Severe Thunderstorm Warning 62, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
138 PM EDT Mon Jul 15 2024

.SYNOPSIS...
Dangerous heat and humidity is on tap for our region today through
Wednesday, with the chance for mainly afternoon and evening
thunderstorms each day, and especially later Wednesday into
Wednesday night. A cold frontal passage late Wednesday into
Thursday will give way to a much more comfortable airmass to end
this work week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1105 AM Update:

The heat and humidity, as well as the risk for scattered
t-storms which could be strong, remain the two weather-related
stories for today.

On the heat/humidity front...no changes made with standing heat
advisories in effect. There's been some residual midlevel cloud
cover around associated with last night's storms which occurred
to our south and west but these are rapidly eroding to mostly clear
skies. Temps were already into the upper 70s to mid 80s with
dewpoints in the upper 60s to mid 70s. We could have a couple
locations in the CT and Merrimack Valleys see heat indices flirt
with excessive heat warning-criteria heat index values but felt
it wasn't significantly widespread enough to warrant an upgrade
in headlines. We continue to encourage everyone to practice
heat-related precautions such as staying hydrated, staying
indoors in an air conditioned room and to check in on more
vulnerable populations sensitive to heat including the elderly
and pets/animals.

Regarding the risk for scattered strong t-storms...strong
heating should push afternoon CAPE values to sizable numbers,
with mixed-layer (e.g. temps/dewpoint in lowest 100-mb) CAPE
values around 1500-2000 J/kg. It is a pulse-storm enviroment
with shear values around 30 kt or less, so if storms develop,
what comes up will come down fairly quickly. Torrential
downpours and isolated instances of straight line wind damage
from water-loaded downdrafts would be the main risks, probably
in that order. The bigger issue is on storm coverage. Models
are pretty scattershot on locations but there's something of a
loose majority depicting scattered storms within a few miles
either side north or south of the Mass Pike to the eastern coast
during the 18-00z timeframe along a weak/subtle convergence
boundary (SSW winds south of the Pike, with WSW winds north of
it), gradually sagging southward and weakening thereafter.
There is also a far more apparent MCS that is taking shape over
Lower Ontario, and while models are more robust with storms over
NY today, its possible as this feature moves into NY later this
afternoon it could give a little more support to storm
coverage. In sum, the risk for afternoon storms appears to be
increasing and if storms materialize, some storms could become
strong with torrential rain capable of street flooding and
produce instances of wind damage. With the timing being around
the 2-9 PM window and centered in some of our larger urban
areas, the risk for street flooding coinciding with the PM
commute is a possibility.

Previous discussion:

* Dangerous Heat & Humidity Today
* Isolated strong-severe t-storms possible later today

Upper level ridge over the Atlantic coupled with shortwave
energy in south Central Canada will bring dangerous heat and
humidity to the region today. Westerly flow aloft and at the
surface with 850T near +19C/+20C should allow highs to reach the
lower to middle 90s away from the immediate south coast. This
coupled with dewpoints in the upper 60s to the lower 70s will
result in afternoon Heat Indices of 95 to 100+ degrees. We
continued the Heat Advisory for most of the region and did add
northern Worcester county as well.

The other issue will be the potential for afternoon/early
evening scattered showers and thunderstorms. Anomalous heat and
humidity will result in MLCapes increasing to 1500 to 2500 J/KG.
This will combined with a surface trough which should be enough
to trigger scattered showers and t-storms. 0 to 6 km shear is
rather limited generally only 20 to 25 knots...but low level
lapse rates will become quite steep. The HREF shows some decent
2-5 KM Updraft Helicity swaths in the vicinity of the BDL-ORH-
BVY area. Seems like the guidance is focused on that initial
surface trough. The Nadocast and CSU Machine Learning probs do
indicate some severe weather probs too. While we do not think
this will be a widespread outbreak because of limited
forcing/shear...there will be the risk of isolated strong to
severe t-storms. Main threat will be isolated strong to
damaging wind gusts. Pwats near 2 inches will support brief
torrential rain with any thunderstorms.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/...
Key Points...

* Dangerous Heat & Humidity Continues Tue
* A few strong to severe t-storms possible later Tue

Details...

Tonight...

Once the diurnal convection diminishes early this evening...we
will need to watch a more organized area of showers & t-storms
tracking across NY State. This activity will be weakening as it
moves east out running the better forcing and given the loss of
diurnal heating with poor mid level lapse rates. It is possible
the activity survives in a weakened form late this evening
across western MA/CT...but not sure it will even make it.
Otherwise...dry and muggy tonight with overnight lows only
dropping into the lower 70s. Low clouds and patchy fog should
mainly be confined to the south coast, Cape and Islands.

Tuesday...

The dangerous heat and humidity continues on Tue. 850T are a
tad warmer than today and will be on the order of +20C/+21C.
Given southwest flow thinking highs will mainly be in the middle
90s away from the immediate south coast, Cape and Islands. We
may even see high temps reach the upper 90s in the lower CT
River and Merrimack Valleys. Heat Indices in the 97 to 103
degree range are on tap for the vast majority of the region and
Heat Advisories remain posted. It is even possible a few spots
reach heat indices up to 105 degrees.

The other issue will be again the threat for scattered afternoon
and evening showers & t-storms. We once again will have Capes on
the order of 1500 to 2500 J/KG as a shortwave moves through NY
State. This may end up being a later show with the greatest risk
for convection during the later afternoon and evening hours
especially across the interior. Steep low level lapse rates
along with approaching shortwave will increase effective shear
to between 25 and 35 knots. A few strong to severe thunderstorms
will be possible with the best chance across the interior where
the activity arrives earlier. The CSU Machine Learning guidance
indicates severe weather probs with the main risk being locally
strong to damaging wind gusts. Torrential rainfall and a very
localized flooding risk will also exist given Pwats near 2
inches.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Key Points...

* One more day of dangerous Heat & Humidity Wed
* Scattered showers & t-storms late Wed/Wed night
* Relief from heat & humidity Thu/Fri/Sat with seasonable temps

Details...

Wednesday and Wednesday night...

One more day of dangerous heat and humidity is on tap for
Wednesday ahead of an approaching cold front. High temps should
reach the lower to middle 90s away from the immediate south
coast with Heat Indices between 97 and 103 degrees. The Heat
Advisories continue for the vast majority of the region. We do
expect scattered showers and thunderstorms to develop later
Wed/Wed night as shortwave/cold front approaches. Depending on
the timing...there will be the risk for scattered strong to
severe thunderstorms as forcing & wind fields increase with the
approaching cold front. The CSU Machine learning probs also
indicate modest probs for the potential of severe weather. Some
of this activity may persist well into the night given
increasing effective shear. Torrential rainfall with any of the
thunderstorms will again result in a localized flood threat.

Thursday/Friday/Saturday...

Some showers may linger into the first part of Thu depending on
the timing of the cold front. Otherwise...high pressure building
in from the midwest behind the cold front will result in
significant relief from the heat and humidity. Highs will be
well into the 80s with overnight low temps mainly in the upper
50s to the lower 60s...which we have not seen in quite a while.
Humidity levels will also return to comfortable levels.

Sunday...

A weak cold front dropping south may trigger a few showers...but
the vast majority of the day looks dry at this point. Highs look
to be well into the 80s.

&&

.AVIATION /18Z MONDAY THROUGH FRIDAY/...
Forecast Confidence Levels:

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

18z TAF Update:

Through 00z Tuesday: High confidence.

VFR for most, although potential for SHRA/TS mainly within a few
miles on either side of Interstate 90 from 19-00z. Included
VCTS at BOS, BED, ORH, BAF and BDL where confidence is the
greatest on storms nearby, and will proactively amend if a
direct hit. Risk for TS is more conditional at PVD and left out
for now. SW winds around 10-12 kt with gusts 20-25 kt.

Tonight: High confidence in trends though moderate on timing.

Any TS should dissipate shortly after sundown. We do have to
watch storm activity now developing over far western NY for the
western airports after 03z Tue but would be in a weakened state
if it makes it this far east at all. Otherwise generally VFR,
though IFR-LIFR anticipated again tonight for the Cape airports.
SW winds around 10 kt.

Tuesday and Tuesday Night: High confidence.

Any IFR-LIFR over the south coast, Cape and Islands erodes by
mid morning (~12-15z) with generally VFR conditions. There is
another risk for SHRA/TS again Tue aftn to early night, with
perhaps a better chance from ORH westward, but left out of the
TAFs due to lower confidence on development and coverage.
Weakening after sundown with generally VFR conditions, although
stratus and fog at sub-VFR levels again expected for the Cape
and Islands. SW winds around 10 kt.

KBOS TAF...High confidence in TAF, moderate on storm timing.
VFR, though potential for TS in the 19-23z Mon timeframe. Will
AMD if a direct hit but included VCTS for that 19-23z timeframe.
Risk should end by 00z Tue with VFR anticipated tonight and Tue.
Possible SHRA/TS again on Tue but left out of the TAF due to low
confidence. SW winds around 10-12 kt this afternoon with gusts
to 24 kt, gusts ease after sundown then re-develop again Tue.

KBDL TAF...High confidence in TAF, moderate on storm timing.
VFR, though potential for TS in the 18-23z Mon timeframe. Will
AMD if a direct hit but included VCTS in 18-23z timeframe. Risk
should end by 00z Tue with VFR anticipated tonight and Tue.
Possible SHRA/TS again on Tue but left out of the TAF due to low
confidence. SW winds around 10-12 kt this afternoon with gusts
to 24 kt, gusts ease after sundown then re- develop again Tue.

Outlook /Wednesday through Friday/...

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

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

Thursday: Mainly VFR, with areas MVFR possible. Chance SHRA.

Thursday Night through Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Tuesday...High Confidence.

A ridge of high pressure off the Atlantic coast will result in
persistent SW winds across the waters through Tue. Gusty
nearshore SW winds up to 25 knots are expected in the vicinity
of Buzzards and Cape Cod Bay as well as Nantucket sounds this
afternoon. Therefore...we hoisted small craft headlines for
these waters this afternoon. We should see the same situation
develop Tue afternoon. In fact...the potential for 25 knot
nearshore SW wind gusts may cover a larger area. An additional
small craft advisory will likely be needed.

The other issue will be overnight and early morning fog with the
focus across the southern waters.

Outlook /Wednesday through Friday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 20 kt. Seas up to 5 ft. Chance of rain showers, chance of
thunderstorms.

Wednesday Night: Winds less than 25 kt. Local rough seas.
Chance of rain showers, chance of thunderstorms.

Thursday: Winds less than 25 kt. Local rough seas. Chance of
rain showers, isolated thunderstorms.

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004.
     Air Quality Alert from 11 AM to 11 PM EDT Tuesday for CTZ002.
MA...Heat Advisory until 8 PM EDT Wednesday for MAZ013-016>019.
     Heat Advisory until 8 PM EDT Tuesday for MAZ003>007-010>012-
     014-015-020-021-026.
RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>005.
     Heat Advisory until 8 PM EDT Tuesday for RIZ006-007.
MARINE...Small Craft Advisory until 7 PM EDT this evening for
     ANZ231>234-254.

&&

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