077
FXUS61 KBOX 131436
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1036 AM EDT Sat Jul 13 2024

.SYNOPSIS...
Showers, and some downpours, into early afternoon. Hot on
Sunday, but slightly drier with sunny skies. Dangerous heat and
humidity is expected Monday, Tuesday, and Wednesday with
afternoon heat indices between 95 and 105 degrees. The greatest
risk for scattered showers and thunderstorms will be Monday and
Wednesday, but much of this time will still feature dry weather.
Relief from the heat and humidity arrives by Thursday and
Friday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Still a risk for embedded thunderstorms this morning, but losing
confidence that we will see them. Have not seen active lightning
during the past couple of hours. Latest SPC mesoanalysis showed
a similar picture. Plenty of buoyancy, but poor mid level lapse
rates. Shear was offset to the northwest, so not expecting
severe weather.

Updated rainfall chances using the latest runs of the HRRR,
which seemed to have the best handle on the existing rainfall.
Will reconsider the Flood Watch later today, as will likely be
able to drop portions prior to the existing expiration of 8 PM.
Minor tweaks to temperatures to reflect observed trends.

645 AM Update...

* Showers with a few t-storms and localized torrential rainfall
  into mid afternoon with the greatest risk south of I-90

A cluster of showers and embedded t-storm quickly moved across
areas near the south coast earlier this morning and were
already exiting the Cape. However...an additional band of
showers has set up in northeast axis in the vicinity of a
Springfield to Fitchburg line. So far rainfall rates have been
under control...but given Pwats of 2+ inches in place and
dewpoints between 70 and 75 any of this activity will be capable
of producing torrential rainfall and very localized
flooding...especially if it occurs in a vulnerable urban center.
The mesoscale processes involved though make it very difficult
to locate where this may occur...but latest guidance continues
to indicate areas south of I-90 most at risk. Thinking is that
the flood threat will be localized...but worth continuing the
Flood Watch given the tropical environment in place. Embedded
thunder is also possible with any of the activity especially
across RI/SE MA. Previous Discussion below.

Short-wave energy approaching from the west and convergence
along a stalled frontal boundary is expected to support bands of
showers with heavy downpours and embedded thunder this morning.
These showers will have the potential to produce excessive
rainfall as the atmosphere will hold tropical like
characteristics with 2+ inch PWATs, deep warm cloud layer depths
exceeding 11000 feet, and moderate instability with SBCAPE
values near 1000 J/kg. Given the weakly forced environment,
there remains quite a bit of uncertainty with respect to where
the heaviest rainfall will setup. Latest hi-res model guidance
has trended toward southeastern MA, RI, The Cape and Islands
being at the greatest risk for excessive rainfall. Nonetheless,
several solutions still support bands of heavy precip across
western MA and CT. Given the uncertainty, no changes have been
made to the flood watch headlines currently in place.

As previously forecast, most of the shower/thunderstorm activity
will take place before noon time today. Thereafter, we should
see some clearing from northwest to southeast as the 2+ inch
PWAT plume regresses to the southeast. After noon time
however,the equatorward entrance region of a 100 knot 250 hPa
jet is expected to develop over the northern half of the
forecast area. Some hi-res model guidance has been hinting at
the potential for this feature to provide additional forcing to
support redevelopment of a few thunderstorms across interior
southern New England during the mid- afternoon/evening hours.
Confidence in this outcome is low, but additional thunderstorms
after this morning`s "main event" will be possible before
sunset. Aside from the shower/thunderstorm chances, expect
continued muggy conditions with highs in the mid to upper 80s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Tonight

A surface frontal boundary moves through the region overnight
with northwest flow and a drier airmass. PWATs fall to values
close to an inch. This should result in a quiet evening with
clearing skies from northwest to southeast. There will be some
relief from the humidity as dewpoints fall into the mid to upper
60s across the region, but the urban areas will still struggle
to fall below 70.

Tomorrow

A subtle mid-level ridge builds over southern New England
tomorrow with weak high pressure at the surface. This will
support a sunny/hot afternoon. Not expecting humidity to be as
oppressive as with previous days as light west/northwest flow
will allow dewpoints to drop into the low to mid 60s across much
of interior southern New England. Humidity may linger over the
southeastern areas with dewpoints still in the low 70s. Expect
highs in the upper 80s to low 90s with some mid 90s possible in
the CT River Valley. Coastal areas may be slightly cooler as the
gradient wind may be weak enough to support sea-breezes along
the coast.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Key Points...

* Dangerous Heat & Humidity Mon/Tue/Wed...Heat Indices 95-105
  degrees
* Greatest risk for scattered showers & t-storms Mon and
  especially Wed
* Relief from the Heat & Humidity by Thu and Fri

Details...

Mon/Tue/Wed...

Excellent model agreement in strong upper level ridge in the
Atlantic and shortwave energy in Canada dropping towards the
Great Lakes. This will result in westerly flow aloft with
above normal height fields and high confidence in dangerous heat
and humidity. High temps Mon/Tue/Wed should be well into the
90s in most locations away from any potential localized marine
influences near the immediate coast. Dewpoints in the lower to
middle 70s will result in very humid conditions...bringing
afternoon Heat Indices into the 95 to 105 degree range each day.
Heat Headlines will certainly be needed. In fact...the worst of
the heat appears to be Tue & Wed when we may even need some
Excessive Heat Headlines for the potential of Heat Indices of
105+. Regardless of the specific temps...the main story will be
dangerous heat and humidity in the Mon through Wed time frame.

The other issue will be assessing the potential for scattered
showers and thunderstorms. Westerly flow aloft with the
mesoscale part of it this time of year makes that quite a
challenge. For what its worth...most of the guidance indicates
Mon and especially Wed with the greatest risk for a round of
scattered showers and thunderstorms...but can not rule out a few
on Tue either. That being said...this is typical summertime
convection so much of this time will still feature dry weather.
Given the anomalous hot and humid airmass in place...instability
will likely be high for this part of the country. In
addition...appears we may have some sort of remnant EML to work
with as well. So if wind fields aloft end up strong enough there
will be the potential for severe weather. The CSU Machine
Learning probs also continue to indicate this potential in the
Mon through Wed time frame...so will need to continue to monitor
this potential.

Thu/Fri...

We may be dealing with some lingering showers for the first part
of Thu...depending on the timing of the front. However...the
main story will be High pressure building in from the midwest
behind the cold front. The result will be relief from the Heat &
Humidity by Thu and Fri with seasonable or even slightly below
normal temperatures. Quite the welcomed change from the first
half of the upcoming week!

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:

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

Today...Moderate Confidence

Areas of showers, with some downpours, for most of southern New
England through about 18z. There is uncertainty on the overall
coverage of TSRA, but the greatest risk is south of the MA
Turnpike. Conditions should begin to improve after 18Z with
VFR conditions settling in, but there is a risk for some
redevelopment of scattered showers and a few storms mainly
across the interior in northern and western MA between about 18z
and 21Z. Confidence not high enough to include mentions of
thunder during this time period, but did leave VCSH in the TAF
update through 00Z. As for ceilings, they are rather variable
this morning, but expect a lot of MVFR conditions. Those should
tend to improve to VFR this afternoon, except for areas near
the south coast, where MVFR-IFR conditions will probably
persist.

Tonight...Moderate Confidence

Any remaining scattered showers should diminish during the early
evening. Otherwise...MVFR-IFR ceilings will pretty much be
confined to areas near the south coast. However...light-calm
winds/high dewpoints may allow for patchy ground fog to develop
especially in the typically prone locations.

Tomorrow...High Confidence

VFR. Light northwest winds becoming more westerly during the
afternoon. Winds may be weak enough to support sea-breezes along
the coast.

KBOS TAF...Moderate confidence in TAF.

KBDL TAF...Moderate confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR.

Monday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHRA, chance TSRA.

Monday Night: Mainly VFR, with areas MVFR possible. Slight
chance SHRA, slight chance TSRA.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA, slight chance TSRA.

Tuesday Night: Mainly VFR, with areas MVFR possible. Breezy.
Slight chance SHRA, slight chance TSRA.

Wednesday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHRA, chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Marine


Today

Showers with embedded thunder move over the coastal waters this
morning. This will be followed by some clearing during the
afternoon. Winds remain steady out of the southwest. Seas
generally 3 to 5 feet over the southern marine zones and 1 to
3 feet over the eastern marine zones.

Tonight and Tomorrow

Weak high pressure builds over the coastal waters tonight into
tomorrow. This will support diminishing winds and subsiding seas
with relatively calm conditions over the marine zones.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Winds less than 25 kt.

Monday: Winds less than 25 kt. Slight chance of rain showers,
slight chance of thunderstorms.

Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers, slight chance of
thunderstorms.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of seas approaching 5 ft. Slight chance of
rain showers, slight chance of thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch until 8 PM EDT this evening for CTZ002>004.
MA...Flood Watch until 8 PM EDT this evening for MAZ002>021-026.
RI...Flood Watch until 8 PM EDT this evening for RIZ001>007.
MARINE...Small Craft Advisory until 9 PM EDT this evening for ANZ255-
     256.

&&

$$

SYNOPSIS...Frank/RM
NEAR TERM...Belk/Frank/RM
SHORT TERM...RM
LONG TERM...Frank
AVIATION...Belk/Frank/RM
MARINE...Frank/RM