Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 270220
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1020 PM EDT Mon Sep 26 2016

.SYNOPSIS...
A cold front will bring a period of showers to the area late
tonight into Tuesday morning as it moves into the region. The
front will stall near the coast keeping the threat of showers
along the south coast and Cape/Islands through Tuesday night.
Low pressure dives from the Great Lakes to Virginia midweek,
pauses, then lifts northeast through New England during the
weekend. This will support showers, fog, and drizzle through the
weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

1005 PM Update...

Mid and high clouds have continued to progress eastward over the
last few hours, covering most of the region except for S coastal
Mass, Cape Cod and the islands at 02Z. Lower clouds have reached
into central and western areas, with bases down to 3500-4500 feet,
as seen on latest obs as well as IR satellite imagery, and
continue to move steadily E-NE.

Leading edge of an area of showers extended from W of KGFL-KMSV-
KABE, which was moving E-NE. Appears that most of the showers will
push into northern New England, but the southern extent out of S
central PA should push into the E slopes of the Berkshires by
07Z-08Z then continue across the region through daybreak.

Current forecast was in pretty good agreement with satellite and
radar trends, though a bit slow. Have updated to bring conditions
current and incorporated trends into the overnight forecast.
Temps were running a few degrees milder than forecast, while
dewpts were a bit low. Have adjusted those trends as well.
However, with the S-SW wind flow in place, expect temps and
dewpts to slowly rise overnight, which was the previous forecast
trend.

Previous Discussion...

Modest low level jet develops which will advect anomalous PWAT
plume around 1.75" northward into SNE. Excellent moisture
transport along the low level jet which is somewhat coupled with
right entrance region of upper jet will result in deep layer
forcing for ascent so expect a band of widespread showers to move
across the region, 06-10z western New Eng and 10-14z eastern New
Eng. Models do show some elevated instability south of New Eng so
cant rule out isold thunder, mainly near the south coast.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tuesday...
Approaching shortwave energy ahead of the mid level low in the
Great Lakes will push a cold front into the region Tue morning
with the front likely stalling near the south coast as flow aloft
becomes parallel. Showers continue along and ahead of the front,
but quickly diminish behind the front.

Rapid mid level drying behind the front so showers will end
quickly Tue morning and likely by daybreak in the west followed
by partial sunshine late morning through the afternoon from west
to east. However, clouds and a few showers will likely linger
through the afternoon near the south coast and especially the
islands where deep moisture plume persists near stalled front. QPF
should average 0.25-0.50" with locally higher amounts near the
south coast and Cape/Islands. In addition, may see some southerly
gusts to 30 mph in the morning over the Cape/Islands as the low
level jet moves through. A blend of the temp guidance suggests
highs reaching 70-75 degrees and it will be somewhat humid in the
coastal plain with dewpoints climbing into the low/mid 60s ahead
of the front.

Tuesday night...
The northern edge of the elevated PWAT plume and deep moisture
axis will remain over the outer Cape/Islands where showers will
likely continue off and on through the night, especially over the
islands. As upper low digs south through the Great Lakes, the flow
aloft over New Eng will back somewhat and this may push deeper
moisture further inland across SNE. However, the northward extent
of possible showers is uncertain as models differ on how far NW
deeper moisture extends. We will confine PoPs to SE New England
with highest at ACK where best chance of rain. Patchy drizzle
will be possible further north as abundant low level moisture
moves in from the NE below dry air aloft.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Big Picture...

Maritime trough moves off out to sea, allowing West Atlantic upper
ridge to build to our east. Closed upper low over the Northern Great
Lakes dives south to the mid-Appalachians and then stalls as it runs
into the West Atlantic ridge. The associated surface low will linger
in and around Virginia. The closed low sits for a spell. The entire
system then slowly lifts northeast through New England over the
coming weekend.

Models show some differences in detail, but overall pattern
agreement through at least Saturday. The largest differences are in
the breakdown and ejection of the closed low through New England
Sunday and Monday. The GFS is progressive while the ECMWF is farther
west and lifts the system through New England about a day later.

Details...

Wednesday through Saturday...

Closed low to our west and building high to our east should result
in a stalled pattern over Southern New England from midweek to the
weekend. Upper flow starts from the southwest and turns from the
south by late week.  Meanwhile surface low near Virginia and high
pressure over Quebec and the Maritimes will mean a surface flow from
the northeast possibly turning east by late week.

Given the forecast configuration, one would at least expect an
extended period of low clouds, fog, and drizzle. Models show deep
lift on Thursday morning topped by divergence at 250 mb, and an
extended period of low level lift through Saturday generated by
divergence between 700-800 mb. Expect periods of showers embedded in
the clouds/drizzle during this time.

Sunday-Monday...

Upper low and surface low eject up the coast over New England,
driven by a 100 knot upper jet.  This moves a coastal surface low
past us either Sunday morning per the GFS or Sunday night per the
ECMWF.  Expect showers with this passage.  The upper low then passes
overhead either Monday morning per the GFS or late Monday night per
the ECMWF.  Vertical totals associated with the cold air pool at 26-
28.  So partly to mostly cloudy skies during this phase, and
scattered or widely scattered showers are possible.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels.

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

Short Term /through Tuesday Night/...

Overnight and Tuesday...Moderate confidence, mainly due to
timing. Expect VFR through at least 2 AM/06Z. Gradually expect
lowering to MVFR-IFR from W-E especially 08Z-12Z as band of
showers move through. Showers will exit 12Z-15Z followed by CIGS
gradually lifting to VFR. However, occasional showers and lower
CIGS/VSBYS will persist over the islands and possibly across the
Cape through the afternoon. Brief southerly wind gusts to 25 kt
possible over the outer Cape/Islands Tue morning.

Tuesday night...Moderate confidence. Expect MVFR/IFR conditions to
redevelop overnight as stratus expands across the region. Patchy
fog as well. Occasional showers expected to continue across Cape
and Islands. The showers may push inland overnight but uncertain
how far north and west they get and may be confined south and east
of I-95 corridor.

KBOS TAF...High confidence in trends but lower confidence in
timing. A period of IFR cigs possible Tue morning.

KBDL TAF...High confidence in trends but lower confidence in
timing.

Outlook /Wednesday through Saturday/... Moderate confidence.

CT and Western MA... VFR with brief MVFR possible in showers. Local
IFR in late night/early morning fog.

RI and Eastern MA including ORH... MVFR in low clouds and showers.
Areas of VFR possible in the midday and afternoon. Areas of IFR
in fog and drizzle at night.

&&

.MARINE...
Forecaster Confidence Levels.

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

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

Overnight into Tuesday...Expect increasing southerly winds after
midnight and Tue morning as low level jet moves across the
waters. Low level inversion is not that strong so we do expect
some mixing. A period of 25-30 kt gusts possible, diminishing in
the afternoon as the low level jet exits. In addition, seas will
build to 5-6 ft over the southern waters. Small craft advisories
will continue. Vsbys will lower late tonight into Tuesday in areas
of showers and patchy fog, lingering over the SE waters. Low risk
for a t-storm over southern waters.

Tuesday night...Winds turn NE through the night as low pres tracks
well to the south with ridging nosing down from the north. May see
a pulse of 20-25 kt gusts over NE MA waters late. Vsbys reduced in
showers and patchy fog, especially SE waters. Low risk for a
t-storm SE waters.

Outlook /Wednesday through Saturday/... Moderate confidence

East to Northeast winds through the period. Winds to 25 knots,
either sustained or frequent gusts, Wednesday through Friday then
diminishing Saturday. The persistent Northeast wind will push
Gulf of Maine waters toward the coast and build seas of 6 to 9
feet. Expect Small Craft Advisory for much of this period. Also
period of poor vsbys in fog and showers/drizzle.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 4 AM to 2 PM EDT Tuesday for ANZ232.
     Small Craft Advisory until 8 PM EDT Tuesday for ANZ235-237-
     254>256.
     Small Craft Advisory until 2 PM EDT Tuesday for ANZ250.

&&

$$
SYNOPSIS...WTB/KJC/EVT
NEAR TERM...WTB/EVT
SHORT TERM...KJC
LONG TERM...WTB
AVIATION...WTB/KJC/EVT
MARINE...WTB/KJC


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