Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 211722

Area Forecast Discussion
National Weather Service Taunton MA
122 PM EDT Fri Oct 21 2016

Sub-tropical low pressure lifting north toward southern New
England will bring numerous showers and isolated t-storms with
locally heavy rainfall this afternoon and tonight. Additional
showers are likely Saturday as the storm moves into northern New
England along with strong winds developing late Saturday into
Saturday night. Dry but cool and blustery conditions Sunday into
early next week.


1030 am update...

Warm sector continues to surge northward thru the coastal plain of
CT/RI and southeast MA at 10 am. Some brief clearing in the warm
sector has allowed temps to soar into the low 70s at 10 am.
However this will be short lived as visible satellite imagery and
radar show clouds and showers just south of the islands and
streaming rapidly northward. Thus sunshine across eastern CT/RI
and southeast MA will be short lived and replaced by clouds and
showers midday or shortly thereafter.

Warm sector will continue advecting northward but will likely
stall somewhere across northern MA as pres falls and associated
cyclogensis evolve south of New England later today. Thus tricky
temp forecast across northern MA today. So expecting low clouds
and fog to lift across the interior but then may redevelop later
today as showers become more widespread.

Bands of rain showers across the interior...mainly west of the
I-95 corridor. These showers will exit northward into central New
England. However a new area of showers just south of the islands
will come onshore early this afternoon from south to north. More
importantly is area of convection farther offshore south of New
England...which appears to be expanding with time per latest
satellite imagery. This area is associated with moisture
streaming north of a subtropical low north of the bahamas.
Complexity to our forecast is the exact track of this convection
this afternoon. Consensus from the 00z global guidance including
much of the mesoscale guid including the NMM/HRRR and RAP is for
this convection to move across RI and eastern MA. However some of
the mesoscale guidance including the 00z ARW and NCAR ensembles is
to track offshore convection farther west into CT and central MA.
Will need to evaluate 12z guidance and latest satellite and radar
trends before we have a better handle. Thus no major changes to
previous forecast. Earlier discussion below.


Previous discussion...

Amplifying mid level trof moving into the Great Lakes will back
the flow across the NE which will slow the eastward progress of
the frontal boundary in New York state, while allowing subtropical
low to lift north toward New Eng. One area of heavy rainfall will
persist to the west assocd with the slow moving front and right
entrance region of the upper jet. A second area of heavier
rainfall with the subtropical low and high PWAT plume is likely
but location is still somewhat uncertain. Guidance is favoring RI
and eastern MA during the afternoon as area of strong omega lifts
north from the ocean with anomalous PWAT plume focused across SE
New Eng, but it still could be further west per some of the hi-res
guidance. The other factor to consider is the airmass will
destabilize this afternoon with SBCAPES around 500 J/kg and
strong deep layer shear which may lead to isold strong t-storms
developing. The hi-res guidance is indicating potential for
localized heavy rainfall of up to 3 inches but low confidence on
location if this occurs.

Warm front lifts to the north with SNE in the warm sector today.
Temps will warm into the upper 60s to lower 70s and it will be
quite humid with dewpoints in the mid 60s.


Localized heavy rainfall with thunder potential will linger into
the evening as the sub-tropical low lifts north across the
region, then models are signaling a dry slot moving into the
region during the second half of the night. Expect precip to taper
off with areas of drizzle and fog developing overnight. Very mild
and humid into the overnight with temps holding into the 60s, then
falling through the 50s western New Eng late tonight as cold front
approaches from the west. Temps may remain in the 60s all night in
eastern New Eng.

Potent neg tilt trof and mid level low over PA will lift NE into
New Eng during Sat. Strong QG forcing and mid level frontogenesis
with deep moisture moving into SNE will lead to sct to numerous
showers on Sat, especially western New Eng. High temps will occur
in the morning with 50s west and 60s east, then falling into the
40s and 50s in the afternoon as strong cold advection develops.

Strong winds will develop in the afternoon as the low pres deepens
across northern New Eng with strong pres gradient and strong cold
advection developing. Potential for westerly gusts up to 40 mph
by mid/late afternoon and up to 50 mph for Cape/Islands. Wind
advisories will likely be needed for portions of SNE.



* Some rain lingers int Saturday night, along with strong winds.
* Cooler and dry by Sunday.
* Seasonably cool early next week.

Overview and model preferences... The reasonably good agreement that
models have shown for much of the mid and extended terms continue
again with the overnight 00Z update.  Only toward the end of next
week, with a deepening clipper, do they begin to show more extreme
differences at the synoptic scale.  Deep trof/cutoff slowly shifting
into Atlantic Canada suggests seasonably cooler conditions to follow
the rain/unsettled wx ending by Sat night.  These conditions linger
into mid week thanks to a persistent draw of CP air out of central
Canada.  Not entirely dry, as weak reinforcing shortwaves could
deliver a few shra on Mon, but definitely more fall like until the
latter portions of next week.  Update will feature a blend of
operational guidance as well as persistence.


Sat night... Low pres will continue to rapidly deepening as it
shifts into N New England and QC, possibility dipping into the 970s.
Very pronounced trowal/comma head setup wrapping well to the S and W
of this low as the upper lvl cutoff lags the sfc low.   Good lapse
rates above developing inversion will enhance the precip efficiency
in this region as well. Therefore, could see some fairly persistent
wrap-around precip lingering into the overnight hours Sat.  While
the highest risk will be in the upslope regions of the Berkshires
could see some of it spill across the CT valley and into the
Worcester hills as well.  Only eastern areas will begin to see an
end to the precipitation thanks to good downslope component with W-
NW flow.  Dry air begins to entrain, so expect rapidly dropping
dwpts into the early morning hours on Sun.  Mins are likely to hold
in the 40s and low 50s however, mainly due to delayed cold advection
and remnant cloud cover lingering beneath dry inversion aloft.

Also, may need to watch for strong W-NW flow as strong isallobaric
couplet forms overnight.  LLJ is near 45 kt at H92 to as well.
Mixing overnight could see yield winds to 40 kt at least, therefore,
may need to have a wind advisory into Sun morning.

Sun and Sun night... Gradual improvement as the downsloping and
drier air aloft is able to finally force some of the moisture in the
low levels out allowing for some clearing.  Winds remain a factor,
although may drop below wind advisory criteria by late morning,
expect mainly 25-35 mph out of the W through the daylight hours. H92
temps hover around +3C so expect highs in the 50s, while mins drop
into the 40s (held back by the continued pres gradient).

Mon and Mon night... Secondary shortwave rotates through the trof to
the NE, reinforcing the cyclonic flow briefly.  Noting enough
moisture between the sfc and H7 for there to be a few shra during
the day with this passage, enhanced by additional mid lvl cold
advection.  Flow gradually shifts from predominantly W, to the NW
after this frontal passage.  Similar temps to Sun as the strongest
cold advection lags into the overnight hours.

Tue into Wed night... Definite return to cool fall conditions.  H92
temps drop as low as -1C by early Wed morning (H85 as low as -6C).
Winds remain elevated Tue, but shift gradually to the N-NW with
gusts 20-30 mph.  The winds weaken Tue night into Wed as high pres
crests over the region.  Therefore, highs in the upper 40s  and low
50s are likely each day, with overnight mins dropping to near or
below freezing across much of the region thanks to radiational

Thu into Fri... Clipper system approaches from the W, however there
is uncertainty on the final track.  GFS is to the S, leading to S
New England lying on the climatologically dry N tier.  The ECMWF is
stronger and to the N, suggesting a more unsettled/wet passage.  In
any case, will be including some POPs for the tail end of the
extended forecast.


Forecaster Confidence Levels.

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

1030 am update...

Brief improvement to VFR across RI and southeast MA...possibly
into northeast MA but then likely trending back to MVFR and
eventually IFR/LIFR after 21z-22z. Across CT and central-western
MA upward trends will continue for this afternoon but then back
down to IFR/LIFR this evening.

Short Term /through Saturday/...

Today...High confidence on trends but lower confidence on exact
timing and details.

LIFR/IFR will continue across most sites through 15-18Z with some
areas lifting to higher IFR/MVFR through the afternoon. Mostly fog
this morning, but higher risk for showers and even a few
thunderstorms this afternoon, heavier/steadier rain arrives this
evening especially for E MA and RI.

Tonight...High confidence.
More IFR/LIFR likely in showers/drizzle and fog. Isold t-storm

Saturday...Moderate confidence. IFR/LIFR improving to MVFR with
pockets of VFR cigs possible in the afternoon. Improving vsbys.
Sct to numerous showers, especially west. Winds becoming W with
gusts 30-40 kt developing in the afternoon.

KBOS TAF...High confidence in trends.

KBDL TAF...High confidence in trends.

Outlook /Saturday Night through Tuesday/...

Sat night into early Sun...High confidence.
Lingering IFR/MVFR conditions persist even as rainfall gradually
recedes across the east but continues across the west.  Lower CIGS
will be the primary cause of the lower categories.  Other issue is
winds, gusts 30-40 kt possible at times out of the W, sustained 20-
30 kt.  Winds off the deck remain high as well 40-50 kt at about

Sun into Tue...High confidence.
Fairly persistent weather for the period.  W winds continue 15-20 kt
sustained with gusts 25-35 kt at times, diminishing by late Tue.
Mainly VFR.  Although some low clouds during the day light hours may
come close to the upper end of MVFR.


Forecaster Confidence Levels.

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

1030 am update...not much change from previous forecast. Heavy
showers with embedded thunder likely late this afternoon and
evening across the eastern MA waters...possibly westward into RI.

Short Term /through Saturday/...Moderate confidence.

Today...Mainly SE gusts to 20 kt as warm front lifts north of the
region. Low vsbys in areas of dense fog will improve somewhat
toward midday. Potential for locally heavy rainfall, especially
this afternoon.

Tonight...As center of low pres moves across the waters easterly
winds will diminish then become SW/W toward daybreak as the low
moves to the north. Vsby reduced at times in areas of rain and

Saturday...Gale force SW/W winds developing in the afternoon with
gusts 35-45 kt. Gale warnings issued.

Outlook /Saturday Night through Tuesday/...

Sun night into Mon night...High confidence.
W wind gusts continue, reaching 35-45 kt while staying sustained 25-
35 kt.  Gale warnings are likely to continue through this entire
period.  Seas reach 10-12 ft on the ocean waters.

Mon into Tue...High confidence.
W winds continue, although they will gradually shift to the NW late
Mon into Tue.  Gusts reach 25-35 kt.  Small craft advisories will
follow the Gales.  Seas slowly diminish through the period, but
should remain between 5-8ft through much of the period on the ocean


A high astronomical tide and storm surge of about one foot will be
enough to cause some spotty, minor coastal flooding along the
Massachusetts east coast. Have issued a Coastal Flood Advisory for
Nantucket Harbor where the total water level is expected to reach
near or a little over the 5 foot threshold for minor coastal
flooding. Believe that the surge of .9 feet at 1:15 pm may
increase a little to 1.1 or 1.2 feet as a consequence of an
increasing easterly gradient piling some additional water into
Nantucket Sound. In addition, high tide may coincide with a period
of heavy rain that could have difficulty draining due to the high
harbor water level. Models suggest the nose of a 40 knot low level
jet will edge close to the east entrance to Nantucket Sound around
the time of the very late afternoon high tide. Elsewhere, believe
any minor coastal flooding will be spotty enough to be covered by
a coastal flood statement. Anticipate the usual suspects such as
Longs Wharf in Boston, a lane or two of Morrissey boulevard,
Scituate Harbor parking lot, etc. will get briefly wet.


MA...Coastal Flood Advisory until 7 PM EDT this evening for MAZ024.
MARINE...Gale Warning from noon Saturday to 8 AM EDT Sunday for
     Gale Warning from 2 PM Saturday to 8 AM EDT Sunday for ANZ230-


NEAR TERM...KJC/Nocera/Doody
TIDES/COASTAL FLOODING...Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.