatures and not as humid
weather returns later Friday into Saturday. Showers and
thunderstorms are possible each afternoon and evening, from Thursday
thru Monday, however many hours of dry weather are expected. Heat
and humidity looks to return Sunday and Monday, although not as hot
as the current heat.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --
1100 AM Update:

Key Points:

* Hot and humid weather leading to high heat indices away from
  the southern coastline, excessive to those with prolonged
  exposure.

* Low chance for a t-storm north of the Mass Pike no sooner than
  2 PM til sundown. Any storm which develops will be capable of
  frequent lightning, downpours and localized gusty winds, but
  many stay dry.

Main story continues to be the hazy, hot and humid weather.
Current temps were in the upper 70s to the low 80s with SW winds
around 5-10 mph. The breezes will help somewhat, but offsetting
that will be rising dewpoints leading to increasing mugginess.
Currently dewpoints are in the mid 60s, but dewpoints in the
lower 70s are common in NY and those will be advecting in this
afternoon. So by this afternoon it will feel closer to a sauna,
and heat indices in the mid 90s to lower 100s should be pretty
common away from the South Coast, Cape and Islands. No changes
to heat headlines with this update. We continue to encourage
folks practice recommended heat-related precautions including
staying in cooler/air conditioned rooms or buildings, staying
hydrated, and checking in on pets/animals and the elderly.

Although the vast majority of the time today is dry, I did opt
for a 15% to no more than 20% chance for a t-storm for locations
north of the Mass Pike. While isolated strong storms are
already underway in the Adirondacks into the western shore of
Lake Champlain, that activity should pass well to our north.
Some of the latest guidance shows the upper ridge shifting just
enough southward to at least create a window of opportunity for
isolated storms later this afternoon. The bigger issue is that
there isn`t much to trigger storms, other than a progged subtle
windshift that is basically on the mesoscale level north of the
Mass Pike. Mixed- layer CAPE values are progged to be in the
2500 J/kg range north of the Mass Pike per SPC`s mesoanalysis by
20z and flow is weak; steeper low level lapse rates would also
favor large downdraft CAPE values as well. I suspect storm
coverage to be no more than isolated (a couple?). If we do get
an isolated storm to develop, it could become strong with
frequent lightning and downpours along with localized wind
gusts/outflows given the likely high downdraft CAPE. Higher-res
guidance offers mixed opinions about this but it`s in these
weakly forced, pulse- storm settings where higher resolution
guidance often struggles so view this as more of a possible
outcome. I can`t see any more than a couple storms today if
there`s enough lifting to develop them at all, and much of the
area looks dry.

Previous discussion:

Wednesday...

Subtropical ridge continues to build northeast into New England and
peaks at around 598 dam this afternoon, slightly stronger than
Tuesday. Thus, more of a capped environment should support a mainly
dry forecast. Can`t rule out an isolated, brief shower/t-storm over
across northern MA on the periphery of the ridge. Otherwise, most
locations remain dry today along with mostly sunny conditions. Some
late day cloudiness across northern MA in response to convection
just across the border in NY/VT and NH.

Airmass continues to warm with 850 mb temps rising another deg or
two from Tuesday, up to +19C to +20C by late Wed. 925 mb temps also
rise to +24C to +26C, highest values across northern MA. This will
support a swath of highs of 92 to 97 across northern portions of
CT/RI/MA, hottest across northern MA. These air temps combined with
dew pts in the low 70s will yield heat indices ranging from 95 to
104, hottest values in the Merrimack Valley of northeast MA. Hence,
heat headlines remain in effect.

Not as hot along/near the south coast, in response to SSW winds
streaming across cooler SSTs in the low to mid 60s. This will yield
highs in the 80s, except 70s along the immediate coast including
Cape Cod and the Islands.
-- End Changed Discussion --

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
345 AM update...

* Another warm and humid night
* Heat & humidity peak Thu with highs 93-98, not as hot south coast
* T-storms possible northern CT/RI/MA Thursday

Tonight...

No change in airmass, thus not much relief with lows only 65-70.
Remaining humid with dew pts in the 60s. Any early evening isolated
shower/t-storm along or near the MA/VT-NH border will fizzle quickly
with sunset given lack of deep layer shear and poor mid level lapse
rates. Could have some patchy late night/early morning fog along the
south coast.

Thursday...

Subtropical ridge remains over the region with temps aloft warming a
few more degs, with much of northern MA having 925 mb temps warm to
about +26C. This will support highs in the mid to upper 90s with
hottest values across northern portions of CT/RI/MA, including metro
Boston. These air temps will combine with dew pts in the low 70s to
yield heat indices between 95 and 104, highest values once again
over northern CT/RI/MA. Mostly sunny conditions will continue,
except during the afternoon becoming partly cloudy with possibly
convection across northern MA. Surface winds becoming more WSW as
ridge becomes elongated west to east.

As northern stream trough traverses Ontario and Quebec, some weak
height falls occur on the northern periphery of the tropical ridge,
opening the door for afternoon convection across NY/VT/NH, possibly
spilling into northern MA. Low risk for a few strong storms
especially across northern MA with up to 2000 j/kg of SB CAPE
combined with some weak forcing for ascent and eroding cap on
northern periphery of ridge over northern MA. Also, localized heavy
rain threat with PWATs up to 2 inches and deep unidirectional
westerly flow, promoting backbuilding/training cells. HREF PMM field
has 10% probability of 3 inches of rain in 3 hours across northern
portions of CT/RI/MA. Given the lack of deep layer shear and poor
mid level lapse rates, thinking the threat of heavy rain is greater
than the risk for severe storms. Also, lack of shear and instability
aloft will support afternoon convection weakening with sunset and
cooling blyr.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
345 AM update...

* Some relief from the heat on Friday and Saturday, though heat
  headlines may still be necessary for portions of Connecticut on
  Friday.

* Shower/Thunderstorm chances each afternoon/evening through Sunday,
  with the best chance across CT.

* Stronger shortwave/cold front may bring more widespread
  precipitation/thunderstorm chances late Sunday night and Monday

Strong ridging begins to collapse late week which will allow a cold
front to drop south across southern New England during the day on
Friday. While signals of this began to crop up last evening,
confidence continues to increase with significant model consensus
between the global and long ranged hi-res (NAM12km) guidance, in
more tolerable temperatures developing across the region. There does
remain uncertainty in how far south and west the front will
progress, which will likely lead to a significant temperature
gradient Friday afternoon across the region, with portions of the CT
River Valley making (yet another) run at 90F and temps across
eastern MA cooling into the 70s thanks to onshore flow behind the
front. Given growing confidence in cooler conditions, heat headlines
have been dropped away from the CT River Valley on Friday.

Showers and thunderstorms are expected to develop along the
"ridge", well collapsing ridge, axis Friday afternoon, which is
currently expected to be concentrated across central Connecticut.
Low level lapse rates around 8C/km combined with MUCAPE values
approaching 2200J/kg will drive storm development, but with really
subpar mid level lapse rates (around 5C/km) and the best bulk shear
flip-flopped from the greatest instability (bulk shear of less than
20kt across CT Friday afternoon/evening, but higher across eastern
MA where CAPE is much lower), we`ll likely see some pulsey sub
severe storms that could generate a heavy downpours given a "skinny
CAPE" and PWATs exceeding 2". Storms will struggle to survive past
~02Z, however, as instability completely collapses after sunset.
It`s sort of a "copy paste" scenario for Saturday with little change
in the airmass. Isolated to scattered thunderstorms are again
possible across the region, though would hedge fewer localities see
rain Saturday afternoon compared to Friday as both temperatures and
dewpoints around 5F cooler.

Showers/thunderstorms look to grow in coverage Sunday afternoon and
evening as robust shortwave begins to approach from the Great Lakes
region. Will likely see several rounds of precipitation between
Sunday afternoon and Monday evening before trailing cold front
finally moves eastward of the region by late Monday/early Tuesday.
The first round of storms Sunday afternoon will again struggle to
persist past sundown as instability is lost. Beyond Monday, it
appears that another prolonged period of warm and dry conditions
develops for next week.

&&

.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --
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 /Thursday Night through Sunday/...

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...
Forecaster Confidence Levels:

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

345 AM update...

Through Thursday...High confidence.

High pressure just offshore provides fairly tranquil boating weather
this period. Low risk of an isolated thunderstorm this afternoon
across the northern MA waters. Otherwise, patchy fog may lower vsby
1-3 miles tonight along the southern coastal waters. Thu, quiet
except across the northern MA waters where a few thunderstorms may
traverse the waters toward the NH border.

Outlook /Thursday Night through Sunday/...

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.

&&

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

&&

$$

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