-- Discussion containing changed information from previous version are highlighted. --
566
FXUS61 KBOX 201815
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
215 PM EDT Thu Jun 20 2024

.SYNOPSIS...
Dangerous heat and humidity on tap for today. Another round of
thunderstorms is expected this afternoon/evening across
northern MA including the Greater Boston area. Some of the
storms could become strong to locally severe. A backdoor cold
front moves in from northeast to southwest Friday, though there
remains uncertainty how far inland the front tracks. Showers and
thunderstorms are likely along the front Friday afternoon,
especially across CT and western-central MA. Scattered
thunderstorms possible each day this weekend as well as Monday
ahead of cold frontal passage that ushers in drier airmass next
week. Aside from Sunday, more seasonable temperatures can be
expected for the majority of the long term forecast.

&&

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

-- Changed Discussion --
215 PM Update...

* Severe T-Storm Watch for northern MA through 8 PM with
  damaging wind gusts, torrential rain/localized flooding

* A few storms are possible further south especially in the high
  terrain along the Berks this afternoon and evening

A Severe thunderstorm watch has been issued for 8 pm for
northern MA. Despite limited effective shear struggling to
exceed 20 knots...robust instability is in place for southern
New England standards with afternoon MLCapes already over 2000
J/KG in northern MA and downdraft Capes were currently around
1000 J/KG. There are a few storms trying to develop now as a
result of the elevated heating along the Berks...but across the
rest of the region there is just not enough forcing yet. The
biggest question remains the southward extent of the main
activity developing across northern New England. While some of
the radar simulations keep the worst of the activity to our
north...certainly could envision the activity impacting northern
MA as guidance like the HREF suggests given the amount of
instability and Dcapes. The Machine learning probs from the CSU
to the Nadocast and HRRR Neural network are showing a quite
robust signal for localized damaging wind gusts possible. In
addition...Pwats of 2+ inches will support torrential rain with
any of these t-storms and a localized flooding threat especially
with the slow movement of the storms.

So in a nutshell...a Severe T-Storm Watch is in effect for
northern MA until 8 pm this evening. This is on the southern
extent of the main forcing from northern New England...but a few
storms could push further south from outflow/instability.
Also...these storms will be capable of producing torrential
rainfall and a localized flood threat.
-- End Changed Discussion --

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...

350 AM update...

* Early evening storms dissipate after sunset
* Another warm/humid night
* Storms Fri likely focused across CT & western-central MA

Thursday night...

Any early evening storms across northern MA will come to an end by
10/11 pm, given lack of deep layer shear and poor mid level lapse
rates to support nocturnal convection. Otherwise, dry weather
prevails along with another very warm and humid night, with dew pts
in the upper 60s to lower 70s.

Friday...

Northern stream short wave trough moves across eastern Quebec and in
its wake, sends a backdoor cold front traversing our region from
northeast to southwest. This front will break the heat but also
serve as focus for possible convection, especially across CT and
western-central MA, on the warm side of the boundary. First temps,
warm sector airmass is confined to CT and western-central MA where
925 mb temps are still about +23C. If enough sunshine materializes,
this could be a 4th consecutive day of 90+ temps for portions of CT
into western-central MA. Heat indices of 95+ are possible, thus a
heat advisory remains posted there for Friday. Not as hot elsewhere
with highs in the 80s, except mid to upper 70s along the eastern MA
coast, in response to NE flow behind the backdoor front, streaming
across SSTs in the 60s. Humidity not as oppressive as previous days,
but still elevated with dew pts in the 60s.

As for convective potential, modest height falls will be accompanied
by modest deep layer shear and mid level moisture to support another
round of storms. Storms likely most numerous across CT into western-
central MA in the warm sector and also vicinity of the backdoor
front where low level convergence will be maximized. Modest SB
instability of 1000-1500 j/kg of CAPE and marginal deep layer shear
will provide a low risk of storms with strong winds. Perhaps a
somewhat higher risk is heavy rain/flood threat, with PWATs 2+
inches, slow moving frontal boundary and unidirectional flow aloft
for potential back building storms. HREF highlights this potential
with a 30% of 3 inches of rain in 3 hours from 18z-21z Fri and again
21z-00z Saturday.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...

350 AM update...

* Scattered thunderstorms possible Saturday through Monday with best
  chance for storms coming each afternoon/early evening

* Cold frontal passage on Monday will usher in a much drier period
  for the middle of next week

* Brief reprieve from oppressive heat Saturday before heat builds
  back Sunday. More seasonable airmass next week behind the frontal
  passage

Saturday...

Synoptic pattern changes very little for Saturday as we remain on
the northern fringes of the retrograding ridge with a
quasistationary cold front draped somewhere across southern
Connecticut/southeastern New York. The location of this frontal
boundary will again be key in evaluating the scattered thunderstorm
threat for Saturday, as a rather stable airmass will continue to
exist over at least the eastern half of our CWA given onshore
surface flow, with southerly flow to the south and west of the
boundary. It is along and west of this convergence zone, that could
potentially exclude all of southern New England, that some scattered
storms can be expected.

*If* the boundary shifts a bit more "northeast", putting portions of
SNE in the warm sector, storm development will be delayed until
after 15Z or so with a capping inversion in place, thus, we`ll have
to wait for surface instability to build to overcome the cap.
Compared to Friday, bulk shear is a bit more robust, around 30-35kt,
in the area of greatest instability, which could support some longer
lived cells. Lapse rates look, average, around 6C/km and with DCAPE
values between 300-500J/kg and freezing levels at almost 15,000ft,
the damaging wind and hail threats remain low in the absence of very
intense updrafts. The greatest threat associated with any stronger
cells would be pockets of heavy rain given another day of "tall
skinny CAPE" and PWATs around 2". A quick inch or two of rain
leading to isolated poor drainage or urban flooding is possible if
longer lived cells develop.

If we do "warm sector" and see some storms develop, surface based
instability will dissipate very quickly after sunset, which will
mitigate any marginal severe threat beyond 01Z. There will remain
some elevated CAPE, which may allow some showers and perhaps a
rumble of thunder to linger past 03Z Saturday night. The NBM 3
hourly thunder probs give a reasonable estimate of where showers and
storms will be possible, generally isolated to the Connecticut River
Valley.

Sunday/Monday...

Sunday, and in particular Sunday evening, looks to be more active as
low pressure/shortwave moves east from the Great Lakes. A few waves
of energy will provide enhanced lift and with bulk shear values
climbing north of 40kt, expecting to see some longer lived
convection compared to Saturday. Will note, however, that the
shortwave has become increasingly LESS amplified in models over the
last 24 hours, which does introduce some uncertainty regarding storm
coverage, as a weaker wave = less forcing = fewer storms.

MUCAPE values exceeding 2000J/kg will again wane quickly after
sunset on Sunday with the actual frontal passage delayed until
sometime mid-day Monday. Thus, expecting a lull in showers/storms
overnight Sunday before a potential second round of convection
during the daylight hours of Monday. A much more comfortable airmass
develops behind the front late Monday with dewpoints dropping into
the low 60s.

Next week...

Cold frontal passage early next work week looks to usher in a much
more seasonable, albeit still warm, airmass for mix next week.
Precip chances look rather scarce with an apparent lack of forcing
mechanisms.

&&

.AVIATION /18Z THURSDAY THROUGH MONDAY/...

-- Changed Discussion --
Forecast Confidence Levels:

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

18z TAF Update...

This afternoon & evening...Moderate Confidence.

VFR outside of any scattered showers/t-storms...which are the
main concern with this TAF issuance. We have VCTS in the BDL/BAF
terminals and did include a TEMPO for the next few hours in BAF
where storms are forming to their west. We also included TEMPO
groups in the 20z-24z time range for ORH/BED/BOS...but will
focus in more on specific timing once that becomes clear. SW
winds of 5 to 15 knots with some gusts up to 25 knots towards
the Cape and Islands.

Late This Evening & Overnight...Moderate Confidence.

Mainly VFR once the scattered t-storms wind down by late
evening. However...will have to watch for some patchy ground fog
and reduced vsbys /MVFR-IFR levels/ especially where ground
becomes wet from the earlier showers & t-storms. Light SW winds.

Friday...Moderate Confidence.

VFR except for lower conditions in scattered showers &
t-storms. Main risk for a few strong to severe storms will be across
western/central MA & northern CT Friday afternoon/early
evening. Backdoor cold front will shift the winds to the NE as
the day wears along.

Friday night...Moderate Confidence.

Bulk of the scattered showers/t-storms should wind down Friday
evening. Otherwise...light onshore flow coupled with a cooling
boundary layer will allow MVFR-IFR ceilings to develop.

KBOS TAF...Moderate Confidence in TAF. The main uncertainty is
timing any threat for t-storms impacting the terminal.
Currently...thinking 20z-24z is the main risk but will need to
evaluate latest radar trends and fine tune that.

KBDL TAF...Moderate Confidence in TAF. The main issue revolves
around t-storms currently to the north of the terminal. Greatest
risk is from 20z-24z...but again will need to fine tune that.


Outlook /Friday Night through Monday/...

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

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

Sunday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA, isolated TSRA.

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

&&

.MARINE...
Forecaster Confidence Levels:

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

930 AM Update...

Through Friday: High confidence.

High pressure just offshore provides fairly tranquil boating weather
much of this period. Scattered thunderstorms this afternoon across
the northern MA waters. A few strong storms possible into early Thu
evening. SW winds 10 to 15 kt, then shifting to the NE Friday.
Thunderstorm probability shifts to the RI waters Fri. We did opt
to hoist a short-fused small craft advisory for the nearshore
waters towards the Cape/Islands and Buzzards Bay this afternoon
and early evening. Good mixing over the land should yield some
SW wind gusts of 25 knot nearshore resulting in some choppy
seas.

Outlook /Friday Night through Monday/...

Friday: Winds less than 25 kt. Isolated rain showers.

Friday Night through 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. Areas of seas approaching 5 ft. Slight 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: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Rain showers likely, 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 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 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 MAZ007-
     010>021.
     Heat Advisory until 7 PM EDT this evening for MAZ002>007-
     012>019-026.
RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>005.
     Heat Advisory until 7 PM EDT this evening for RIZ001>005.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
     ANZ231>234-254-255.

&&

$$

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