Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 210720
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
320 AM EDT Mon Aug 21 2017

.SYNOPSIS...
High pressure moves offshore today. Southwest winds bring warm
humid air Tuesday night, which will feed developing showers and
thunderstorms. The wet weather will continue into Wednesday,
until a cold front sweeps across Southern New England. High
pressure brings dry weather Thursday through Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Clear skies and light winds have allowed temperatures to drop
down into 50s and lower 60s early this morning, along with
patchy valley fog.

Temperatures will rebound quickly this morning thanks to plenty
of sunshine. Model cross sections show enough low level
moisture to result in diurnal cumulus once again today, but
coverage should not be enough to affect viewing of the partial
solar eclipse this afternoon (unless one of those clouds ends up
in the wrong place at the wrong time!) Any higher clouds will
remain thin enough to allow good viewing. Remember to wear safe
glasses in order to protect your eyes.

The only other issue today is whether or not a weak sea breeze
develops for a few hours from late morning into early afternoon,
before gradient tightens and SW flow gets underway. Forecast
soundings show deep enough mixing in Boston which should prevent
sea breeze from making it onto land, although it`s certainly
possible any sea breeze could stay parked over Boston Harbor.

Highs will top out in mid to upper 80s across much of area.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Mid level short wave is expected to dampen out as it reaches SNE
tonight. This will cause any convection that develops to our
west today to weaken as it reaches out area, especially given
loss of daytime heating and little overall support for
convection this far east. It`s still possible that a few showers
survive the trip, especially given increase in low level
moisture, mainly in western New England and near South Coast.

Cold front approaches from Great Lakes Tuesday, but will be far
enough to our west to keep better chances of convection outside
of SNE. Models are fairly aggressive in trying to bring at least
scattered showers/storms into region, especially given surface-
based CAPE in excess of 1000 J/kg and plenty of low level
moisture as precipitable water values climb to nearly 2 inches.
However better large scale lift should be located closer to cold
front, so we`re not convinced we`ll see much in way of
showers/storms until very late in day across western New
England. It`s possible a few showers develop due to heating and
moist airmass in place however.

Other issue is with high temperatures. NAM keeps much more low
level moisture locked in which keeps highs in low to mid 80s,
while GFS does not and highs end up closer to 90. Persistent SW
flow certainly favors NAM and it also has support from high-res
models (NMM in particular). It may end up that south coastal
areas hang onto clouds/fog for much of day, while areas farther
inland are able to break out into sunshine. Based upon model
consensus, nudged highs down slightly (mid to upper 80s).

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Big Picture...

Low pressure from Central Canada digs over Quebec Tuesday night and
swings the trough axis across the Northeast USA during the late
week. Cyclonic flow lingers over New England through next
weekend. With upper heights lowering after midweek, expect
temperatures to trend seasonably cool during the period.

GFS and ECMWF mass and thermal fields are similar through the
period, although with small differences developing Saturday and
Sunday. Thus, forecast confidence is moderate to high.

Details...

Tuesday night and Wednesday...

Upper shortwave moving up the St Lawrence Valley will swing a cold
front across the Northeast USA. Consensus of timing would bring the
front across New York State Tuesday night and across Southern New
England on Wednesday morning. Deep moisture along with PW values of
2+ inches will be acted on by favorable dynamics in the right
entrance region of the upper jet.  This suggests potential for
thunderstorms with locally heavy downpours. Winds at 850 mb are
forecast at 35-40 knots and 500 mb winds around 50 knots. So there
is also a potential for a few strong wind gusts.

Thursday through Sunday...

High pressure builds in at the surface while an upper trough digs
south from Quebec. The trough will carry a cold pool with cold
advection aloft on Friday. Temperatures at 500-mb will minimize at
-16C to -18C Saturday morning. Moisture fields show a moist layer
between 850 and 700 mb during this period, but dry air above and
below. This looks like a partly cloudy and dry pattern, but the
instability shows some potential for a few showers. For now we will
maintain pops at less than 15 percent.

The mixed layer reaches to between 850 and 800 mb, with temps at the
top of the layer supporting max sfc temps in the 70s. Dew points
will be in the 50s Thursday-Friday and 45-55 over the weekend...with
light winds allowing min temps in the 50s each night.

The light wind flow will allow sea breezes to develop each late
morning and afternoon Friday through Sunday.

&&

.AVIATION /07Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Patchy valley fog will bring local MVFR visibilities through
sunrise. Otherwise VFR today as W winds become SW this
afternoon. It`s possible a weak sea breeze develops near eastern
MA coast 15z-18z, but confidence is not high enough to include
in forecast.

VFR conditions persist through midnight, but MVFR/IFR ceilings
should develop overnight, especially from western New England
to South Coast. Ceilings should slowly lift Tuesday to VFR,
except along South Coast, Cape, and Islands where IFR may last
much of the day due to persistent SW flow.

Scattered thunderstorms are possible Tuesday afternoon across
interior, mainly near Berkshires.

KBOS Terminal...High confidence in TAF. Low chance of a weak sea
breeze 15z-18z but currently expecting it to remain offshore.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Areas of MVFR cigs/vsbys in showers/thunderstorms Tuesday night and
Wednesday morning.

Otherwise VFR all areas, with patchy IFR cigs/vsbys in fog Wednesday
night/early Thursday and Thursday night/early Friday.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...High confidence.

SW flow gets underway today as high pressure moves offshore.
That said, gradient may weaken enough to allow for a sea breeze
to develop on eastern MA waters from late morning into early
afternoon before winds shift to S/SW.

Small Craft Advisories are being posted from Tuesday afternoon
into Tuesday night for south coastal and outer coastal waters.
Gusts should reach 25 to perhaps 30kt, along with building seas
up to 6-7 ft offshore.

Outlook /Tuesday Night through Friday/...

Tuesday night...

Southwest winds near 25 knots Tuesday night will maintain building
seas with heights 5 to 7 feet on the outer waters and on RI Sound
and Block Island Sound.

Wednesday through Friday...

Cold front moves across the waters, turning winds from the west-
northwest. High pressure then builds in with winds diminishing to
less than 20 knots.  Winds further turn from the north on Friday.
Diminishing wind will bring diminishing seas, with some leftover 5
foot seas Wednesday followed by seas 3 feet or less Thursday and
Friday.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 2 PM Tuesday to 6 AM EDT Wednesday
     for ANZ231>235-237-250-254>256.

&&

$$
SYNOPSIS...WTB/JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...WTB
AVIATION...WTB/JWD
MARINE...WTB/JWD



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