Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 191403
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1003 AM EDT Sat Aug 19 2017

.SYNOPSIS...
A cold front will slowly cross the region today, shifting
offshore by evening. High pressure brings fair seasonable
weather Monday and Tuesday. Another cold front sweeps through
New England Wednesday, followed by more dry weather Thursday and
Friday.

&&

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

10 AM update...

Not much change from previous forecast. Post frontal airmass
continues to slowly overspread the region with NNW winds across
CT-RI and western-central MA. SPC mesoanalysis shows winds at
925 mb westerly all the way to the coastline. So as solar
heating increases westerly winds just above the surface will
help mix down drier air aloft to the surface.

However the process will be slower across eastern MA as very
moist low level airmass with dew pts into the lower 70s across
southeast MA will remain trapped given drier air aloft. Although
eventually westerly winds and drier air aloft will win out with
low clouds eroding late morning into early afternoon with at
least partial sunshine developing across all of eastern MA. Cape
Cod and Islands will likely be the slowest regarding this trend.

Given southwest flow aloft cold front either gets hung up or
washes out across eastern MA this afternoon. This will result in
an area of surface convergence enhanced by afternoon seabreeze
along the eastern MA coastline. Thus low risk for a few low top
(given dry air aloft) showers later this afternoon across
eastern MA. All hi res guid including the NCAR ensembles, ARW,
NMM and HRRR in agreement on this.

Otherwise a warm summer afternoon with highs well into the 80s.
Less humid across western MA/CT but remaining somewhat humid
across eastern MA with seabreeze precluding drier/less humid air
from advecting seaward.

Previous discussion...

Otherwise, cold front still very slowly moving into S New
England just within the last hour or so, and has likely only
just recently reached the BOX CWA per latest MSAS/OBS. WV
imagery suggests a slight shift to the mid-upper flow such that
its likely now more parallel to this steering flow. Therefore
expect a very slow passage of the sfc front through the day
today, and it may wash out in-situ, as drier air is already
filtering in aloft per GOES-16 low-upper lvl WV products. This
is supported by LAPS/RAP data as well, with K-indices already in
the 20s across W-MA and CT. While the dry air aloft continues
to build, it will trap the lower lvl moisture, maintaining the
high dwpts through the day today. While this will maintain the
humid conditions across S New England, the capping implied by
the dry air should limit further redevelopment of convection
today in spite of CAPE holding thanks to rising mid lvl lapse
rates under cyclonic curvature. There is one short window this
afternoon, when the remnants of the front reach E MA/RI where
modest moisture pooling could allow for a few SHRA to develop,
hinted at in latest mesoscale guidance. Given the dry air
however, capped POPs at slight chance.

Warm and humid today, even with early cloud debris, gradual
clearing and afternoon mixing should still allow temps to reach
well into the 80s, with dwpts even at peak mixing remaining in
the mid-upper 60s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
Cold front continues to move offshore and wash out, but drier
air at the sfc will still lag well behind due to the inability
to mix it out during the daylight hours. Meanwhile, low pres
continues to wrap up in Quebec as fairly robust upper lvl
shortwave rotates toward New England during the late evening
hours, shifting E of the area by early AM Sun. This is
accompanied by a secondary spike in K-indicies and PWATs through
the column as it shunts the initial dry slot to the E, similar
to comma-head style wrap-around. While forcing attendant to the
wave is relatively strong, the remnant dry air should mitigate
shower development, but the risk is non-zero. Some QPF on recent
model runs, but given the initial drying, and the moisture spike
mentioned above is quite modest, will maintain a generally dry
forecast with this update and only suggest slightly more clouds
than previous forecast. Day crew can reassess if POPs might be
needed.

With little change in low-lvl moisture, maintaining dwpts in the
60s, and increased cloud cover expected, overnight mins should
once again only fall into the mid 60s at the coolest spots.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Big Picture...

Subtropical high maintains sultry summer weather over the
Southern USA. Zonal flow over the Northern USA and Canada early
week amplifies during the mid and late week. Contour heights
are normal to a little above normal early week, but fall below
normal by late week. This supports warm summery weather early
week, followed by cooler, drier air moving into New England late
week.

Model upper contour fields are similar through much of next
week. Thermal fields and surface pressure fields show a similar
distribution. This maintains confidence in the forecast data
for our area.

Details...

Sunday through Tuesday...

High pressure builds over the region during this period. Upper
trough is directly overhead Sunday morning with lingering
moisture below 800 mb but dry air above. Mid level air is at
-10C or warmer, while the destabilizing part of the cold pool is
near the Canadian border. Expect dry weather Sunday with some
diurnal clouds but otherwise a fine day. Temps aloft at 13-14C
should support max sfc temps in the low to mid 80s.

Expect dry weather Sunday night to Tuesday, although increasing
mid and high clouds during Tuesday. Light wind flow will allow
sea breezes on Monday afternoon, but gradient on Tuesday
suggests a general southwest flow.

Mixing should reach between 800 mb and 850 mb each day. Temps
of 14-16C Monday and 16-18C on Tuesday suggest max sfc temps in
the 80s Monday and 85-90 Tuesday...cooler along the coast. Slow
increase in humidity with dew points in the 60s. Winds in the
mixed layer Tuesday suggest southwest gusts 15-20 knots.

Tuesday night-Wednesday...

Closed low digs over Quebec and the Northeast USA with
shortwave driving a cold front into/through New England. Expect
showers/scattered tstms with the front Tuesday night/Wednesday.
Cold fropa Wednesday brings an end to the showers, but
sufficient mixing to bring northwest gusts to 20 knots behind
the cold front.

Thursday-Friday...

Surface high pressure builds over the region while upper trough
moves overhead. Cold pool in the upper trough moves in with
500-mb temps forecast to reach -16C to -18C by Friday. Temps at
850 mb will be 8-10C. Moisture profiles show lots of dry air
during that time but with a moist layer at 850 mb. Expect dry
weather with potential for some diurnal clouds. Expect max temps
in the 70s, possibly near 80 in spots.

&&

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

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

Short Term /through Tonight/...

10 AM/14Z update...

Only change to the forecast is for a slightly slower erosion to
the low clouds across eastern MA. Earlier discussion below.

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

Through 15Z...Moderate confidence.
Mix of IFR/MVFR with the continuation of low CIGS and areas of
fog. After sunrise, conditions will gradually improve especially
across W MA/CT, but this may take until mid morning to fully
break out to more widespread VFR. SHRA/TSRA mainly Cape/Islands
continue through 10Z this morning, then shift offshore. IFR
remains through that area through 15Z. Winds gradually shift to
W-SW and recede.

After 15Z through tonight...Moderate confidence.
Gradual improvement to VFR all terminals Winds mainly W-SW,
with sea breezes possible E coast.

KBOS Terminal...Moderate confidence.
Mix of IFR this morning will gradually give way to improvement
to VFR. Sea breeze possible but with a late start.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday through Tuesday...High confidence.

VFR. Areas of IFR in early morning fog/low clouds, then improving
to VFR by each mid morning and continuing the remainder of each day.

Tuesday night-Wednesday...Moderate confidence.

VFR with areas of MVFR cigs/vsbys in showers and scattered
thunderstorms. IFR cigs possible in early morning fog/low clouds
especially in areas that have had some rain.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tonight/...Moderate confidence.

Winds and seas will diminish this morning. Timing of current
small craft advisories seems reasonable, although conditions may
drop off more quickly than current advisories suggest. After
advisories are dropped boating conditions should remain
relatively quiet into the overnight hours, with the only issue
being some localized marine fog which may restrict visibilities,
especially on the S waters.

Outlook /Sunday through Wednesday/...

Sunday through Tuesday...High confidence.

High pressure builds over the waters. Winds remain 15 knots or
less, and seas 3 feet or less. Winds pick up a little on
Tuesday, but remain 20 knots or less with seas 4 feet or less.

Tuesday night-Wednesday...Moderate confidence.

A cold front approaches from the Great Lakes, crossing the
waters late Wednesday or Wednesday night. Southwest winds
increase with gusts 20 to 25 knots during Wednesday. Winds shift
from the northwest Wednesday night. 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. A Small Craft Advisory may be needed on some of
the waters during this time.

&&

.CLIMATE...
Highest recorded dew points on August 18 in any year
PVD   76 in 2002
BDL   82 in 2002
BOS   77 in 1994
ORH   75 in 1987

Highest dew points yesterday
PVD 76 from about 1130 to midnight EDT
BDL 76 from about 630 PM to midnight EDT
BOS 74 from about 11 PM to 1 AM EDT
ORH 72 from about 740 PM to 1220 AM EDT

So the PVD value ties highest for August 18.

Highest value at PVD for August is 80.
Highest value at PVD for anytime is 81.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ235-
     237.
     Small Craft Advisory until 5 PM EDT this afternoon for ANZ250-
     254.
     Small Craft Advisory until 8 PM EDT this evening for ANZ255-
     256.

&&

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



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