Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 201420
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1020 AM EDT Sun Aug 20 2017

.SYNOPSIS...
High pressure over Pennsylvania this morning will circulate
dry and less humid air into the region today and Monday. The high
moves offshore Tuesday bringing warm southwest winds. The
returning warm air triggers showers Tuesday night, with showers
and thunder Wednesday as a cold front sweeps through New
England. High pressure brings dry weather Thursday through
Saturday.

&&

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

1015 AM update...

Not much change from previous forecast. Post frontal cold air
advection giving way to some scattered to broken
cumulus/strato-cu clouds over the high terrain. However as mean
mid level trough moves offshore heights begin to rise across the
region resulting in warmer temps aloft. Therefore clouds across
the high terrain should be less numerous this afternoon
especially late in the day.

Cold front just cleared Nantucket with a wind shift to the west
and dew pt falling from 71 to 65. This post frontal airmass
continues to overspread the region combined with deep blyr
mixing will promote dew pts falling into the 50s. So while temps
will be warm this afternoon in the 80s, it will be fairly
comfortable given the less humid airmass. Northwest winds will
keep the beaches just as warm. Earlier discussion below.

==================================================================

Plenty of sunshine expected today as upper flow flattens out and
column remains fairly dry. Forecast soundings show rather deep
mixing this afternoon, which will bring some 20-25 mph gusts
over land and also transport drier air down, as dewpoints fall
into 50s. Given prevailing W/NW flow, went a little higher than
guidance with highs in mid to upper 80s for much of area, even
Cape Cod.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Clear skies and diminishing winds will allow temperatures to
drop off quickly after sunset. Stayed closer to NAM/GFS MOS for
lows tonight which tend to perform best, which gives lows in 50s
and lower 60s.

High pressure moves offshore Monday, resulting in S/SW flow
developing during afternoon. Gradient may remain weak enough for
a few hours of sea breezes along E MA coast, but even if that
occurs it should erode by mid afternoon. Once again column
remains fairly dry, aside from some high clouds, so those
looking to get a view of solar eclipse shouldn`t have any issues
(just remember to wear safe glasses!) Airmass remains similar to
today so expect highs once again to reach mid to upper 80s, but
it will be a little cooler near South Coast, Cape, and Islands
with more of an onshore flow.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Big Picture...

Zonal flow amplifies over the Northern USA and Canada during
mid and late week, replacing warm air early in the week with
cooler than normal air late week.

GFS and ECMWF mass and thermal fields are similar through the
period, although with small differences on Saturday. Meanwhile
the GGEM develops a tropical system that moves up the USA East
Coast mid to late week. Satellite imagery does show a
disorganized system north of Puerto Rico moving west, but the
GGEM is the only model to develop it into a threat to the East
Coast while the other models develop no system. This will be
monitored, but for now left out of our long range forecast.

Details...

Monday night and Tuesday...

High pressure shifts offshore with a developing west to southwest
flow over Southern New England. Moist layer between 800 and 850 mb
suggests some low cloud formation under a cirrus canopy. The lack of
deep moisture, including dry air below the moist layer, will work
against shower formation. Data does show a developing theta-e ridge
along the New York border, and this may provide a focus for a few
showers to form late Tuesday in our far west.

Precipitable water values rise through the day. The core of high
values will remain west of us in New York, but values of 1.5 to 1.75
inches arrive during the afternoon. Expect increasing humidity
during the day, with dew points climbing to the upper 60s and near
70. Temps at 850 mb reach 18C, which with full heating would support
max sfc temps either side of 90.

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. 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 strong
thunderstorms along with locally heavy downpours, especially
Wednesday just ahead of the front.

Thursday through Saturday...

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 Thursday and Friday. Temperatures at 500-mb will
minimize at -16C to -18C early 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.
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.

&&

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

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

Short Term /through Monday/...

1030 AM update...

Not much change from 12z TAFs, dry runways, VFR along with a
a modest WNW wind. Earlier discussion below.

===================================================================

VFR through the period with northwest winds today coming around
from the west-southwest Monday.

Winds this afternoon will gust to 15-20kt near South Coast. Low
chance of a sea breeze along East Coast for a few hours early
in afternoon, but right now we are more confident in no sea
breeze occurring. Light winds tonight with patches of IFR in
fog, most likely in the CT Valley. Winds again gusting 15-20 kt
near the South Coast Monday.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Generally VFR through the period. Areas of IFR cigs/vsbys in fog
late Monday night and early Tuesday. Areas of MVFR cigs/vsbys in
showers/thunderstorms Tuesday night and Wednesday. Areas of IFR
cigs/vsbys in fog Wednesday night and early Thursday.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

1030 AM update...

Very pleasant boating weather with WNW winds 10-15 kt, a few
gusts up to 20 kt near shore. Offshore wind will provide
excellent vsby and dry weather. Earlier discussion below.

====================================================================

Winds and seas below Small Craft Advisory levels.

Cold front moving offshore this morning will bring W/NW winds to
coastal waters today. Should have some 15-20kt gusts nearshore
this afternoon, along with choppy seas, before winds diminish
this evening.

High pressure becomes centered over region tonight before
heading offshore Mon with S/SW flow prevailing. Gradient may
weaken enough to allow for sea breeze along East Coast Mon
afternoon, but chances seem low right now.

Outlook /Monday Night through Thursday/...

Monday night-Tuesday...High confidence.

High pressure shifts east of the waters. Winds 20 knots or less, and
seas 4 feet or less.

Tuesday night-Thursday...Moderate confidence.

A cold front approaches from the Great Lakes, crossing the waters
late Wednesday or Wednesday night. Showers and scattered
thunderstorms will move across the waters Tuesday night and
Wednesday.

Southwest winds increase with gusts 20 to 25 knots. Winds shift from
the northwest Wednesday night and diminish to 15 knots or less. Seas
build Tuesday night and Wednesday reaching 5 to 6 foot heights on
the outer waters. The southwest flow may also nudge those higher
seas into RI Sound as well. Once the cold front moves through and
winds become northwest, seas will subside to 3 feet or less.

A Small Craft Advisory may be needed on some of the waters Tuesday
night and Wednesday.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

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


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