Area Forecast Discussion
Issued by NWS Boston, MA

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097
FXUS61 KBOX 261204
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
804 AM EDT Mon Sep 26 2016

.SYNOPSIS...
High pressure will center over New England tonight and allow for
efficient radiational cooling. A low pressure area and associated
front will bring showers to the area late tonight into Tuesday
morning. Forecast confidence remains low for the remainder of the
week, though an east to northeast wind flow looks to persist into
next weekend. There will also be the chance for showers as well as
periodic drizzle and fog.

&&

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

745 AM Update...

With clear skies and mainly calm winds thanks to high pressure
ridge overhead early this morning, several ASOS sites saw temps
fall at or below freezing, including KOWD, KTAN, KORE and KCEF.
Patchy fog also developed in some spots during the pre dawn hours.
Noted a report of 28 degrees at Barre Falls mesonet site this
morning as well.

IR satellite showing mainly clear skies across the region at 11Z,
though some manned sites reporting some patchy high clouds. More
mid and high clouds across central NY into southern Ontario
working SE as seen on latest satellite loop.

Updated temps/dewpts to bring conditions current, as well as local
low clouds/patchy fog that developed after 06Z. Remainder of
forecast on target, but did refresh to bring current as well.

Previous Discussion...

The light pres gradient will give allow for sea breezes to develop
even as light return flow picks up later in the day with the crest
moving of high pres moving offshore. This will lead to slight warm
advection and increasing low lvl humidity throughout the day and
in spite of mixing, which will hold back the sfc dwpts somewhat in
spite of the full column moistening. Increasing H85 temps through
the day to +6C by late day. Mixing should be able to tap this lvl
in spite of the increasing moisture and clouds through the
afternoon. Therefore, expecting more widespread low 70s and upper
60s, near normal for this time of year.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Tonight into tomorrow...
Meandering cutoff will begin to settle across the Great Lakes
region as strong shortwave energy rotates on the S and E periphery
late tonight into tomorrow. This shortwave is associated with an
upper lvl jet streak whose equatorward entrance region sets up
across the NE during the late night/morning hours tonight into
tomorrow. As it stacks with its attendant sfc low, expect
occlusion process which will dray moisture out of the gulf region,
yielding PWATs near 1.5 inches and K-vales between 30-35. This
occlusion approaches late, so suspect rain band associated with
the mid-lvl F-gen will not arrive in the W until around midnight
local, sliding E into the daylight hours, with partial improvement
Tue W-E from 12Z (8AM local) to 18Z (2PM local). As the front
approaches the S coast it is expected to slow as it begins to
parallel the flow aloft. Looking at QPF values to range between
0.3 inches to possibly as high as an inch on the S coast as the
front slows. Once again, much needed rain, but not drought-
busting.

LLJ around 40 kt at H85 will be moving across the region during
the early AM hours and go along with a narrow corridor of elevated
instability. This will be maximized along the S Coast/Cape/Islands
especially after sunrise. Will include a risk for TS with heavier
rain.

Temps overnight cool to the low 50s but hold thanks to upper lvl
warming and increasing dwpts at the sfc. By day, especially with
some clearing in the W, highs increase into the low 70s to mid 70s
in the warmest locations.

&&

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

* Showers with a few thunderstorms linger Tuesday night, mainly
  across the coastal plain
* May see scattered showers at times into next weekend, but low
  confidence mainly on timing
* Temperatures near or slightly below normal with raw northeast
  winds

Overview...

Medium range models and ensembles continue to signal anomalous
high amplitude Omega blocking pattern setting up across the
central and northern U.S. during this timeframe. Timing with this
type of set up lends to low confidence on timing. Big question is
where the H5 cutoff low will eventually set up, though 00Z model
suite have a fairly good handle in slowly shifting the low south
through the Great Lakes through mid week, then to the southern
Appalachians by late week. Some placement issues come into play,
mainly with the ECMWF after Thursday. However, when it comes to
the surface features, much less confidence due to rather wide
variance in location and track over time. Another question will
be QPF amounts due to timing of the precip and whether the mid
level S-SW flow will allow some tapping of the moisture feed up
the eastern seaboard.

One thing that does have some certainty is the prolonged NE wind
flow that will be in place once the front passes Tuesday night
into Wednesday. This will set up a prolonged fetch off the ocean,
which will also keep low level moisture in place. Patchy drizzle
can not be ruled out at times along the coast with the onshore
flow.

With this complex pattern, low confidence on exact timing and
trends continues especially beyond Wednesday.

Details...

Tuesday night-Wednesday...

Cold front pushes from N-S across the region Tuesday night, but
most of the showers should be mainly along the south coast at
least through midnight. Some instability also in place, so can not
rule out a few thunderstorms through around 04Z-06Z before moving
offshore. Noting the PWAT plume of 1.6 to 1.7 inches lingers along
the south coast, but rather sharp drying line pushes in behind.
High dewpts look to linger especially along the coast, mainly in
the lower 60s overnight, so could also see patchy fog develop.

Questions come into play during Wednesday, as models having
trouble with area of showers moving westward mainly during the
afternoon as low pressure passes offshore in the SW flow aloft.
Drier air in place early Wednesday away from the coast, but PWATs
increase late Wed/Wed night with onshore flow which will aid in
scattered shower development with unsettled upper pattern in
place. Highs on Wednesday will be in the mid 60s to lower 70s,
mildest across the CT valley where it will be driest for most of
the day.

Thursday through Sunday...Low confidence.

As H5 cutoff low pres slowly shifts S over time, the upper flow
becomes mainly S-SW bringing deeper moisture feed up the coast.
This will combine with the persistent NE low level flow to help
develop spotty showers and possibly patchy drizzle at times mainly
along the coast. Will also see some late night/early morning
patchy fog. Question through the remainder of the week is the
placement and timing of any showers.

Big question for late this week into the upcoming weekend will be
the position of high pressure trying to build south out of Quebec
and eastern Ontario, and whether it brings drier air into the
region. At this point, it looks unlikely as the models are
tending to bring better organized moisture northward in the S-SW
mid level flow.

Overall, high uncertainty remains in play with this pattern. Have
kept low POPs going at this point. With the persistent NE low
level flow, expect temperatures to run near or a few degrees below
seasonal normals.

&&

.AVIATION /12Z 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/...

Today...High confidence. VFR. Light/variable winds early, shifting
to SW through the day. Sea breezes expected on coasts.

Tonight and Tuesday...Moderate confidence, mainly due to timing.
VFR to start the overnight. Gradually expect lowering to MVFR/IFR
from W-E especially 06Z through 12Z, first due to lowering CIGS,
then due to mix of RA and BR with lower vsbys. Expect this to
gradually lift from W-E 12Z-20Z, although these lower categories
may linger longer across the S Coast/Cape/Islands into Tue
evening.

KBOS TAF...High confidence in TAF through 06Z Tue, including sea
breezes.

KBDL TAF...High confidence in TAF through 06Z Tue.

Outlook /Tuesday Night through Friday/...

Tuesday night...Moderate confidence. Showers linger with MVFR-
IFR conditions likely, with the best chance across the coastal
plain. Conditions should improve to mainly VFR across the
interior, but could lower to MVFR-IFR in patchy fog in the
normally prone inland areas.

Wednesday through Friday...Low confidence. Scattered showers will
linger from time to time with patchy MVFR conditions at times, but
should see VFR conditions where it is dry. May see patchy late
night/early morning fog each day with local MVFR-IFR CIGS/VSBYS.

&&

.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.

Today...High pres moves across the waters. Winds and seas remain
below small craft advisory thresholds.

Tonight into tomorrow...
SW winds increase during the overnight hours as a front approaches
the waters. The increase in winds reach 25-30 kt late evening,
with seas building 5-6 ft by Tue morning. Small craft advisories
will be issued mainly for the outer waters where seas/winds will
be the strongest. Front moves over the waters tomorrow afternoon,
with rainfall and possibly even an isolated thunderstorm.

Outlook /Tuesday Night through Friday/...

Tuesday night...High confidence. Light SW winds will shift to
N-NE as cold front passes. Seas around 5 ft on the outer waters
early, then subsiding. Reduced visibilities in showers and fog,
mainly from Cape Cod southward. Low risk for a few thunderstorms
on the southern waters to near Cape Cod and Nantucket.

Wednesday through Friday...High confidence in an extended period
of E-NE winds. Wind speeds mainly below small craft criteria, but
may gust to around 25 kt Thursday and Thursday night on the outer
waters, lingering into Friday south of Block Island. Moderate
confidence of reduced visibilities at times in showers and patchy
fog. May also see increasing seas due to prolonged NE fetch,
especially across the eastern open waters.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 11 PM this evening to 2 PM EDT
     Tuesday for ANZ235-237.
     Small Craft Advisory from 11 PM this evening to 8 AM EDT
     Tuesday for ANZ250.
     Small Craft Advisory from 11 PM this evening to 5 PM EDT
     Tuesday for ANZ254>256.

&&

$$
SYNOPSIS...Doody/EVT
NEAR TERM...Doody/EVT
SHORT TERM...Doody
LONG TERM...EVT
AVIATION...Doody/EVT
MARINE...Doody/EVT



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