699
FXUS61 KBOX 201333
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
933 AM 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/...

930 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 severe t-storms likely this PM...mainly in northern MA
  capable of locally damaging wind gusts & torrential rainfall

Previous forecast is on track. Another day of dangerous heat and
humidity with afternoon highs in the middle to upper 90s away
from the south coast and a low risk a few towns hit 100 degrees.
Afternoon Heat Indices in the 95 to 105+ range and a few
locations may briefing exceed 105 degrees. Heat Headlines
continue away from the south coast. 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.

The other big issue revolves around scattered t-storms this
afternoon and evening with the focus in northern MA. Despite
limited effective shear struggling to exceed 20 knots...robust
instability for our part of the country with afternoon MLCapes
exceeding 2000 J/KG and perhaps up to 3000 J/KG will develop.
Steep low level lapse rates will be in place as well. The
Machine learning probs from the CSU to the Nadocast and HRRR
Neurel network are showing a fairly robust signal for localized
damaging wind gusts possible. This makes sense given the unusual
amount of instability coupled with steep low level lapse rates.
In addition...SPC Meso- Analysis indicates DCape to exceed 1000
J/KG which is another solid signal. Main risk will be across
northern MA...but may see a few storms migrate further south
given the instability in place. In addition...Pwats of 2+ inches
will support torrential rain with any of these t-storms and a
very localized flooding threat.

&&

.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 /13Z 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.

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...Nocera/KS
MARINE...Frank/Nocera/KS
CLIMATE...BL