Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 191412
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1012 AM 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/...

1015 am update...

Satellite imagery and surface observations continues to indicate
lots of clouds across the area. 12z observed soundings from
ALY/OKX AND CHH show this moisture in the 800-600 mb layer. SPC
meso analysis indicates this mid level moisture and spotty light
sprinkles is in response to weak convergence from the surface up
to about 850 mb. This may delay afternoon convection as this area
of cloudiness will take some time to erode and move offshore.

Latest water vapor imagery shows an impressive vortex/mid-upper
level low just north of Lake Ontario. Given mostly clear skies
just downstream of this feature over upstate NY expect convection
to fire there by 16z or so. Critical to our forecast will be how
quickly clouds over the region now dissipate and/or move offshore
during the afternoon.

Still anticipating scattered showers and isolated thunder with
risk of small hail and gusty winds. However if cloud cover lingers
areal coverage of afternoon/evening convection may be less than
earlier thought. Stay tuned.

Previous forecast was verifying nicely this morning so no major
changes with this update.

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 /14Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

Short Term /through Friday/...

1415z update ... not much change from 12z tafs. Mid level
cloudiness may delay onset of afternoon convection and perhaps
limit areal coverage as well. Scattered showers and isolated
thunder risk is after 17z inland to after 19z Coastal plain
including Boston-Providence corridor. Peak of activity likely
20z-24z. Earlier discussion below.

====================================================================

Today... High confidence.
General VFR with scattered MVFR cigs/vsbys in showers and
scattered thunderstorms mainly this afternoon and evening. A few
storms may contain small hail. Initial showers in western CT/MA
around midday. Showers shift east reaching BOS Metrowest and
Northern RI around 3 pm.

Tonight... High Confidence. Continued risk of scattered
showers/tstms in the evening with scattered MVFR cigs/vsbys.
Conditions improve to VFR all areas by midnight. Light NW flow
overnight.

Friday... High Confidence. VFR. Sea Breezes developing late
morning and afternoon along the coast.

KBOS TAF...High confidence. Scattered showers and isolated
Thunder roughly 22z-00z. Most activity will be focused to the
west across the Worcester Hills.  Winds remain mostly easterly.

KBDL TAF...High confidence. Scattered showers after 16z with
isolated thunder. Low risk for a few storms containing small hail
and gusty winds 18z-21z.

Outlook...Friday night into Monday

Friday night and 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/Doody
SHORT TERM...WTB
LONG TERM...Doody
AVIATION...WTB/Nocera/Doody
MARINE...WTB/Nocera/Doody



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