Area Forecast Discussion
Issued by NWS Boston, MA

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730
FXUS61 KBOX 291752
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
152 PM EDT Mon May 29 2017

.SYNOPSIS...
A fast moving low pressure system will bring a period of
showers to the region today, however a washout is not expected.
Onshore winds, clouds and showers will result in unseasonably
cool temperatures today. A couple of weak cold fronts may
trigger scattered showers and possibly thunderstorms for the
remainder of the week from to time to time. Otherwise fair
weather with near seasonable temperatures are expected into next
weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
145 PM update...
Rain assocd with the coastal wave will be moving offshore.
However, the second area of rain will continue to move across
SNE next few hours as mid level shortwave lifts NE across
central New Eng. Rain will end from west to east late afternoon
and early evening as the mid level dry slot moves in.

Previous discussion...
Continue to have two areas of rain, one moving into western New
Eng assocd with a mid level shortwave lifting NE from NY/PA
border, with second area along the south coast. Dry air above
the boundary layer holding the rain off across northern and NE
MA. Coastal low off the DE coast forecast to move east today
with core of low level jet remaining offshore so rain area along
the coast will struggle to move northward. We do expect some
light rain to eventually fill in across northern and NE MA this
afternoon as mid level shortwave lifts NE across central New Eng
and the column moistens. However, the rain will likely be
scattered in this region where least amount of QPF expected,
likely less than 0.10". Mid level dry slot moves into the region
from the west late afternoon and evening which will bring an
end to the rain. Updated PoPs to reflect near term trends.
Unseasonably chilly day with temps remaining in the 50s. But
close to 50 degrees east coastal MA which is about 20 degrees
below normal for this region.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Tonight...

Shortwave and deeper moisture will have exited the region by this
evening. However, moist northeast low level flow will probably allow
scattered light showers/drizzle to persist into the overnight hours.
Highest confidence is across eastern MA but cannot rule out for
points west due to upslope flow. Aside from drizzle, anticipate
areas of low clouds and patchy fog to persist as low level moisture
may remain trapped below the inversion. Low temps will mainly be in
the upper 40s to the lower 50s.

Tuesday...

Weak ridging aloft on Tuesday will keep the region in southwesterly
flow as weak surface high pressure moves south of the region. The
area will remain mostly dry but cannot rule out a few isolated
showers out west.

A quick moving shortwave will pass through the flow during the day
which could trigger showers and perhaps a thunderstorm or two.
Highest confidence is out west where better instability resides.
Some hi-res guidance supports the storms that develop in update NY
will moves into the region by late afternoon/evening. This is in
conjunction with SPC marginal risk.

Tricky temperature forecast for Tuesday as 850 mb temps support
warmer conditions then what guidance is indicating. However 925 mb
temps show that the Maritime airmass will remain locked in over SNE
until the later half of the day. Despite some heating in a mix of
sun and clouds, temperatures will remain in the upper 60s to low
70s.

&&

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

* Temperatures near normal this period with summer heat and humidity
  remaining suppressed well south of New England

* Several episodes of showers but also many hours of dry weather too

Synoptic Overview...

Both GFS and EC ensembles support a slow moving omega block pattern
moving across Canada this week into next weekend, resulting in
cyclonic flow and below normal heights over New England. This
suppresses the subtropical ridge across the southern states,
precluding summer heat and humidity from advecting northward into
New England. Thus temps here close will be near normal this period
for southern New England. As for precip, cyclonic flow into the
northeast will yield several short wave troughs and attending cold
fronts sweeping across the region resulting in a few chances for
showers and thunderstorms, but also many hours of dry weather too.
Above average forecast confidence on the overall pattern/theme but
high uncertainty on timing each individual short waves/fronts and
associated timing of shower threat.

Daily details...

Tue night...

Short wave trough accompanied by modest forcing for ascent coupled
with sufficient deep layer moisture to support numerous showers,
especially late evening and overnight. South to Southeast winds will
result in onshore winds and weak low level warm advection. Thus
temps will not be as cool as previous nights.

Wednesday...

Warmer than today and Tue with highs well into the 70s as maritime
airmass moves offshore. Deep layer moisture and forcing for ascent
over the area in the morning resulting in a risk for showers.
However by late morning and especially the afternoon K indices fall
rapidly indicating mid level dry beginning to stream across the
region and dry the column from the top down. This drier air aloft
does steepen mid level lapse rates however it appears the dry air
becomes too deep to support convection. Meanwhile at the surface dew
pts remain somewhat elevated in the MU50s to near 60. This yields
MUcapes of 1000-1500 j/kg. Although deepening mid level dry air will
likely preclude convection from developing. However still 2+ days
away so will have to monitor later trends. Otherwise models in good
agreement on a fairly nice day with the trend being morning showers
giving way to partial afternoon sunshine.

Thursday...

Good model agreement that short wave trough exits east Wed night
with slightly cooler and drier air post frontal airmass for Thu with
dew pts falling from the 50s Wed to the 40s Thu. Seasonably warm
with highs 70-75. Could be the pick of the week.

Friday...

WAA pattern ahead of next short wave trough with a risk of showers
during the day and/or at night. Obviously model timing differences
at this time range.

Weekend...

Surprisingly both deterministic and ensemble guidance in good
agreement for the start of next weekend with mean mid level trough
axis over or east of the region. This would suggest dry weather to
start the weekend. However models differ on how quickly the
northeast trough reloads with upstream jet energy. Thus Sunday`s
forecast becomes highly uncertain. Temperatures look to be near
normal next weekend...highs in the 70s and lows in the 50s.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday night/...Moderate confidence.

Through tonight...MVFR/IFR conditions will continue. Patchy LIFR
fog developing tonight, especially near the coast. Rain will
end from west to east 20-23z.

Tuesday...MVFR/IFR cigs will lift to VFR/MVFR in the afternoon.
A few showers or an isold t-storm possible across western MA by
late afternoon.

Tuesday night...More MVFR/IFR conditions with sct showers and
isold t-storms moving across the region.

KBOS TAF...Moderate confidence in TAF. Lowering vsbys likely as
fog redevelops but timing uncertain.

KBDL TAF...Moderate confidence in TAF.

Outlook /Wednesday through Friday/...Moderate confidence.

Generally VFR. Potential for MVFR cigs/vsbys in afternoon/evening
showers and scattered t-storms. Best chance for the latter will be
in Western/Central Mass and Northern Connecticut Tue night into Wed
morning. Thursday looks to be mainly dry with another risk for
showers and isolated thunder Fri/Fri night.

South winds Tuesday night, becoming southwest Wednesday, west
Thursday, and southwest Friday. All winds should be less than 20
knots.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Today...Moderate to high confidence. High pressure building
across the Canadian Maritimes coupled with a wave of low
pressure passing to our south will generate easterly wind gusts
around 25 knots for the waters. Thus will continue with SCA.
Easterly fetch will also build seas between 3 and 6 feet across
our open waters. The strongest wind gusts/seas will be across
our southern waters. Rain and patchy fog is also expected to
develop and an isolated t-storm or two can not be ruled out
across our southern waters.

Tonight...Moderate to high confidence. Winds and seas will
begin to diminish this evening and especially after midnight as
disturbance moves away from the region and pressure gradient
weakens. Areas of fog may persist into early Tue am reducing
vsbys for mariners.

Tuesday...High confidence. Improving conditions as weak high
pressure moves over the southern waters. Could see areas of fog
or drizzle but seas and winds will remain below SCA.

Outlook /Tuesday Night through Friday/...Moderate confidence.

Tuesday night through Friday...

Winds less than 25 kt most if not the entire forecast period. Vsby
may be restricted Tue night into Wed with scattered showers and
isolated thunder possible. Dry/tranquil likely beginning Wed
afternoon into Thu then another round of showers and isolated
thunder possible Fri/Fri night.

&&

.TIDES/COASTAL FLOODING...
Latest observations continue to show a storm surge just over a
half foot along the eastern MA coast for this current high tide.

For tonight, another high astro tide of 11.5 feet occurs just
after 3 am in Boston. While that is lower that previous nights,
the onshore component will remain with a storm surge of a half
foot or even a bit more. While no significant problems are
expected, very minor nuisance coastal flooding is possible along
the most vulnerable shore roads along the eastern MA coast. Will
hold off on coastal flood statement at this time.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
     ANZ231>235-237.
     Small Craft Advisory until 2 AM EDT Tuesday for ANZ250-254>256.

&&

$$
SYNOPSIS...Nocera/Dunten
NEAR TERM...KJC
SHORT TERM...Dunten
LONG TERM...Nocera
AVIATION...KJC/Nocera
MARINE...Nocera/Dunten
TIDES/COASTAL FLOODING...



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