192
FXUS61 KBOX 130703
AFDBOX

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

.SYNOPSIS...
Showers with heavy downpours and embedded thunder this morning
into early afternoon. Hot on Sunday but slightly drier with
sunny skies. Dangerous Heat & 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/...
3 AM Update...

Today

KBOX radar picking up on some light returns across western CT
and the coastal waters south and west of ACK/MVY. 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 & 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 /07Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:

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

Through 12Z...Moderate Confidence

MVFR/IFR conditions persist through 12Z. Showers with embedded
thunder begin to overspread southern New England from southwest
to northeast during the hours leading up to 12Z.

Today...Moderate Confidence

Showers with heavy downpours and embedded thunder for most of
southern New England through about 18Z. Confidence in TSRA
is highest at PVD and Cape/Islands terminals. Moderate
confidence that BED/BOS will be impacted by storms near or over
the terminal between about 12-16Z. Lowest confidence in TSRA is
at BED and BDL. Conditions should begin to improve after 18Z
with VFR conditions settling in, but there is a risk for some
redevelopment of showers and storms mainly across the interior
between about 18 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.

Tonight...Moderate to High Confidence

Any remaining showers or storms should diminish shortly after
00Z. VFR across the interior with MVFR/IFR ceilings over The
Cape/Islands terminals. Winds remain steady out of the
southwest through 06Z and shift to light out of the northwest
thereafter.

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...Low to Moderate confidence in TAF.

Conditions deteriorate to MVFR through 12Z. Showers with heavy
downpours and embedded thunder are expected to be in the
vicinity of the terminal this morning, but there is a chance the
greatest activity could stay to the south. Improvements to IFR
likely after 18Z.

KBDL TAF...Moderate confidence in TAF.

MVFR/IFR ceilings this morning. There is a risk for showers with
heavy downpours and embedded thunder to be in the vicinity of
the terminal, but latest model guidance has been trending toward
shower/thunderstorm activity staying to the south and east of
BDL. Nonetheless, a risk still remains so we included a PROB30
group for TSRA between 10-14Z. Improvements to VFR likely after
16Z.

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 through this evening for CTZ002>004.
MA...Flood Watch through this evening for MAZ002>021-026.
RI...Flood Watch through this evening for RIZ001>007.
MARINE...Small Craft Advisory until 5 AM EDT early this morning for
     ANZ231>237.
     Small Craft Advisory until 9 AM EDT this morning for ANZ254-
     255.
     Small Craft Advisory until 9 PM EDT this evening for ANZ256.

&&

$$

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