098
FXUS61 KBOX 091059
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
659 AM EDT Tue Jul 9 2024

.SYNOPSIS...

Oppressive heat and humidity are expected to continue today and
tomorrow with isolated shower/thunderstorm chances in the
afternoon/evening hours mainly in western and northern
Massachusetts. Quite warm and very humid weather will continue
through the end of the work week. While at least isolated
showers and thunderstorms are possible at anytime...it appears
the main risk for more widespread activity with torrential
rainfall/localized flash flood potential will be sometime
Friday/Friday night along a frontal boundary.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

655 AM Update...

Previous forecast is on track. We can not rule out a brief
elevated spot shower/t-storm this morning across west and
northwest MA...but dry weather will generally prevail through
mid-afternoon. The main risk for isolated to scattered
thunderstorms will be mid-late this afternoon into this evening
across northern and western MA...which the prior forecast
discusses below.

Fog and low-stratus persists over the south coast and southeastern
MA. A dense fog advisory remains in effect for these areas
through 11AM. Expect fog/low stratus to lift by mid to late
morning giving way to partly sunny skies. Heat and humidity
continues today as a broad Bermuda High Pressure system
continues to support a southwest stream of warm/moist air into
The Northeast. Heat indices peak in the upper 90s to near 100
across portions of interior MA this afternoon. Also, with steady
southwest winds likely to fend off a sea-breeze, heat advisory
criteria may be reached all the way to the coast with
consecutive days of 95+ heat indices possible today and
tomorrow. Thus, the heat advisory has been expanded to the coast
to include the rest of Plymouth, Suffolk, and Essex counties.

In addition to potentially dangerous heat this afternoon, there is
also a marginal risk for a strong to severe thunderstorms mainly
across northern and western MA. Surface temperatures in the upper
80s/low 90s and dewpoints in the low 70s will result in surface
based CAPE values between 1000 and 2000 J/Kg. Additionally, an
increasing pressure gradient aloft will support a stronger wind field
with a 40 to 50 knot jet developing at 500 hPa. This will support
deep layer shear values between 30 and 40 knots which would be
sufficient to sustain organized convection. Moisture values will
surge this afternoon as well with PWATs rising above 1.75 inches.
The lacking ingredient today will be lift as only modest forcing for
ascent is in the forecast with a weak upper-level disturbance moving
across the northeast. The greatest forcing looks to be focused just
north of southern New England, but hi-res model guidance suggests
this feature will sag far enough south to trigger some showers and
storms across northern and western MA, mainly along route 2.
Damaging wind gusts and heavy rainfall/flooding would be the primary
risks as weak mid-level lapse rates will limit hail growth
potential. Nonetheless, some small hail can`t be ruled out either.
Timing for storms would be the typical 2 to 8 PM time frame when the
most instability will be available.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...

Tonight

Persistence forecast tonight with very little change in the air
mass. Thus, we can expect another round of fog/low stratus
developing across the south coast, Cape, Islands, and portions of
southeast MA. Another dense fog advisory may be needed. No relief
from the heat overnight as low temperatures will be bounded by
dewpoints in the low 70s. Light south/southwest winds.

Tomorrow

Warm/moist air mass remains in place tomorrow with an uptick in
surface dewpoints to the mid 70s. This will be accompanied by
increased deep layer moisture as PWATs are likely to exceed 2 inches
across much of southern New England. With the increased moisture
will come increased cloud cover, but the overall tropical air mass
will continue to support heat advisory conditions despite less
sunshine. Confidence in this aspect of the forecast is on the
moderate to high side. The lower confidence portion of tomorrow`s
forecast resides with scattered shower/thunderstorm potential. There
will remain a suitable environment for thunderstorms tomorrow
afternoon with sufficient deep layer shear (30+ knots), 1000 to 2000
J/kg of CAPE, and a surplus of moisture. Forcing for tomorrow will
be associated with a surface warm front lifting north with the
remnants of what is now Tropical Depression Beryl lifting into The
Midwest. The strongest atmospheric forcing will reside near the
remnant circulation, but model guidance supports shower/thunderstorm
activity across portions of southern New England. Similar to today`s
forecast, hi-res model guidance is hinting at the greatest
shower/thunderstorm potential generally along and north of I-90/.
Once again damaging wind gusts and heavy downpours would be the
primary threat with hail being a secondary threat. Areas south of I-
90 can expect hot/muggy conditions with modest south winds and more
clouds than sun.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

Key Points...

* Quite Warm & Very Humid Thu & Fri

* Very Warm Sat/Sun/Mon with above normal temps continuing and still
  humid but a bit of an improvement from Thu and Fri

* Main threat for widespread showers/t-storms with torrential
  rainfall & localized flash flooding will be sometime Fri/Fri night

Details...

Thursday and Friday...

Strong upper level ridging across the Atlantic will direct the
remnant circulation of Beryl across the eastern Great Lakes and then
into northern New England on Thu. This will result in Thu being a
very humid day with dewpoints in the 70s as deep tropical moisture
continues to be dragged northward. Highs should be well into the 80s
to the lower 90s Thu depending on the amount of solar insolation
realized. So certainly may need to expand Heat Advisories into Thu
for a good portion of the region.

Thinking that the main focus for showers and thunderstorms on Thu
will be to the north and west of our region...in the vicinity of the
forcing from the remnant circulation of Beryl. That being
said...enough heating/instability will be around for the threat for
at least an isolated convective threat at least in our western and
northern zones. While forcing will be rather limited in our
region...any storm that is able to develop will be capable of
producing torrential rainfall and a very localized flood threat.

Appears though that the main risk for more widespread showers/t-
storms will be sometime Fri and/or Fri night as shortwave energy
gets pulled up from the south and a surface frontal boundary drops
down from the north. These two factors combined with Pwats in excess
of 2 inches will bring the potential for showers and t-storms with
torrential rainfall. We certainly will need to watch for a localized
flash flood potential given the above factors...but that will depend
upon the mesoscale factors which are unknown at this point in the
forecast.

Sat/Sun/Mon...

If the frontal boundary slows down a bit...Sat could feature another
decent risk for showers and t-storms. At this time though...the
consensus of the guidance shifts the main forcing with the shortwave
off the coast. Regardless...modest instability will result in at
least an isolated convective risk at times but thinking the majority
of the weekend into Mon features dry weather. It still will be very
warm and humid...but dewpoints will be a bit lower than what we are
expected Thu/Fri as upper level flow turns somewhat more westerly.
Still looking at above normal temps though with highs well into the
80s to the lower 90s away from the south coast.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...

Forecast Confidence Levels:

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

12z TAF Update....

Today...Moderate confidence.

Fog/low stratus across southeast New England will lift by mid-
late morning...but may still flirt at times with parts of the
very immediate south coast & Cape. As for ACK...IFR/LIFR
probably hang tough for most of the day but there may be a brief
window where conditions temporarily scour out. Elsewhere expect
VFR conditions with a chance for thunderstorms west of I-495 at
BED, ORH, and BAF. Thinking BDL may be displaced far enough
south to escape thunder chances, so didn`t include a PROB30
group in BDL TAF due to a lack of confidence. Similarly, while a
thunderstorm reaching BOS is not out of the realm of
possibility, confidence is currently not high enough to include
a PROB30 group for -TSRA with this update. Southwest winds
increasing to between 10 and 15 knots with some 20+ knot gusts
possible by afternoon.

Tonight...Moderate confidence.

Any isolated to scattered convection that is able to
develop...mainly in western and northern MA should diminish by
mid-late evening. Otherwise...IFR/LIFR conditions in low
clouds and fog likely return to the south/southeast coast
tonight as the boundary layer cools. But the northward extent
of the MVFR-IFR conditions remain uncertain...but thinking
primary risk will be near and especially south of I-90.

Wednesday...Moderate Confidence.

Any lower cigs/vsbys should improve to mainly VFR away from the
south coast. MVFR/IFR may linger into the afternoon near the
immediate south coast, Cape and Islands. A few showers or
t-storms are possible again Wed with the main risk probably
across western and northern MA again. Winds becoming more
south/SSW at 10 to 15 knots.

KBOS TAF...Moderate to High confidence in TAF.

High confidence in VFR conditions today and moderate to high
confidence that strong southwest winds will fend off the sea-
breeze today. Scattered showers and thunderstorms are expected
west of I-495 today so there is a low chance that a storm could
make it to the terminal this evening (22-01Z). Confidence in
this actually occurring is too low for any mention of thunder in
the latest TAF update.

KBDL TAF...Moderate to High confidence in TAF.

High confidence in VFR conditions today, but brief lower
conditions possible if thunderstorms/heavy downpours impact the
terminal. Confidence in thunderstorm activity is higher across
north/west MA and less so for CT so there is no mention of
thunder in the latest TAF update, but there is a low risk.
Mentions of -TSRA may appear in future updates if confidence
increases.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: MVFR/IFR conditions possible. Breezy. Slight
chance SHRA, slight chance TSRA.

Thursday: Mainly MVFR, with local IFR possible. Breezy. Chance
SHRA, chance TSRA.

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

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

Friday Night: Mainly MVFR, with local IFR possible. Breezy.
Chance SHRA, chance TSRA.

Saturday: Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA, isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Tomorrow

Warm/humid air mass will support areas of fog over the coastal
waters, especially during the overnight hours. Otherwise expect
winds to prevail out of the south/southwest with modest seas up to
three feet over the outer marine zones. Can`t rule out an isolated
afternoon/evening storm over the eastern marine zones both Tuesday
and Wednesday afternoons.

Outlook /Wednesday Night through Saturday/...

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

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

Thursday Night: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers, slight chance of thunderstorms.

Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers, slight chance of thunderstorms.

Friday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of
rain showers, chance of thunderstorms.

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, slight chance of thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004.
     Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for CTZ002.
MA...Heat Advisory until 8 PM EDT Wednesday for MAZ005-006-011>013-
     017-018.
     Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for MAZ005>007-010>016.
     Heat Advisory from noon today to 8 PM EDT Wednesday for MAZ007-
     014>016-019.
     Dense Fog Advisory until 11 AM EDT this morning for MAZ020>024.
RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>004.
     Dense Fog Advisory until 11 AM EDT this morning for RIZ006>008.
MARINE...None.

&&

$$

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