Special Weather Statement, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Taunton MA
203 PM 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/...

Minor adjustments to the ongoing forecast for the 18z update. Cu
field has developed across southern New England as mid-level
lapse rates continue to steepen to 6 C/km. 0-6 km Bulk shear
values remain abut 40kts, but CAPE values are lacking a bit
with SB Cape values around 500 J/kg. While low level lapse rates
are quiet steep, appears we continue to be missing the moisture
as K values remain below 30. Both GFS and NAM show a thin
ribbon of K values exceeding 32 after 19z along the CT/MA/RI
border and points south. This may be the focus on any convection
later this afternoon as shown in the Hi-res guidance. Still
think any convection will be mainly isolated to scattered as the
lack of moisture will limit any widespread outbreaks. Best
timing is still after 3PM as a few isolated showers are
beginning to pop up across Central PA.

Previous Discussion...

Weak wave moving through the flow today will push a weak
boundary south into the area. Latest guidance indicated good
mixing north of the Pike per steep lapse rates which would
result in dewpoints dropping into the 50s. This may limit
thunderstorms activity in that region.

However, south of the Pike, more south to southwest flow will
keep surface dewpoints in the 60s and with temps rising into the
mid 80s, instability values will rise above 1000 J/KG. 0-6km
shear values will also increase to 40 kts especially across the
southern half of the region. Soundings show some mid- level
moisture in combination with higher theta-e values especially
across the south coast which will help aid in thunderstorm
development as upper level jet streak moves through. As prev
forecaster noted, soundings are more in an inverted-V profile so
gusty winds are the main threat. Still cannot rule out the risk
for hail. Agree with SPC's marginal risk, but would not be
surprised if it gets pushes eastward into RI and SE MA.
Initiation time appears after 2-3 PM and will last until
evening.

&&

.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 /18Z WEDNESDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday Night/...

Before 00z...High confidence. WSW flow gusting between 20-25 kts
area wide. Mainly VFR with isolated MVFR between BDL and HYA and
points south due to hit or miss -TSRA. Stratus and fog will
continue over the outer Cape and Islands with vsbys less than 1
mile.

Tonight...High confidence. Improving conditions to VFR for Cape
and Islands. Winds becoming light.

Thursday...High confidence. VFR. Winds generally light but
backing to the south. Low confidence on sea-breezes along the
E-coast.

Thursday night...Moderate confidence. CIGs lowering and
thickening, potentially to MVFR, as South winds increase along
with -RA chances. Low chance of iso -TSRA.

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

KBDL Terminal...High confidence. VFR. A stray -TSRA is possible
after 19z.

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/...

1000 AM Update...

Winds appear to mix out just enough early this afternoon across
Boston Harbor and Narragansett Bay to issue a SCA for wind
gusts 25 kts.

Previous Discussion...

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 until 8 PM EDT this evening for ANZ230-
     232>234-236.
     Small Craft Advisory until 11 PM EDT this evening for ANZ235-
     237.
     Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256.

&&

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

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE Taunton MA
344 PM EDT WED JUN 21 2017

CTZ002-212045-
Hartford CT-
344 PM EDT WED JUN 21 2017

...Strong Thunderstorm Approaching Western Hartford County...

At 343 PM EDT, Doppler radar was tracking a strong thunderstorm over
Norfolk, in Litchfield County, moving east at 20 mph.

Dime size hail and winds in excess of 40 mph will accompany this
storm as it reaches Hartford County.

This strong thunderstorm will be near...
  Hartland around 415 PM EDT.
  Granby and Canton around 435 PM EDT.
  Simsbury and East Granby around 445 PM EDT.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Get indoors when this storm approaches. Winds may be strong enough to
produce minor damage, such as a few downed branches.

&&

LAT...LON 4204 7301 4204 7282 4200 7281 4200 7277
      4183 7279 4186 7295 4189 7294 4190 7291
      4197 7289 4197 7303
TIME...MOT...LOC 1943Z 283DEG 19KT 4200 7317

JWD