Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 210730
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
330 AM EDT Wed Jun 21 2017

.SYNOPSIS...

Another cold front sweeps S New England Wednesday with some
isolated to scattered shower / thunderstorm activity. Thereafter
high pressure builds over the region Thursday. Warmer and more
humid air will return Friday. A cold front will cross the region
late Friday night and Saturday followed by seasonably warm but
less humid conditions for the weekend. Cooler and somewhat
unsettled weather returns early next week.

&&

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

Into sunrise...

Relatively clear and quiet. Increasing presence of cyclonic flow
with mid-level ascent ahead of the parent H5 trough axis entering
the region, as discussed in more detail below, should see low to
mid level clouds on the increase from the N/W. S winds prevailing
remaining light. A final note, lots of moisture just offshore of
Nantucket. As we see heights lower this morning, quite possible
that low clouds return for Nantucket along with reduced visibility.
It`s a low risk at this time.

Today...

Shower and thunderstorm activity possible, more likely around the S-
coastal region of S New England. Sweeping cool front associated with
slight height falls across the region and cyclonic flow. A SW draw
of higher theta-E air beneath steep lapse rate profiles as we mixing
out up to H7 as the column cools around H5 with mid-level moistening
associated with the mid-level H5 trough axis. However, dynamics and
differential vorticity advection sweeping through early. Prefer to
focus outcomes in areas of more favorable column moisture with high
theta-E, lesser H5 dewpoint depression contributing to higher total
column instability, and where positive vorticity advection resides.
Agree with the 21.0z high-res NAM and WRF-ARW with outcomes around
the S-coast, though perhaps the high-res NAM is too robust, a wet-
bias. Also prefer the region given SW flow and likely higher surface
dewpoints. Especially with stronger storms, any cores lifted within
indications of mid-level dry air, obtaining negative buoyancy, and
along with inverted-V boundary layer profiles, can`t rule out some
gusty winds.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/...

Tonight...

Overall cool, comfortable, and dry. Perhaps some shower activity
lingering over the S-waters early evening. Otherwise mid to upper
level heights rising as flow aloft becomes anticyclonic yielding
increasing subsidence. High pressure building across the region
behind the cool front resulting in light winds and clearing. An
opportunity for radiational cooling, leaning towards coolest MOS
guidance. Dewpoints dropping into the upper 40s to low 50s.

Thursday...

Pleasant day on tap. High pressure remains in control, pushing S/E
allowing for light winds to veer southerly. Would expect sea-breezes
along the E-coast. Mixing up to H8, likely will see few to scattered
clouds around 6 kft over the interior early on prior to increasing
mid to high clouds late with enhancing SW flow of warm, moist air
ascending out ahead of the mid-level H925-7 warm front. Looking at
high seasonable for this time of year, upper 70s to low 80s.

Thursday night...

Cloudy and mild with a chance of showers towards morning. Isentropic
ascent of higher theta-E air with a measure of elevated instability
as the column moistens aloft. Increasing K-indices. With convergent
forcing along the H925-7 mid level warm front now lifting N/E across
the region, there`s the chance for shower activity, hold of on any
thunder mention. Increasing surface dewpoints back up into the 60s
with increasing surface S winds as cloud decks lower and thicken by
morning. More than likely mild overnight, perhaps visibility issues
with mist along the S-coast with higher dewpoint air traversing N
over cooler ocean waters. Lows in the 60s.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...

Highlights...

* Very warm and humid Fri with sct showers/t-storms interior
* Gradually turning less humid this weekend with seasonably warm
  temps
* Cooler early next week with showers possible at times.

Overview...

Much of the extended period will feature a subtropical ridge in the
SW and SE CONUS with a broad longwave trof over the Gt Lakes. Polar
jet in the vicinity of New Eng with multiple shortwaves moving
through the flow will bring shower chances to New Eng from time to
time but no washouts are expected. A brief return to very warm and
humid weather Fri, otherwise heat and high humidity will be
suppressed to the south with temps cooling early next week as
mid level trof approaches.

Details...

Friday and Friday night...

Warm sector airmass overspreads SNE Fri with increasingly warm and
humid conditions. Clouds should give way to partly sunny skies with
highs reaching well into the 80s and close to 90 in the interior
valleys. Gusty southerly flow will hold temps in the 70s along the
south coast. Dewpoints will be climbing through the 60s and may
approach 70 in some spots. S/SW winds may gust to 30 mph at times
Fri afternoon, strongest in the interior where deepest mixing
expected.

There may be some elevated convection early in the morning as higher
theta-e air moves in, otherwise airmass destabilizes in the
afternoon with CAPES increasing to 1000-2000 J/kg in the interior.
Expect a few t-storms developing in the interior in the afternoon
but lack of synoptic forcing will likely limit areal coverage. Given
high KI and PWATs and marginal deep layer shear, locally heavy
rainfall will be the primary threat from any t-storms that develop
but a wet microburst is always possible in environments with high
PWATs and CAPES exceeding 1000 J/kg.

Cold front approaches Fri night with theta-e ridge axis and elevated
instability in place so threat of showers and t-storms will
continue Fri night.

Saturday...

It appears cold front may get hung up near the south coast Sat and
there is some uncertainty with how quickly drier air moves in from
the north. It is possible showers may linger into the morning
depending on timing, especially near the coast. Otherwise the trend
should be for skies becoming partly sunny and gradually turning less
humid in the afternoon but higher dewpoints will probably linger
near the coast. Cooler mid level temps lagging to the north and west
so a warm day expected with high into the low and mid 80s.

Sunday...

Will have to watch remnants of tropical moisture to the south. Latest
guidance keeps this south of New Eng but close enough that it could
bring wet weather to SNE. If it remains to the south, expect
seasonably warm temps with comfortable humidity levels.

Monday and Tuesday...

Mid level trof approaches from the west which will lead to increased
risk of showers and isold t-storms but timing and amplitude of
shortwave energy is uncertain. Temps likely trending cooler.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...

Forecaster Confidence Levels...

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

Short Term /through Thursday Night/...

Today...Moderate confidence.

Increasing SW winds with gusts up to 25 kts ahead of possible
SHRA / TSRA though feel activity will be isolated to scattered
and confined around the S-shoreline of MA / RI / CT. Low-end VFR
CIGs SCT-BKN 5-7 kft with a lower risk of MVFR / IFR along with
any RA / +RA.

Tonight...High confidence.

CIGs eroding. SKC. Winds becoming light. VFR.

Thursday...High confidence.

VFR. Winds becoming S though remaining light. Potential sea-
breezes along the E-coast. Mid to high cloud increasing late.

Thursday night...Moderate confidence.

CIGs lowering and thickening, potentially to MVFR, as S winds
increase along with -RA chances.

KBOS Terminal...Sea-breeze possible Thursday. VFR for the entire
TAF period.

KBDL Terminal...High confidence. VFR.

Outlook /Friday through Sunday/...

Friday into Friday night...Moderate confidence.
Some MVFR/IFR cigs/vsbys possible early Fri, otherwise VFR but brief
lower conditions possible in any showers/t-storms in the afternoon
and evening. MVFR/IFR stratus and patchy fog developing Fri night.

Saturday...Moderate confidence.
MVFR/IFR may linger into the morning, especially near the coast,
otherwise trend should be for improving conditions from north to
south.

Sunday...Moderate confidence. Mainly VFR.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Thursday Night/...

Another cold front will sweep the waters late today into tonight
ahead of which there is the possibility of showers and thunder-
storms, while in all likelihood some locations seeing gusts up
to 25 kts. Seas remain heightened on the outer waters around 5
to 6 feet. Behind the front, high pressure settles in with winds
becoming light enough to allow sea-breezes for Thursday and
wave action to diminish. Winds veering S, will see wet- weather
return to the region as a warm front lifts N across the area.

Outlook /Friday through Sunday/...High confidence.

Friday...S/SW gusts to 25 kt expected to develop over Boston Harbor
and Narragansett Bay where best mixing. Some of the higher gusts may
spill into the nearshore waters but lighter winds outer waters. Seas
building to 5 ft over the outer waters.

Saturday...Expect SW winds mainly below SCA but ow risk for G25 kt
lingering. 5 ft seas lingering over the southern waters.

Sunday...Winds and seas below SCA.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 11 AM this morning to 8 PM EDT this
     evening for ANZ232>234.
     Small Craft Advisory until 11 PM EDT this evening for ANZ235-
     237.
     Small Craft Advisory until 4 AM EDT early this morning for
     ANZ250.
     Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256.

&&

$$

SYNOPSIS...KJC/Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...KJC
AVIATION...KJC/Sipprell
MARINE...KJC/Sipprell



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