861
FXUS61 KBOX 201119
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
719 AM EDT Thu Jun 20 2024

.SYNOPSIS...
Hot and humid weather continues Thursday. Another round of
thunderstorms is expected Thursday 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/...

7 AM Update:

Very minor changes made to bring forecast in line with
observations and expected convection this afternoon.

350 AM Update:

* Dangerous heat & humidity again today with near record highs away
  from the south coast. Low prob a few towns briefly hit 100

* A few strong to severe PM storms possible in northern MA

Subtropical ridge remains in place one more day with dangerous heat
and humidity remaining over SNE. Airmass warms a few more degs today
with multiple models offering 925 mb temps up to 26C to 27C, hottest
across northern MA. This combined with at least partly to mostly
sunny conditions (before afternoon convection fires), will yield
near record highs in the mid to upper 90s across much of northern
MA, low 90s for the high terrain. Low to mid 90s for northern
portions of CT/RI and into interior SE MA.  These dangerous
temperatures will combine with oppressive dew pts in the low 70s to
yield heat indices of 95 to 104 across much of MA away from the
south coast and into northern portions of CT/RI. Southwest winds off
the relatively cooler ocean will provide some relief once again and
limit highs to the 80s across southern RI/MA, including Cape Cod and
mid to upper 70s for the Islands.

As for afternoon thunderstorms, weak short wave energy traversing
the backside of the ridge is firing convection early this morning
across Lake Erie/Ontario and into western NY. This short wave energy
will traverse NE into central and northern New England, helping to
erode the northern periphery of the ridge via height falls. This is
the greatest area of concern for strong to severe storms, across
NYS/VT/NH/ME, which is supported by CSU machine learning probs and
HREF max potential updraft. Northern MA will be on the southern
periphery of these height falls and marginal shear. Low confidence
convective forecast given hi res guid typically struggles in these
weakly forced environments. However, the wildcard will be strong SB
instability with 1500-2500 j/kg, which may be sufficient in breaking
the mid level cap where weak deep layer shear and height falls
overlap across northern MA. Farther south across CT/RI and southeast
MA, deep layer shear decreases to less than 20 kt along with
weakening height falls. Hence, lower probs of storms across CT/RI
and southeast MA today. Given magnitude of SB instability combined
with inverted / sounding signature, kept previous enhanced wording
in the forecast for northern MA, gusty winds and brief heavy
downpours possible, with PWATs up to 2 inches along with weak WNW
unidirectional flow aloft.

&&

.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 /11Z THURSDAY THROUGH MONDAY/...
Forecast Confidence Levels:

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

12z TAF Update:

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

VFR and modest SW winds except MVFR in thunderstorms with cloud
bases around 3000ft and vsby reductions to around 3SM. Greatest
risk for thunderstorms is across northern MA into VT/NH/ME. A
few strong to severe storms are possible 18z-00z. Storms would
be capable of frequent lightning, heavy downpours and localized
strong gusts. SW winds around 10-15 kt. Farther south across
CT/RI and southeast MA, much lower probability of any storms.
Brief MVFR possible with any storm.

Thursday night: high confidence on trends, but lower on exact
timing.

Any early evening thunderstorms across northern MA will dissipate
with sunset. Easing SW winds early Thu night could shift to N/NE
under 10 kt by daybreak.

Friday: high confidence on trends, lower on exact details.

Greatest risk for storms is across CT into western-central MA. A few
strong to severe storms possible in this region during the
afternoon. Elsewhere, risk of storms is much lower. Mainly VFR but
lowering to MVFR in PM storms. NE winds around 10 kt except SW winds
across CT, eventually becoming NE.

KBOS TAF...High confidence in TAF. Low probability of seabreeze 15z-
18z with direction of 160 degs, but higher probability winds remain
SW. Chance of thunderstorms as early as 17z and possibly as late as
02z. Although, many hours of quiet/dry weather in between.

KBDL TAF...High confidence in TAF. TSRA Thu PM, although timing
uncertain. Could begin as soon as 18z but more likely after 20z.

Outlook /Friday Night through Monday/...

Friday: Mainly VFR, with local MVFR possible. Scattered SHRA,
scattered TSRA.

Friday Night through 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.

&&

.MARINE...
Forecaster Confidence Levels:

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

350 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.

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 from 11 AM this morning to 11 PM EDT this
     evening for CTZ002-003.
MA...Heat Advisory until 7 PM EDT Friday for MAZ008>011.
     Air Quality Alert from 11 AM this morning to 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 from 11 AM this morning to 11 PM EDT this
     evening for RIZ001>005.
     Heat Advisory until 7 PM EDT this evening for RIZ001>005.
MARINE...None.

&&

$$

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