NWS Technical Forecast Discussion

000
FXUS61 KBOX 232328
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
728 PM EDT Fri Jul 23 2021

.SYNOPSIS...
Scattered showers and thunderstorms should weaken and dissipate
after sundown, leading to clearing skies and a seasonable
evening. High pressure then builds overhead Saturday allowing
for dry and seasonably mild weather. A warm front brings an
increasing risk for showers later Saturday night, along with
increasing humidity levels. Unsettled pattern returns Sunday
with chances for showers and thunderstorms each afternoon into
midweek.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
A few showers and thunderstorms still remain on radar this
evening, but we should start to see convection settle down over
the next couple of hours as diurnal heating diminishes.For
tonight, should see clearing occur with light winds and weak
ridging in place. While confidence in coverage is currently
fairly low, areas that saw rain this afternoon could be a
potential breeding ground for patches of fog. But again very hit
or miss. Also, in coordination with GYX, have issued a coastal
flood statement for the eastern MA coast around tonight high
tide period. Impacts should be marginal/mainly spashover. For
more detail see the Coastal Flood section below. Lows tonight in
the 50s in the outlying sheltered valleys that radiate well,
and the upper 50s to near 60 in the urban areas.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Saturday:

Trough aloft shifts eastward into the waters east of Cape Cod, with
shortwave ridging and related background subsidence aloft building
over SNE. Looking to be a generally dry and seasonably mild
Saturday. A few convection-permitting models do show some pop-up
showers or rogue thunderstorms on the Narrangsett Bay breeze during
the afternoon but thinking this is overdone given the subsidence
aloft. Highs upper 70s to mid 80s, with cooler mid 70s near the
eastern MA and southern RI/SE MA coast where sea-breezes are
expected to develop.

Saturday Night:

Saturday evening features increasing and lowering/thickening
clouds. This is due to a warm front that will be moving in from
eastern NY. Rain, perhaps with a rumble or two of thunder should
be on the increase during the second half of the night and
mainly across the western half of the forecast area. Sided
toward the SREF-based PoP which brings fairly solid Likely PoP
into our western zones. Turning more humid as well with mild
lows in the mid 60s, with upper 60s near the South Coast/Cape
and Islands.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Highlights...

* Best shot of rain and thunderstorms comes Sunday and Sunday night.
  Drier conditions return later in the day Monday

* Warmer and more humid air returns starting Sunday and into next
  week

Details...

Sunday and Sunday night...

A mid level shortwave moves out of the Great Lakes into New England
on Sunday, flattening the ridge and bringing with it a surge of
moisture that will fuel showers and thunderstorms. As the warm front
lifts through on Sunday surface dewpoints surge back into the upper
60s or low 70s but moisture also returns through the entire column;
PWATs rebound to over 1.75 inches. Notably, though dewpoints surge
Sunday, temperatures do not. Even with a much warmer airmass
overhead (850 mb temps rising from ~10C on Saturday to ~15C on
Sunday) high temperatures will be near or even slightly below what
they were on Saturday thanks to abundant cloudcover and poor mixing
of the boundary layer. The difference will be a much muggier feel to
the air on Sunday. This poor mixing may also limit how much of the
low level jet overhead mixes down; it will be breezy, but the
strongest gusts will be over the Cape and Islands, 20 to 30 mph
though can`t rule out some higher gusts to 40 mph. Showers and
thunderstorms move in with the warm front`s arrival, overspreading
most of southern New England by late morning. We have plenty of
moisture available with good forcing for ascent, and instability is
there as well, on the order of 1000-1500 J/kg of CAPE. Bulk shear
values are marginal, near 30 kts, but lapse rates are poor which
would help to limit strong updrafts and storm strengthening. Gusty
winds and downpours are possible with any storms.

Monday through Thursday...

Shower and thunderstorms will come to an end behind the cold front
which drops through sometime on Monday ushering in a marginally
drier airmass, but won`t see much relief from the humidity. Showers
come to an end later Monday but we remain beneath an essentially
stationary broad upper trough through at least Thursday; no
uninterrupted stretch of high pressure in sight. Thus, off and on
rain chances will continue as shortwaves periodically rotate through
the trough. Fortunately it`s looking like a "more dry than wet"
situation through around Wednesday with high temperatures back in
the 80s. Confidence is low beyond this timeframe, but there are
signals that our next more formidable frontal system may approach
around Wed/Thu bringing a better chance of rain.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels.

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

00Z TAF Update:

Through 06Z: High confidence.

VFR with VCSH/VCTS diminsing by the 01-02Z time frame. Showers
and thunderstomrs are currently confined to southeastern MA and
the south coast. Have left VCSH through 01Z for PVD, HYA,FMH.
Once these showers clear out we`ll be left with VFR ceilings and
visibilities through tonight.

Saturday and Saturday Evening

VFR. Light and variable winds. Sea-breeze develops along the
coastal areas around 14-15Z.



KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Sunday through Wednesday/...

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

Monday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA, isolated TSRA.

Monday Night: VFR.

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

Tuesday Night: Mainly VFR, with areas MVFR possible. Chance
SHRA, isolated TSRA.

Wednesday: Mainly VFR, with areas MVFR possible. SHRA likely,
isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Showers and thunderstorms should dissipate shortly around
sundown.

Through Saturday, winds and seas look to remain below small
craft advisory levels. Wind gusts mainly under 20 kt from the
N/NW through tonight, becoming E/ESE on Sat. Seas remain 3 ft or
less through Sat.

Later Saturday night and particularly overnight, a warm front
will bring about an increase in southerly winds with gusts on
the outer eastern waters close to SCA levels. Seas also should
build into the 2-4 ft range.

/Outlook /Sunday through Wednesday/...

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

Sunday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers,
chance of thunderstorms.

Monday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain
showers, isolated thunderstorms.

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers.

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

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

&&

.HYDROLOGY...
No active river flood warnings are in effect for rivers and
waterways in BOX`s hydrologic service area.

However, after having a tremendous amount of rainfall from early
to mid July, many rivers and streams in Southern New England are
running much higher than normal. This is resulting in swift
currents that would not typically be seen in the summer. These
swift currents can catch those swimming or tubing/kayaking on
area waterways off guard. There have been 3 total reports of
drownings in the last week on the Farmington River in CT and
Squannacook River in MA given the higher than usual flows and
swift currents.

As we move into the weekend, those with interests swimming,
tubing or kayaking on area rivers and waterways should be aware
that swift currents due to high flows are occurring. This can
result in dangerous conditions, and it is generally discouraged
to swim, inner-tube, or kayak until flows decrease.

&&

.TIDES/COASTAL FLOODING...
400 PM Update:

Tides are reaching their monthly astronomical peak. This may
bring a period of minor splashover with the evening high tide
around midnight tonight. Weak wave action and light offshore
winds should mitigate a larger coastal flood threat, but minor
splashover on typically vulnerable shoreline roads are possible.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Loconto/RM
NEAR TERM...RM
SHORT TERM...Loconto
LONG TERM...BW/RM
AVIATION...RM
MARINE...Loconto/RM
HYDROLOGY...
TIDES/COASTAL FLOODING...

NWS BOX Office Area Forecast Discussion

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