Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 231424
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1024 AM EDT Wed Aug 23 2017

.SYNOPSIS...
A cold front crosses most of the region this morning bringing
beautiful weather today an end to the oppressive humidity,
except for the Cape and Islands where low clouds and muggy
conditions will persist into this afternoon. Otherwise, high
pressure brings dry weather with very comfortable humidity
levels Thursday through early next week.

&&

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

No major changes to the forecast this morning. Brought the
forecast back in line with observed trends. Showers have
largely ended, except for the few showers around the Cape and
islands. These showers should move offshore by noon at the
latest. Cannot completely dismiss the possibility of a shower
redeveloping in this region until a cold front finally moves
offshore.

Speaking of this cold front, it was almost aligned with the I-95
corridor in MA and RI at 10 AM. Further SE progress of this
front will be rather slow, perhaps take the rest of today to
clear the Cape and islands. West of this front, dewpoints drop
noticeably. East of this front, still dealing with dewpoints in
the 70s.

Previous Discussion...

*/ Highlights (7 am update)...

 - Much lower humidity will bring beautiful weather today except
   across the Cape/Islands where low clouds and muggy conditions
   linger into the afternoon

*/ Discussion (7 am update)...

Low clouds and showers continue to linger across S/SE areas of
New England, with a few isolated along and ahead of a cool front
presently sweeping in from the N/W with winds across N/W portions
of MA and CT becoming northwesterly.

Lower dewpoints and humidity immediately following, clouds
scouring out should allow for abundant sunshine towards midday
into the early afternoon hours aside from the Cape and Islands
where low clouds and showers will linger the longest as the cool
front becomes hung up. Will monitor closely as to possible heavy
shower / thunderstorm activity develops. Anticipate it will but
however remain offshore just clipping Nantucket to the SE.

The environment becoming well mixed with diurnal heating behind
the cool front, will see highs warm into the low to mid 80s with
winds becoming a bit breezy out of the W with gust potential up
around 20 to 25 mph.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
Tonight...

Much drier air along with mainly clear skies/diminishing wind
will result in a much cooler night. The cold front will also
finally clear Cape/Islands this evening, allowing any left over
low clouds to exit the region. Low temps across much of the
region will bottom out well down into the 50s by daybreak
Thursday. Patchy fog may develop late in the typically prone
locations, but not expecting anything widespread.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
The persistence of low pressure across the Gulf of Alaska rebuilds
the monsoonal high over the 4-corners region of the CONUS. The flux
of milder, Pacific air into Central Canada enhances the shearing
flow equatorward across SE Canada and NE CONUS with reinforcing
shots of cooler, drier air. Keeping the tropical-moist axis at bay,
well S/E of the region draped E off the Mid-Atlantic, looking at
slightly cooler than average conditions with comfortable conditions
under a continual influence of 1025 surface high pressure and light
winds.

While the atmosphere devoid of moisture, will need to watch two
periods in particular of spot shower activity given stronger mid-
level ascent across the region as individual shortwaves rotate
through towards the rear of the prevailing H5 troughing pattern
across SE Canada into early next week. This in addition to steep
lapse rates via a maintained cold pool aloft with H85 temperatures
hovering around +6-8C beginning roughly Friday and continuing into
early next week as well. However restrained, agree with the prior
forecaster that within the upper level jet axis with predominant
confluent flow, in the left entrance region which, ageostrophically,
is a region more so of subsidence.

Consensus of forecast guidance through 12z Monday, then forecast
guidance diverges on mid-upper level features and will thus focus on
ensemble means between the GEFS and ECens. Overall, similar thinking
as earlier, 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
50s.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tonight/...

Today...High confidence, except across the Cape and Islands.
Conditions improving to VFR, if not already, NW to SE. Exception
remains the Cape and Islands where MVFR-IFR CIGs will linger
into the afternoon hours with an early threat of -RA. SKC N/W.
Will see W winds pick up with gusts up around 20 kts across
N/W areas of MA and CT.

Tonight...High confidence.
Mostly VFR, however a risk of MVFR-IFR for Nantucket during the
evening hours. Clear conditions across the interior, may see
some patchy MVFR-IFR ground fog develop in the typically prone
locations toward daybreak, but nothing widespread is expected.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday through Sunday/...

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

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tonight/...High confidence.

Today and Tonight...
SW low level jet weakens through 12z allowing small craft wind gusts
to come to an end. However, seas will be slower to diminish but
should drop below criteria across all waters by mid afternoon.
Winds and seas will remain below small craft advisory thresholds
tonight as cold front clears the entire region this evening.

Outlook /Thursday through Sunday/...

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.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ235-
     237-250-254>256.

&&

$$
SYNOPSIS...Frank/Sipprell
NEAR TERM...Frank/Sipprell
SHORT TERM...Frank
LONG TERM...Sipprell
AVIATION...Frank/Sipprell
MARINE...Frank/Sipprell



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