Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 291058
AFDBOX

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

7 AM update...

Two areas of rain this morning, first area is across NY state
and is associated with parent low over the eastern Great Lakes.
Second area of rain moving onshore to the CT/RI coastline
including Marthas Vineyard and Nantucket. This area of rain is
associated with a coastal low off the DE shore. SPC mesoanalysis
shows lot of dry air at 925 mb and 850 mb over New England.
Therefore the northern edge of the rain shield along the south
coast will erode somewhat as it tracks northeast. Updated hourly pops
to reflect this thinking. Expecting rain shield to become more
fragmented this afternoon as short wave trough deamplifies as it
climbs up and over the east coast ridge. Otherwise previous
forecast verifying nicely.

Temp not rising much today from current morning values as
clouds, onshore winds and showers will keep temps only in the
50s. Normal high temp for today should be around 70. Thus much
cooler than normal. Earlier discussion below.

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

A chilly and damp Memorial Day will be in store for southern New
England today. Coastal low off the Mid-Atlantic coast will continue
to develop and move eastward for today. This low will strengthen as
it passes south of Long Island and SNE. Meanwhile, an upper level
trough will pass to the west. The combination of these two systems
will bring scattered to widespread showers for today.

Latest guidance has trended downward in qpf amounts as coastal low
has trended farther south than previous guidance. This may actually
cut off some of the moisture for the western system. Thus, locations
across eastern Massachusetts may experience just showers than heavy
rainfall. The chances for thunder also seems slim as the region
remains on the cool side of guidance.

The bulk of the rainfall will move through the region late this
morning into the afternoon hours. 700 mb dry slot will swing through
during the later half of the day resulting in more drizzle than
rainfall this evening.

Temperatures for the region will remain well below average as
Maritime airmass spread westward. This easterly flow combined with
cloud cover will result in temperatures in the mid 50s.

&&

.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 /12Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

7 AM update...

IFR cigs over eastern MA and RI will continue to move westward
into CT and western-central MA today. Rain overspreads the
region from the south and west today. Not much change from
previous TAFs. Earlier discussion below.

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

Today and tonight...Moderate confidence. MVFR to IFR conditions
overspread the region through the morning and into the early
afternoon in a period of rain. Easterly wind gusts around 20
knots are expected on the coast, with up to 25 knots across
portions of the Cape/Islands.

The rain will taper off to light showers/drizzle from west to
east late this afternoon/early evening. However, MVFR-IFR
conditions may persist for much of Mon night with the lowest
conditions most likely on the coast.

MVFR on Tuesday with a few sites reaching VFR. Isolated shower
or two is possible, with a better chance for showers/thunder
across western terminals by the evening.

KBOS TAF...Moderate confidence in TAF.

KBDL TAF...Moderate confidence in TAF.

Outlook /Tuesday Night 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 from 11 AM this morning to 2 AM EDT
     Tuesday for ANZ250.
     Small Craft Advisory until 2 AM EDT Tuesday for ANZ254>256.

&&

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



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