Area Forecast Discussion
Issued by NWS Boston, MA

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767
FXUS61 KBOX 272311
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
711 PM EDT Thu Apr 27 2017

.SYNOPSIS...
Another mild night with some more patchy dense fog overnight
will lead to a mix of showers and isolated thunderstorms during
the morning. Warmer highs Friday will give way to another mild
night Friday night. A cold front crosses the region Saturday
night, bringing much cooler weather for Sunday, especially along
the coast. Another cold front will likely bring a period of
showers sometime Monday into Tuesday. Another storm may approach
New England late Thursday.

&&

.NEAR TERM /UNTIL 3 AM FRIDAY MORNING/...

7 PM Update...

South flow across CT and Western Mass, as well as across RI and
the Cape/Islands. Meanwhile east flow along the Eastern Mass
coast north of Plymouth and extending to the Worcester Hills.
Lots of clouds in place with patches of low stratus along the
coast but two cloud layers with bases 1500-3000 feet.

The weak flow and dew points in the low to mid 50s suggest that
as temperatures dip back close to dew point we will see fog
developing. No changes in forecast thinking at this time, with a
trend maintained to lowering clouds and fog. Min temps still
progged at low to mid 50s.

Previous discussion...

Within the stratus, marine fog also possible once again, but
this time likely limited more to the S coast initially. Will be
hoisting another Dense Fog Advisory for S coastal locations
which may need to be expanded depending on how efficiently it
is able to move inland with the S-SW flow. Not as confident
further inland, but overnight crew can monitor its progress. In
any case some clearing of this fog is likely toward the AM
commute time thanks to the influence of mechanical mixing with
the approaching SHRA (more on this below). Therefore, am capping
the end time of the current Dense Fog Advisory to 09Z (5AM
local).

&&

.SHORT TERM /3 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Early this morning and the daytime tomorrow...
Robust shortwave and attendant front will be approaching from
the W and crossing the region mainly between 09Z an 18Z, slowing
as it begins to parallel the W flow aloft, and possible
stalling somewhere near S coast CT/RI and MA by mid day. Lapse
rates are already going to be near 6.0C/km by the time of the
arrival of this front thanks to a mix of the low-lvl warming
with return flow and cooling in response to the approaching
shortwave. However initially, column is too dry. This is
remedied after 06Z as PWATs begin to approach 1.25-1.50 inches
or nearly 2 std deviations above normal. This will increase the
elevated CAPE profile such that MU Cape values by the time of
the approach are generally around 500j/kg and only increasing as
the front pushes E. Therefore, will continue to highlight the
risk for showers with embedded elevated TSRA between 08Z (4AM
local) and 15Z (11AM local) until drier air and subsidence
pushes into the W. Although 0-6km shear is marginal at around 40
kt, the lack of overall instability will limit the TSRA to
generally isolated and non-severe.

As the front settles near the S coast, it will be a race between
the drier air moving in from the W and NW and the remnant low-
lvl moisture which will become trapped beneath the encroaching
subsidence inversion. Although the drier air aloft and some
capping inversion development should limit the secondary
redevelopment of TSRA through the afternoon, am a bit concerned
with the front hanging nearby as a source for lift even though
it will be washing out given that lapse rates and CAPE profiles
remain somewhat elevated. Later shifts may want to re- evaluate
this threat. It is a low risk given the drying/subsidence
mentioned above, but it`s something to watch and models are not
producing any QPF during the afternoon which is a typical red
flag.

Temps will be warmer tomorrow thanks in large part to clearing
and better mixing with the subsidence inversion across the W.
H85 temps are near +12C, which should allow for spots of upper
70s to low 80s. One caveat may be S coastal areas if the front
does indeed stall over land, as it could allow for the
continuation of some clouds much later into the day, limiting
the heating in spots along the coast of RI and SE MA. Current
highs may be too warm if this occurs.

Tomorrow night...
In spite of the spots of clearing during the evening, skies will
once again be increasing as yet another weak front approaches
from the W and winds shift to the SW once again. This should
limit mins overnight to the mid 50s widespread. Given the
lingering subsidence inversion will be holding POPs back until
after sunrise in W MA/CT for another round of SHRA.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Big Picture...

Longwave pattern shows ridges off each coast with a trough over the
Midwest.  Shortwave pattern shows three systems of interest in the
long term. The first is a shortwave racing across Quebec Saturday as
high pressure builds north from the ocean. The second is a broad
closed low over the Plains this weekend that ejects through the
Great Lakes Tuesday. Trailing shortwave energy from the North
Pacific generates a second closed low over the Plains midweek.

Model mass fields are similar through early next week with some
differences by midweek.

Details...

Saturday...

Shortwave crossing Eastern Canada driven by a 160 knot upper jet.
Upper venting along this jet seems focused over Northern New
England through the day although some signs of it shifting to
the northern MA border by evening. Axis of the shortwave passes
late aftn/evening with signs of a surface wind shift over Srn
New England 18Z-24Z. Stability parameters show 500-1000 J/Kg and
totals of 48- 50. Precip water values are forecast to reach 1.6
inches, well above normal for the end of April.

With the best forcing to our north, we will mention scattered
showers N/W of I-495/I-84 and include a risk of thunder. Could be a
few heavier showers embedded in any wet weather.  Mixing to 900 mb
and possibly 850 mb, which could bring down wind gusts of 20-25
knots. Temps in the mixed layer support max sfc temps in the upper
70s and low 80s.

Saturday night through Sunday night...

High pressure builds south from Canada Saturday night bringing drier
weather, but also a trend to an east wind on Sunday.  This will keep
temps cooler near the eastern coast while warmest temps will be in
the CT Valley. Expect a lower mixed layer Sunday, but temps near 925
mb should support interior sfc temps reaching the mid and possibly
upper 60s while the coast is in the mid 50s.

Winds turn from the south Sunday night as warmer air moves north.
This leading edge of warm air may generate low clouds and drizzle
and possibly a few light showers.

Monday-Tuesday...

Broad closed low over the Plains ejects through the Great Lakes.
This system will sweep a cold front through New England Tuesday
morning. Strong low level southwest jets develop, one inland ahead
of the main low...and the second along the coast. The coastal jet
begins to affect our area early Monday with a 65 knot core moving up
across the South Coast and Islands by Monday night. Expect showers,
especially Monday night and Tuesday morning.  Precip water values
climb to 1.7 inches, quite high for early May, and may contribute a
few locally heavy downpours. Stability parameters also suggest
potential for a thunderstorm.

Wednesday-Thursday...

High pressure builds over the region with mainly dry weather.  Upper
shortwave and cold temps aloft may generate diurnal clouds
Wednesday.  Models show another Plains low spreading clouds and
precip our way but with 18-24 hour differences in timing. For now we
will show chance pops developing from the southwest Thursday evening
and spreading across the region overnight. But this timing is low
confidence and will need to be monitored.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Tonight...Moderate confidence.

Lingering break in the clouds over Boston and Northeast MA, but
cigs 1500-3000 in the interior and cigs 200-400 feet in patches
along the coast. Expect east winds in Northeast MA to trend from
the south during the night but remain light. This should allow
cigs to reform where absent and lower where present. Low stratus
is most confident along the South Coast and Islands where the
wind is directly off the water.

This trend should bring another round of fog tonight, but not
as widespread as last night, and should mainly be focus around
the S coast of MA/RI and possibly snaking up the CT Valley late.
In any fog LIFR vsbys possible at times. Some showers and
isolated TSRA possible mainly between 09Z and 14Z from W-E. This
will help improve conditions to mainly IFR.

Friday and Friday night...High confidence in trends, but lower
confidence in exact timing in TAF.

IFR/MVFR CIGS and fog, along with showers and isolated
thunderstorms end between 12Z and 16Z in the morning, with
fairly rapid improvement to VFR in clearing. Winds turn W, with
gusts to 15 kt at times. VFR continues into the overnight thanks
to W winds, although there is a low risk for showers, but
mainly after daybreak Sat.

KBOS TAF...Moderate confidence in TAF. VFR conditions early will
trend to IFR overnight, but lower confidence in the LIFR risk.

KBDL TAF...Moderate confidence in TAF. There is a low risk for
LIFR fog after midnight and early Friday.

Outlook /Saturday through Tuesday/... Moderate confidence.

Saturday... Moderate-High confidence.

Mainly VFR, but with brief MVFR cigs/vsbys in showers and isolated
thunderstorms in the afternoon/evening.

Sunday... High confidence.

VFR.

Sunday night to Tuesday... Moderate confidence.

MVFR/IFR cigs/vsbys in low clouds/drizzle and light showers Sunday
night.  VFR and areas of MVFR in showers Monday and Tuesday.
Possible low level wind shear especially over RI and SE Mass Monday
night and Tuesday morning.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Small Craft Advisories will remain in effect mainly over the
offshore waters through Friday night as a lingering 5-7ft swell
remains. Winds shifting from S tonight to W during the day
Friday then back to the SW Friday night should remain below the
25 kt threshold through the period.

Otherwise, another round of overnight dense fog across the
waters tonight and again Friday night. A low risk for a few
thundershowers moving offshore during Friday morning.

Outlook /Saturday through Tuesday/... Moderate confidence.

Saturday-Sunday...

West winds remain less than 25 knots.  Seas briefly near 5 feet on
the southern outer waters Saturday.

Monday-Tuesday...

Cold front approaches Monday and crosses the waters Tuesday. This
will bring showers and possibly a thunderstorm Monday night and
Tuesday. Seas build to 5-8 feet on the outer waters and exposed
southern waters. Expect increasing southwest winds with gusts to at
least 25 knots Tuesday. There is low potential for gale force gusts
to reach down to the waters Monday night and Tuesday.  Absent the
development of southwest gales, a Small Craft Advisory may be
needed.

&&

.TIDES/COASTAL FLOODING...
* Isolated minor coastal flooding impacts during astronomically
  high tide tonight along the east coasts

The astronomical high tides remain very elevated through the
end of the week.

Given a lingering swell offshore and its remnant surge, with
high tide values only a few tenths shy of typical minor flooding
levels, a Coastal Flood Statement continues for the high tide
near midnight tonight for east coastal locations. At this time
we anticipate only minimal impacts at typically prone locations.

Also, due to the breadth of the fetch, we may encounter enough
wave action with lengthening periods on top of an elevated tide
to cause pockets of erosion tonight along the Martha`s Vineyard
south coast, Nantucket south and east coasts, and the outer Cape
ocean side from Truro to Chatham.

Boston High Tides (flood stage at Boston 12.5 feet)...

12.15 feet / Friday 12:15 am

Providence High Tides (flood stage at Providence 7 feet)...

6.25 feet / Thursday 9:50 pm

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 5 AM EDT Friday for MAZ020>024.
RI...Dense Fog Advisory until 5 AM EDT Friday for RIZ005>008.
MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ235-237.
     Small Craft Advisory until 11 PM EDT Friday for ANZ250.
     Small Craft Advisory until 2 AM EDT Saturday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Doody
NEAR TERM...WTB/Doody
SHORT TERM...Doody
LONG TERM...WTB
AVIATION...WTB/Doody
MARINE...WTB/Doody
TIDES/COASTAL FLOODING...



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