Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 220643

Area Forecast Discussion
National Weather Service Boston/Norton MA
243 AM EDT Mon Jul 22 2024


Mostly dry today with a few isolated showers or thunderstorms in the
interior this afternoon. The weather pattern turns more active
tonight with the first in a series of storm systems and increasing
humidity through Thursday. Things dry out Friday through the weekend
with a return of hot and humid conditions.



Kicking off the new work week with a pleasant day thanks to high
pressure which builds in at the surface through mid day beneath
zonal flow aloft. The cold front stalled along the south coast this
morning makes its way north as a warm front and provides enough lift
to kick off a few isolated showers or thunderstorms over the
interior, but subsidence and weak mid level lapse rates should keep
coverage of these garden variety storms isolated at best. Though the
airmass will be warming overhead slightly, thickening clouds
associated with the warm front will help keep temperatures in the
mid to upper 80s.



Tonight will be the most unsettled period of the next 48 hours. The
upper trough over the Great Lakes deepens and sends a shortwave up
the mid-Atlantic coast and southern New England tonight as a surface
reflection strengthens crossing just south of Long Island and
Nantucket by Tuesday morning. There is a lot of moisture in the
atmosphere accompanying this system with PWATs nearing 2.3 inches
and this would spell the potential for heavy downpours and localized
flooding. Any potential impact, however, will be decided by the
eventual track of the low which, at the moment, looks to be far
enough south to keep the swath of heaviest rain offshore. While it
is within the realm of possibility that this swath of 2+ of rain
will pass over Nantucket/Martha`s Vineyard or even Cape Cod, we
continue to think the heaviest rain occurs offshore. Rainfall
amounts of 0.25-1" are more likely along the immediate south coast,
less further north. Secondarily, given the track of the low and
associated shear/helicity (40-60 kts bulk shear and 300-400 m2s2 0-
1km helicity) can`t rule out that this, together with marginal
instability, leads to some rotating thunderstorms offshore or
impacting the Cape and islands overnight into early Tuesday.

The bulk of this activity moves offshore by 10am leaving a mostly
dry Tuesday with a continued mix of clouds and sun but less
cloudcover than we`ll start with Tuesday morning. Oppressive
humidity sets in with dewpoints back into the low to mid 70s.
Fortunately, temperatures themselves will be cooler, in the upper
70s. Can`t rule out a few interior showers Tuesday be height rises
will be working against that.



Key Points

* Seasonable temps with increasing humid conditions.

* Unsettled weather pattern continues Wed and Thur with chances for
  heavy rain and thunderstorms.

* Hot and humid conditions return for next weekend.

Wednesday and Thursday

A more active pattern will continue Wednesday and Thursday as deep
SW flow continues to bring tropical like moisture and humidity.
Temperatures remain near or slightly below normal Wed and Thur due
to thick cloud cover. Dewpoints will remain on the rise into the mid
70s through Thursday.

Another weak shortwave and surface low looks to follow a very
similar track to the one on Tuesday as models trend it further south
making a pass south of SNE. Assuming the low doesn`t trend back
north again, this would bring the heaviest rain near the south shore
along with the Cape and Islands. With PWATS near 2 inches and warm
cloud depths of 14kft, any convective showers and thunderstorms that
do develop will bring heavy rain. Depending on the track of the low,
that heavy rain could bring flash flood concerns if it falls over an
urban area. There is also a conditional severe weather threat
depending on the track of the low. If it stays further south like
models are trending, then the severe threat should stay low being
north of the warm sector. If the low tracks further north, SNE could
enter the warm sector where upwards of 1000 J/kg of instability
combined with strong sfc-6km bulk shear around 35 knots would bring
the risk for severe weather.

Heading into Thursday, the main shortwave finally exits the Great
Lakes. The primary forcing mechanism for thunderstorms looks to be a
cold front moving SE from northern New England. Instability ahead of
the front only looks marginal at this point around 700 J/kg, but mid
level lapse rates actually look halfway decent around 6.0 C/km. With
strong jet dynamics from the shortwave, Sfc-500 mb bulk shear is
impressive at 40-45 knots. Depending on the timing of the cold front
and if more instability can build, there could be a severe threat.
PWATS will still between 1.75-2.0 inches and any thunderstorm would
produce a heavy rain threat and possibly a flash flood threat.

Looking ahead to Friday through the weekend, the pattern appears to
bring back drier conditions with an upper level ridge building in.
Temperatures will be on the rise into the mid to upper 80s Friday, to
upper 80s and low 90s for the weekend. Dewpoints behind Thursdays
cold front drop into the low 60s, but slowly build back to the mid
to upper 60s by Sunday.


Forecast Confidence Levels:

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

06Z TAF Update...

Today... High Confidence.

VFR. Light and variable winds becoming S. A sea breeze ESE
winds developing along the east coast of Massachusetts.

Tonight... Moderate Confidence.

VFR, becoming MVFR with localized IFR along with -SHRA and
-TSRA. S wind becomes SE less than 10 knots.

Tuesday...Moderate Confidence.

MVFR/IFR to start in SHRA/TSRA but these will be moving offshore
by 12-15, improving to VFR.

KBOS TAF... High Confidence in TAF. VFR through the TAF period a
sea breeze has developed and continues through 00z/02z, then
wind direction becomes SSW.

KBDL TAF... High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday: MVFR/IFR conditions possible. Chance SHRA, isolated

Tuesday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.

Wednesday: MVFR/IFR conditions possible. Chance SHRA, isolated

Wednesday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.

Thursday: Mainly VFR, with local MVFR possible. Chance SHRA,
isolated TSRA.

Thursday Night: VFR. Chance SHRA, isolated TSRA.

Friday: VFR. Slight chance SHRA.


Forecaster Confidence Levels:

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

Today and tonight...High Confidence.

Dry conditions most of today, increasing chances of rain tonight
as weak low pressure system exits the Mid Atlantic coast and
moves towards southern New England. Seas 2-3 feet.

Tuesday...High Confidence.

Showers and thunderstorms exiting the waters by late
morning/early afternoon. Seas 2-4 feet.

Outlook /Tuesday through Friday/...

Tuesday Night through Wednesday: Winds less than 25 kt. Seas
locally approaching 5 ft. Chance of rain showers, isolated

Wednesday Night through Thursday Night: Winds less than 25 kt.
Areas of seas approaching 5 ft. Chance of rain showers,
isolated thunderstorms.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.