653
FXUS61 KBOX 202038
AFDBOX

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

.SYNOPSIS...

Scattered strong to severe thunderstorms remain possible this
evening...mainly across northern MA where a Severe Thunderstorm
Watch is in effect. A backdoor cold front moves in from northeast to
southwest Friday...bringing another round of showers and
thunderstorms. The bulk of this activity should be across interior
Massachusetts and Connecticut...where some of the storms may become
severe and also result in a localized flash flood threat. Warm
and muggy this weekend with rain and thunderstorm chances both
Saturday and Sunday. A cold front sweeps across the region Monday
with additional showers and storms, briefly less humid on Tuesday,
but summer warmth and humidity returns for midweek.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

435 PM Update

* Severe T-Storm Watch for northern MA into this evening

* Main risks are localized damaging wind gusts & torrential rainfall
  which may result in a very localized flash flood threat

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

Scattered t-storms that developed from elevated heating along the
Berks have pushed east and become strong to severe. The main severe
weather threat will be locally damaging straight line wind gusts
given steep low level lapse rates and DCape values of 1000+ J/KG.
Some hail is also possible with any big cores. In addition...these
storms will be capable of producing torrential rainfall given Pwats
of 2+ inches. The storms will also be moving rather slow...so there
will be a localized flash flood risk into this evening.

There also was a more organized area of convection to our north
across central VT/NH in association with a shortwave. The guidance
has been struggling with how far south this activity will survive as
it moves away from the stronger forcing. We think that northern MA
is most at risk from this activity from 5 to 10 mph this
evening...given high resolution CAM simulations along with MLCapes
on the order of 2000 to 3000 J/KG

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 across northern MA into this
evening...but can not rule out a few storms working further south
given the amount of instability.

So in a nutshell...the main risk of scattered severe t-storms will
be this evening through about 10 pm in northern MA. 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 flash flood threat.

Once this activity winds down later this evening...much of the
overnight hours should feature dry/muggy weather. Overnight low
temps will mainly be in the upper 60s to the lower 70s. We also may
see some patchy ground fog develop overnight given light winds/high
dewpoints especially where rain falls earlier in the evening.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...

Key Points...

* Showers & t-storms anticipated Fri PM with the severe weather
  threat/very localized flash flood threat across interior MA/CT

* Much cooler across eastern MA but still hot & humid towards the CT
  River Valley where Heat Advisories remain posted

Details...

Friday...

A backdoor cold front will be moving westward across the region
Friday morning and into the afternoon. This will hold high temps in
the 80s across much of eastern MA and temps along the immediate
coast will probably fall into the 70s by afternoon. Further
inland...high temps will top off in the upper 80s to the lower 90s
with the warmest of those readings in the CT River Valley. The Heat
Advisory for Friday will only cover portions of southwest
MA/CT...where Heat Indices are expected to top off between 95 and
100 degrees.

The other concern will be for another round of showers & t-
storms...which looks to be Friday afternoon/early evening. The focus
for this will be along the backdoor cold front and will be enhanced
by shortwave energy in west to northwest flow aloft. While a few
showers & t-storms may impact the entire region...thinking the main
focus for the potential of severe weather/localized flash flooding
will be across western/central MA & northern CT. This is where the
greater instability will reside on the order of 1500 to 2000 J/KG.
These backdoor cold fronts can be a recipe for training and a
localized flash flood threat. The CSU Machine learning probs
highlight this risk nicely with again the focus across the interior.
There also is a severe weather risk with the main concerns being
locally damaging wind gusts with hail a secondary risk. This is
depicted nicely by the machine learning probs as well as the HREF
Updraft Helicity Swaths/Radar Simulations.

Friday night...

The convection should dissipate Friday evening. Otherwise...we will
have to watch for areas of low clouds and fog developing. Overnight
low temps will be mainly in the 60s.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

Highlights:

* 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.

The mid-level ridge continues to deamplify, as 500mb heights lower
from north to south, which is a consistent theme for the upcoming
weekend. At the surface an area of higher pressure near Bermuda
wanes and surface pressure lowers across the northeast. This area of
low pressure developing over the northern Great Lakes, drives our
weather conditions late this weekend into Monday, as the trailing
cold front, south of the low, sweeps across the northeast sometime
Monday. Beyond Monday, high pressure builds back Tuesday through
Thursday, with a cold front possibly pushing across the region later
on Friday/Saturday.

In addition, most of next week trends warmer than normal, and at
this point, next week doesn`t look to be as extreme as this current
week. Though the ensemble situational awareness tables indicates
Monday through Thursday, surface temperatures could be running
between the 90th and 99th percentile (of climatology), with a return
of average temperatures post cold front, sometime late Friday into
Saturday.

Saturday & Sunday:

Getting closer to home, there is a quasi-stationary frontal boundary
across the region, running from east to west through southern New
England and through Upstate New York on Saturday. Weak area of low
pressure tries to develop along the boundary, with the greatest area
for showers and storms along and north of the Mass Pike during the
mid to late afternoon. At this point, not expecting anything severe,
and SPC has most of the region under general thunder. A slightly
more elevated issue would be the potential for excessive rainfall.
The ERO from WPC highlights much of central/western Massachusetts
and northern Connecticut for a `Marginal` risk. Given PWATs are
above two inches, isolated and locally heavy downpours could lead to
urban street and poor drainage flooding.

Aforementioned frontal boundary becomes a warm front Saturday night
into early Sunday, lifting northeast, and placing us in the warm
sector. Scattered showers into the overnight hours, though the best
forcing appears to be concentrated further north. A prefrontal
trough and shortwave pivot through, with showers and thunderstorms
later Sunday afternoon/evening.

As for temperatures, Saturday will be the `cooler` day, the warmest
location will be the Connecticut River Valley, highs here are in the
mid 80s. Much cooler in northwest Massachusetts where highs are low
to middle 70s with the northeast flow. Nightly lows are still mild
which isn`t ideal for sleeping with windows open, lows are in the
middle to upper 60s to 70 degrees in the Connecticut River Valley on
Saturday night, then 2-4 degrees warmer Sunday night.

Next Week:

Warm and muggy day ahead of a surface cold front with temperatures
climbing into the 80s and dew points in the 60s. Storms develop mid
to late afternoon with the surface cold front, a bit a relief behind
the front with lowering dew points into the 50s and leads to a nice
Tuesday with comfortable, albeit warm day, with dew points in the
upper 50s to 60F. High pressure reestablishes across the northeast
leading to a drier week with return of summer warmth. Highs are in
the middle and upper 80s and lows in the 60s with a sign of cooler
temperatures by late in the week.

&&

.AVIATION /21Z THURSDAY THROUGH TUESDAY/...
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 with the main threat
north of the MA Turnpike through 02z. 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 another round of 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 with locally torrential rainfall. 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.

KBDL TAF...Moderate Confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: MVFR/IFR conditions possible. Chance SHRA, isolated
TSRA, patchy BR.

Saturday Night: Mainly IFR, with local MVFR possible. Chance
SHRA, isolated TSRA, patchy BR.

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

Sunday Night: Mainly VFR, with areas IFR possible. Breezy. SHRA
likely, isolated TSRA, patchy BR.

Monday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA, isolated TSRA, patchy BR.

Monday Night: VFR. Breezy. Chance SHRA.

Tuesday: VFR.

&&

.MARINE...

Forecaster Confidence Levels:

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

Tonight through Friday night...High Confidence.

Nearshore small craft wind gusts will linger from areas near the
southeast New England coast into early this evening...so small craft
headlines continue in this region through 8 pm. Other risk tonight
will be a few strong t-storms which may threaten our northern waters
through about 10 pm. We also will need to watch for some fog
developing overnight.

The backdoor cold front will move across our northern waters Fri
morning...but likely stall across our southern waters Fri afternoon
and night. Pressure gradient will be weak enough to keep winds/seas
below small craft thresholds. We will need to watch for areas of fog
developing during the overnight hours.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Chance of rain showers, patchy
fog, isolated thunderstorms. Visibility 1 to 3 nm.

Saturday Night: Winds less than 25 kt. Chance of rain showers,
isolated thunderstorms, patchy fog. Visibility 1 to 3 nm.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. Slight chance of rain showers,
isolated thunderstorms, patchy fog. Visibility 1 to 3 nm.

Sunday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Rain showers likely,
isolated thunderstorms, patchy fog. Visibility 1 to 3 nm.

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Rain showers likely, isolated
thunderstorms, patchy fog. Visibility 1 to 3 nm.

Monday Night: Winds less than 25 kt. Local rough seas. Chance
of rain showers, isolated thunderstorms, patchy fog. Visibility
1 to 3 nm.

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.

&&

.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...Frank/Dooley
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Dooley
AVIATION...Frank/Dooley
MARINE...Frank/Dooley
CLIMATE...BL