Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 191805
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
205 PM EDT THU MAY 19 2016

.SYNOPSIS...
Upper level low pressure rides over the top of surface high
to bring scattered afternoon and evening showers with isolated
thunderstorms to Southern New England today. High pressure
regains control later tonight and brings dry mild weather for
Friday and much of Saturday. The high gives way to some rain
especially across the south Saturday night into Monday. Cooler and
unsettled conditions linger into early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...

2 PM update...

Convection beginning to fire inland especially across the high
terrain closer to the cold air aloft. Weak convergence across the
CT river valley helping to sustain convection with light easterly
flow from Worcester Hills eastward...while light westerly winds at
ORE/PSF and AQW. As mentioned in previous discussions magnitude of
cold air aloft (-23c at H5 already into Northwest MA) will
support a risk for a few small hailers. Meanwhile temp/dew pt
spreads of 20-25 degs will support the risk for gusty winds in the
stronger cells. Greatest risk for thunder/small hail and gusty
winds will be across the higher terrain such as the Worcester
Hills and East Slopes of the Berkshires.

Convection probably doesn/t reach RI and Eastern MA until 5 pm or
later. Showers and isolated thunder may dissipate before reaching
the coastline of RI and MA as instability wanes toward sunset.

Otherwise typical mid May day with highs in the upper 60s to lower
70s away from cooling seabreezes where the shoreline remains in
the upper 50s to low 60s.

Earlier Discussion...

Today...

Upper low pressure over the St Lawrence Valley with supporting
upper jet along the Mason-Dixon Line. Left exit region of the jet
lines up over Southern New England. Upper low and associated cold
pool will move east with the core of cold air over Northern New
England, but noticeable cooling aloft over Southern New England
with temps aloft reaching -22c to -24c. This along with some
daytime heating of the surface will create sufficient lapse rates
and instability to support rising air. The rising air will have
sufficient moisture to generate showers. Stability values will be
low enough to suggest a few tstms. Wet bulb zero values around
6500 feet and cold temps higher up would suggest possible small
hail. Can/t rule out strong wind gusts but confidence in wind is
very low. Forecast chance pops for showers/tstms and will mention
small hail.

Daytime heating will also help generate sea breezes on all coasts
starting later this morning through the afternoon.

Mixing is indicated to reach 800 mb. Temps at that level would
support max surface temps in the mid to upper 60s. With enough sun
the mixing could go a little higher and generate slightly higher
max temps. We will aim at the mid 60s to lower 70s with cooler
values on parts of Cape Cod and Islands.

&&

.SHORT TERM /FRIDAY/...

Tonight...

Cold pool aloft lingers overhead through the first part of the
night. Even with nighttime cooling this should maintain some
instability during early night. Continue scattered showers/tstms
before midnight. The airmass then stabilizes and dries. This will
mean clearing skies and an end to any showers. Not much of a
change in the airmass and dew points will again be in the 40s.
Therefore min temps expected in the 40s and lower 50s.

Friday...

High pressure surface and aloft will bring fair dry weather.
Temperatures aloft will be about 6C milder than today and so the
airmass will be more stable. Some lingering moisture around 800 mb
should support some diurnal cumulus development but by itself not
enough to support any showers except as noted below.

Northwest winds are expected to be light. If so then the daytime
heating may create another afternoon of sea breezes especially
along the east MA coast. It is possible that convergence along a
sea breeze front may be enough to set off a local shower.

Daytime heating will again build the mixed layer to near 800 mb
with temps at that level supporting max surface temps in the low
to mid 70s. If mixing goes above 800 mb then some upper 70s may be
possible.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...

Highlights...

 * Dry and warm Sat.
 * Unsettled late weekend and early next week.

Overview and Model Preferences... Highly amplified flow pattern will
progressively develop late this week and into the weekend.  Weakly
positive AO/NAO with strongly negative PNA are featured.  The
negative PNA is a reflection of slow moving, large and persistent
cutoff which will remain over the Pacific CONUS through the period.
Meanwhile downstream will be a gradually amplifying ridge continuing
to deepen/dig trof downstream.  This trof will be the defining
feature for the sensible wx across our area through early next week,
as it gradually deepens and cuts off completely.  The ridge to the w
will fold over the cutoff, locking it in place from late Saturday
into early Tuesday.  Therefore, after a quiet start to the weekend,
an unsettled and occasionally wet pattern looks to follow.
Reasonable synoptic-scale agreement here between operational models,
so a blend of these will work initially, however ensembles still
uncertain regarding the track of low pres Saturday night into
Sunday, which will ultimately define who does and does not receive
precipitation.  Will lean more on a probabilistic approach here,
using ensemble probabilities to help finalize pop/temps.

Details...

Fri night into Sat... Weak ridge of high pres gains control Fri and
then quickly loses it on Sat as upper lvl cutoff begins to develop
thanks to phasing of N and S streams.  Final period of definite dry
wx expected yielding cool min temps /thanks to radiational cooling/
Fri night, and warm highs on Sat.  Only risk here is that highs may
under-achieve a bit on Sat especially further S as increasing clouds
may limit mixing from fully realizing the H85 temps around +8C.
Still, will feature highs in the low-mid 70s across much of the area.

Sat night into Sun... Low pres looks to pass S of the 40/70
benchmark during the early AM hours on Sun morning.  How far N the
precip shield ultimately reaches will be a factor of this exact
track and the drier/cold advection settling in from the N.
ECENS/GEFS ensemble probabilities are highest /50-70 percent range/
mainly along the immediate S coast then dropping to about 30 percent
at the Mass Pike.  Will yield pops quite near these numbers given
the envelope of solutions between both the operational and ensemble
guidance.  In any case modest NNE flow develops on Sun, locking in
some of the low lvl moisture off of the Gulf Of Maine and yielding
colder than normal temperatures for much of the region.  Expect
clouds to remain in place much of the day thanks to this onshore
flow.

Even with the cloud cover, indications for instability trof to
develop during peak heating Sun afternoon which may spawn some more
showers and possibly even a rumble of thunder or two.  Especially
over the interior, where TT reach near 50 and H5 temps drop quite
close to -20C.  At odds will be the lack of sfc based instability
with NNE flow, but feel there is enough moisture/instability to
feature another round of shra later in the day Sun.

Mon and Tue... Cutoff remains in place across much of New England
through the early half of the next work week.  Although it will warm
aloft and gradually fill, the combination of colder H5 temp
anomalies, leftover moisture and cyclonic flow will yield unsettled
period especially defined by diurnal warming.  Still not a washout
here, as there are likely to be periodic breaks between the shra/ra
risk.  Overall temps show variance in the mid lvls and will likely
be dependent on cloud cover and final precipitation coverage.  Will
mainly go near to slightly below seasonal.

Mid next week... Cutoff to progressively shift E so depending on
timing, suspect there will be a transition to drier and warmer
conditions as it does so.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

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

Short Term /through Friday/...

2 PM update ...

Thru 00z ... high confidence on trends but some uncertainty how
far south and east showers and isolated thunderstorm track.
Activity may struggle to reach BOS-PVD and points SE.

Mainly VFR but brief MVFR in scattered showers and isolated
thunder. A few of the stronger cells may contain small hail and
gusty winds with greatest risk across the higher terrain. Light
East to Southeast winds across RI and Eastern MA. Elsewhere light
and variable becoming light westerly.

After 00z ... high forecast confidence.

Mainly VFR with any lingering MVFR showers and isolated thunder
ending between 01z and 03z. Light and variable winds becoming
light WNW.

Friday ... high forecast confidence.

VFR and dry weather. Diurnal clouds develop but will be in the VFR
category. Light WNW winds but seabreezes may develop along the
coast.

Friday night ... high forecast confidence.

VFR...dry weather and light winds.

KBOS TAF ... High confidence. Scattered showers and isolated
Thunder approach Logan/s airspace after 21z. However most activity
will be focused to the west across the Worcester Hills. Winds
remain mostly easterly.

KBDL TAF ... High confidence. Scattered showers and isolated
thunder. Low risk for a few storms containing small hail and gusty
winds thru 22z.

Outlook...Sat into Monday

Saturday...High Confidence. Mainly VFR with winds WSW.

Sat and Sun...Moderate Confidence.
Periods of MVFR/IFR possible through the period with rain/showers
possible as low pressure churns to the south.  This will be
intermingled with some VFR conditions as well.  Northeast winds
expected.

&&

.MARINE...
Forecaster Confidence Levels.

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

Today... High confidence.

Light winds and seas. There is a potential for a few light
showers or isolated thunder moving off the land mid to late
afternoon.

Tonight... High Confidence.

High pressure rebuilds over the waters. Any early night showers
or thunderstorms will quickly dissipate. Winds and seas will
remain below 15 knots and 5 feet.

Friday... High Confidence.

High pressure over the waters. Winds remain below 15 knots and
seas below 5 feet.

Outlook /Friday Night through Monday/...

Fri Night into Sat...High Confidence.
High pressure spills over the waters and allows for a period of
mainly quiet boating weather.

Sun and Mon...Moderate Confidence.
Low pres south of the region will bring a period of rain, especially
across the southern waters.  Northeast winds will gust to around 25
kt at time Sun, while a building swell reaches 5-6 ft and lingers
into Mon. Small craft advisories are likely to be needed.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...WTB/Doody
NEAR TERM...WTB/Nocera/Doody
SHORT TERM...WTB
LONG TERM...Doody
AVIATION...WTB/Nocera/Doody
MARINE...WTB/Nocera/Doody



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