Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 291117
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
717 AM EDT Sat Jul 29 2017

.SYNOPSIS...
A coastal storm will track south of New England today bringing
gusty winds and light rain to the Cape/Islands and possibly the
south coast, but dry weather further north. The storm will
slowly move offshore tonight and Sunday with improving
conditions. A couple of low pressure areas passing across the
offshore waters southeast of Nantucket will keep a persistent
northeast flow in place into Monday. A slow moving cold front
may push toward the region around mid week, and may affect the
region through late next week.

&&

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

7 AM Update...
Band of showers central RI to PYM and PVC areas has diminished
over the past hour, and that is consistent with high resolution
model trends. Northern edge of main rain shield edging onto
Martha`s Vineyard and may at least graze the south coast but
unlikely to progress any further north. May have a period or two
of moderate rain over Martha`s Vineyard and Nantucket, but
otherwise anticipate heavier rain to remain offshore per current
trends and high resolution models. NE winds have started to
increase but will probably not max out until afternoon/evening.
Still anticipate some sunshine expanding across the NW interior
this afternoon with areas north of the immediate south coast
remaining dry. Only very minor adjustments needed on
temperatures.

Prior Discussion...
Isolated showers continue to develop and redevelop along
surface dewpoint boundary along the south coast. Dewpoints
still in the upper 60s to the south and east of the boundary
but falling into the 50s to the north and west. These showers
will dissipate through daybreak as northerly winds increase and
lower dewpoints advect to the south.

Rather anomalous coastal storm along the mid Atlc coast with
water vapor imagery indicating a potent shortwave digging south
from WV. Very anomalous for late July. Consensus of the guidance
tracks this storm just south of the benchmark by this evening
but there is still some uncertainty regarding the northern
extent of the rainfall. ECMWF is the furthest north of the
guidance and brings steady rain along the south coast with
measurable rain up to BOS. However, the rest of the global
guidance and hi-res guidance keep most of the rain confined to
the islands and possibly up to Cape Cod and adjacent SE coastal
MA. There is a sharp moisture gradient along the northern fringe
of the storm and even ECMWF keeps the deeper moisture just
south of New Eng similar to the other guidance so we are
inclined to follow a non ECMWF blend. As a result, we have
highest PoPs confined to the Islands and particularly ACK where
a rainy and windy expected, with decreasing PoPs to the north.
Rain may clip the south coast but expect mainly dry north of
PVD-TAN, with sunshine further in the interior.

Increasing N/NE winds today as coastal storm tracks to the
south. Mixing is shallow but soundings support gusts 25-30 mph
in the coastal plain with 30-40 mph gusts possible over the
islands and particularly ACK. Highs should reach low to mid 70s
for much of the region, warmest interior valleys, but mid/upper
60s Cape/Islands and possibly SE coastal MA.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Tonight...
Bulk of rain should be offshore by evening but may linger over
the Cape and especially Nantucket into the evening. Otherwise,
excellent drying in the column behind departing storm will lead
to clearing near the coast. Gusty N/NE winds will persist over
the Cape/Islands but diminishing further inland where good
radiational cooling will result in a chilly night. Lows ranging
from the upper 40s to mid 50s, except closer to 60 Cape/Islands.

Sunday...
Storm continues to pull away with lots of sunshine expected as
the column remains quite dry. Gusty winds over the Cape/islands
will gradually diminish as the gradient relaxes. Highs upper 70s
to lower 80s, except cooler Cape/Islands. Very dry airmass with
dewpoints in the 40s and lower 50s.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Highlights...

* Air mass becomes unstable with risk of scattered afternoon/evening
  thunderstorms mid to late week

Overview...

Ridge persists over the western USA and over the west central
Atlantic while the eastern USA remains prone to troughing. As is
usually the case at this time of year, there is some uncertainty
with regard to strength and timing of individual short wave energy
passing through the long wave pattern. Next week begins quietly with
dry weather and warm, but not extreme, temperatures. The air mass
becomes more unstable mid to late week, and consensus of medium
range models, both operational runs and ensemble runs, depict a
rather pronounced upper trough for this time of year digging into
the Great Lakes and Ohio River Valley by late week. The potential
exists for a few days of unsettled weather late in the week into the
next weekend with potentially locally heavy rain. This is toward the
end of the longer term forecast period, however, and a lot can
change between now and then.

Periods of Focus...

Monday...Weather itself for Monday does not appear threatening with
warm temperatures and no rain. However, a long duration and
significant northeast gradient through the weekend will likely have
generated a significant incoming swell and a continuation of a
potentially elevated rip current risk.

Wednesday...The air mass begins to destabilize enough for a risk of
widely scattered or scattered late afternoon/early evening
thunderstorms. Vertical shear is not that significant and so main
threats would be locally heavy rain and lightning.

Thursday and Friday...Cold front approaching an unstable air mass
may trigger scattered thunderstorms during this period. Vertical
shear begins to increase but still not all that high. With
precipitable waters on the increase and deacceleration of the
frontal boundary as it becomes parallel to the upper level flow,
main concern at this time would be for the potential of locally
heavy rain. It is possible that surface cold front hangs back too
far west to be much of a player. In that circumstance, coverage of
any convection will depend upon whether or not any short wave energy
passes through the SW flow aloft.

&&

.AVIATION /11Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...

Today...Moderate confidence. MVFR/IFR cigs/vsbys in rain
expected across the Islands, possibly extending into Cape Cod
and adjacent south coastal region. Dry conditions expected along
and north of PVD-TAN-PYM. May have some areas of MVFR across
southern RI and SE MA. Rest of area should remain VFR. NE winds
will continue to increase with gusts to 25 kt developing in the
coastal plain and gusts to around 30 kt across the Cape and Islands.

Tonight...Moderate confidence.
Areas of MVFR/IFR cigs and light rain may linger into the
evening across Cape/Islands. Otherwise VFR conditions expected with
clearing skies. Patchy late night fog may develop in the upper
CT valley. Will continue to have N/NE wind gusts to 25 kt across
the Cape/Islands.

Sunday...High confidence. VFR. Diminishing wind Cape/Islands.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Sunday Night through Wednesday/...

Confidence...High.

Sunday night through Tuesday...VFR.

Wednesday...VFR except local MVFR ceilings/visibilities possible in
scattered late afternoon thunderstorms.

&&

.MARINE...
FORECASTER CONFIDENCE LEVELS...

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

Short Term /through Sunday/...High confidence.

We upgraded to a gale warning for the outer S/SE waters and ACK
sound where highest confidence of seeing gale force gusts to 35
kt today into tonight. Elsewhere small craft advisories for
gusts 25-30 kt. Seas building to 8 ft outer waters. Winds and
seas slowly diminish later tonight through Sunday. Vsbys reduced
at times in rain and fog today for outer southern waters.

Outlook /Sunday Night through Wednesday/...Moderate confidence.

Confidence...Moderate

Seas above SCA thresholds likely persisting into Monday as a
consequence of a few days of persistent NE flow. Seas should subside
Tuesday through Wednesday. Areas of fog possible Wednesday night as
air mass becomes more humid.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 2 PM this afternoon to 2 AM EDT Sunday for
     ANZ232-254.
     Small Craft Advisory until 2 PM EDT Sunday for ANZ231-233>235-
     237-250-251.
     Small Craft Advisory from 11 AM this morning to 8 PM EDT this
     evening for ANZ230.
     Small Craft Advisory until 2 AM EDT Sunday for ANZ236.
     Gale Warning from 11 AM this morning to 2 AM EDT Sunday for
     ANZ255-256.

&&

$$
SYNOPSIS...KJC/Thompson
NEAR TERM...KJC/Thompson
SHORT TERM...KJC
LONG TERM...Thompson
AVIATION...KJC/Thompson
MARINE...KJC/Thompson


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