Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 221919

Area Forecast Discussion
National Weather Service Taunton MA
319 PM EDT Tue Aug 22 2017

A cold front will bring the potential for scattered showers and
thunderstorms late tonight, which may persist into Wednesday
morning along the coastal plain. High pressure brings dry
weather with very comfortable humidity levels Thursday through


Mostly sunny across southern New England this afternoon, except
for the south coast of RI and MA, including the Cape and
islands, where marine stratus lingered. Minor tweaks to the
forecast this afternoon to bring it back in line with observed

Tricky forecast tonight. The high heat and humidity resulted in
very high CAPE values for our region. Most unstable CAPE values
were between 2,500-3,000 J/kg, with mixed layer CAPE values
between 1,500-2,000 J/kg. Despite this, our region remains
fairly capped this afternoon due to poor mid level lapse rates
and low shear.

The complication in the forecast tonight will be how quickly the
CAPE values drop after sunset, and how quickly stronger shear
arrives. It will be delicate balance. At this time, thinking
strong convection may arrive east of the Berkshires between 8
and 10 PM, then weaken as it moves east from there. Helicity
values in the 0-1km AGL layer range between 150-300 m2/s2. This
would mean at least a very low risk for a tornado as these
storms move across our region overnight.

The other risks from any strong thunderstorms tonight will be
gusty winds and locally heavy rainfall. Thinking any flooding
risk tonight will be very localized as storms should be moving
rather quickly. Training storms also appear to be unlikely.

Well above normal temperatures continue tonight.


The threat for scattered showers and thunderstorms may persist
into Wednesday morning across the coastal plain, and perhaps
even into early afternoon across the Cape and islands. Much
drier air will arrive behind a cold front, which should move
offshore by late afternoon. Near normal temperatures expected
through this period.


A long break from Summer heat. The return of the ridge-trough dipole
stretched from Siberia into the Gulf of Alaska, ridge amplification
across the N-Central CONUS into Central Canada shears energy equator-
ward downstream promoting trough persistence across the NE CONUS.

Several waves through the broader trough reinforcing cooler, drier
air across the region. With available moisture and diurnal heating,
collocated cold pool and steep lapse rates, can`t rule out isolated
to scattered shower activity in the vicinity of S New England ahead
of favorable mid-level ascent. Higher confidence over Upstate NY and
N New England. Lot of dry air and low theta-E behind early week
low pressure persisting over SE Canada. The influence of a 1025
surface high throughout the forecast period, the quasi-
stationary tropical frontal boundary remains well S/E off-shore.

Overall, looking like a prolonged period of dry weather with low
humidity, low dewpoints, making for comfortable conditions. Highs
around the mid to low 70s with lows down into the 50s. Scattered
cloud decks at times. Dewpoints ranging in the upper 40s to low


Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Through 00Z...High confidence. Mainly VFR. Areas of MVFR-IFR
towards the south coast, including the Cape and islands.

Tonight...Moderate confidence.
SCT SHRA/TSRA should move east of the Berkshires between 00Z-
02Z. Storms should weaken as they move east. Mainly VFR, with
areas of IFR towards south coast. Strong SW low level jet could
result in LLWS, especially toward Nantucket.

Wednesday...High confidence in trends, but lower confidence in
overall timing. SCT SHRA/TSRA may persist near and especially
southeast of the I-95 corridor into early Wednesday afternoon.
Improvement to VFR conditions expected from west to east, but
timing somewhat uncertain.

Wednesday Night...High Confidence. VFR.

KBOS Terminal...High confidence in TAF trends. Moderate
confidence in timing.

KBDL Terminal...High confidence in TAF trends. Moderate
confidence in timing.

Outlook /Thursday through Saturday/...

VFR. Daytime SCT low-end VFR CIGs 4-5 kft agl. N/W winds
throughout, light at times to allow onshore E sea-breezes.


Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...High confidence.

Strong southwest low level jet moves across the waters tonight
into Wednesday morning. Gusty southwest winds ahead of an
approaching cold front. Some risk for thunderstorms, but showers
more likely. Will continue Small Craft Advisory headlines, but
did tweak times.

Winds and seas diminish Wednesday afternoon, with more tranquil
boating conditions starting Wednesday night.

Outlook /Thursday through Saturday/...

Good boating weather throughout. N winds persisting. Reinforcing
shots of cool, dry weather as high pressure builds into the region.
Subsequent slight enhancement in winds, gusts remaining below 20 kts
with seas 4 feet or less.


MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ231>234-
     Small Craft Advisory until 2 AM EDT Wednesday for ANZ230-236.
     Small Craft Advisory until 2 PM EDT Wednesday for ANZ235-237.
     Small Craft Advisory until 8 AM EDT Wednesday for ANZ251.
     Small Craft Advisory until 4 PM EDT Wednesday for ANZ254>256.


LONG TERM...Sipprell
MARINE...Belk/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.