Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 290227
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1026 PM EDT SAT MAY 28 2016

.SYNOPSIS...

A backdoor cold front begins to drop S over E/NE MA. Ahead of this
front on Sunday, scattered showers and thunderstorms redevelop,
especially across western MA and CT. Expecting rain late Sunday
night through Memorial Day, heavy at times, with the chance of
thunderstorms. With tropical moisture streaming north from Tropical
Depression Number 2, there is a potential threat for localized flash
flooding. A renewed strong to severe weather threat is possible over
interior New England late Monday. Dry conditions with warm
afternoons follow Tuesday into Wednesday.  Unsettled weather may return
by the end of next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...

1025 pm update...

Still very mild late this evening with temps mainly in the 70s to even
the lower 80s in Boston. Main story overnight will be a backdoor
cold front that will slip south into north central and northeast MA
by daybreak.  This will allow cooler marine air to work into this
region allowing low temps to drop to between 60 and 65 by daybreak.
Meanwhile, portions of western Ma and northern CT will see overnight
lows mainly in the upper 60s to near 70.

Despite some elevated instability, not much forcing for
precipitation overnight.  Can not rule out a brief spot shower/t-
storm but that vast majority of the night will remain dry.  Patchy
fog may develop in the typically prone locations and may end up a
bit more widespread across northeast MA behind the backdoor cold
front.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...
Sunday...

Cooler conditions push into N/E MA associated with a backdoor cold
front as shower and thunderstorm activity emerges across the S/W
interior. Two airmasses to contend with as there is some trickiness
in nailing down specifically where the dividing line will set up
especially with respect to temperature forecasts.

With confidence, behind the backdoor cold front cooler conditions
will prevail with NE flow and low clouds. A definite chill to the
air as it will be roughly some 20 degrees cooler than the day prior
with highs around 70 degrees.

S/W, warmer conditions with highs close to 90, especially in the
lower CT River Valley. Likely scattered shower and thunderstorm
activity with increasing S winds ahead of a strong synoptic
disturbance over the Great Lakes Region. Despite continued mid to
upper level ridging and limited upper level support, albeit
weakening, slightly stronger shear coupled with robust instability
throughout the entire atmospheric layer up to the equilibrium level
yields the potential for strong thunderstorms over N/W interior
portions of MA and CT. Storms have the potential for producing heavy
rain, perhaps even gusty winds and small hail considering the CAPE
and shear profiles. But again, missing the upper level support, much
of the forcing confined to the low to mid levels, especially more so
wherever the increasing S flow converges with high terrain in areas
of high instability. This coinciding with SPC`s Day 2 outlook of a
general thunder risk.

One final note concerning the thunderstorm threat, consensus of
models have some drier air working N along with the increasing S
winds ahead of the tropical moist plume for Sunday night into
Monday. Timing uncertain, it could throw a wrench in the entire cog-
wheel process of convection, pushing the focus area of activity
further N/W outside of MA and CT. Will watch this closely, but aside
the consensus of overall guidance has a potential threat for N/W
portions of CT and MA.

Sunday Night into Monday...

With convection concluding into the evening hours, becoming dry,
focus of attention shifts towards a moderate to heavy rainfall event
developing late overnight and continuing through Monday as moisture
associated with Tropical Depression 2 streams N ahead of a series of
trailing cold fronts associated with a synoptic trough disturbance
sweeping W to E.

Focus is upon areas of deep layer lift, specifically where the right
entrance region of the upper level jet aligns with mid level vortex
energy out ahead of the deamplifying trough pattern, and with
convergent low to mid level forcing along and ahead of an initial
sweeping pre-frontal boundary. This with a deep-layer SW to NE
training environment of warm-moist tropical-origin airmass with
precipitable waters up to around 2-inches, near-record highest for
the end of May looking at atmospheric sounding climatologies. H85
dewpoints in excess of 10C, mixing ratios above 12 g/kg, and some
elevated instability.

In all, and per a consensus of the guidance, expect a sweeping band
of moderate to heavy rainfall, W to E, with embedded thunderstorm
activity. There is perhaps some model forecast struggle with amounts
and location of heavy rainfall / localized flooding, given the
uncertainty as to how fast the cold front will sweep S New England
and subsequent time in which the tropical-origin moist plume
environment will be over any one region. But aside, the overall take-
away is that there is a flash flood potential especially for urban
centers should tropical-origin rains stream SW to NE over any one
area for a prolonged period of time, transitioning through the
period from early Monday morning into evening W to E. Keep in mind
this is quite an infrequent event for this time of year.

Now as this activity moves out, there is still the mid to upper
level synoptic trough disturbance and accompanying surface cold
front ahead of which partial clearing is forecast in wake of a pre-
frontal boundary and tropical moisture. A rebound in instability
beneath stronger W/SW flow and subsequent shear, SPC Day 3 forecast
has a marginal threat mainly N/W of S New England for strong to
severe thunderstorms with threats of heavy rain, strong to
damaging winds, and small hail. Timing is an issue as to how
quickly tropical- origin activity will exit to the E and the
atmosphere over W New England can rebound and destabilize. During
the late afternoon and towards evening, it is quite possible that
thunderstorm activity could enter W New England. Something to
watch out

Monday Night...

Convective activity withers with the lack of diurnal heating. The
associated cold front weakening sweeps SE across S New England
behind which W/NW follows pushing into the region much cooler and
drier air, while shunting tropical moisture out to sea. Will see
lows drop back into the 50s.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Highlights...

* Mainly dry with above normal temperatures Tue/Wed
* Unsettled weather may return toward next weekend

Details...

Tuesday and Wednesday...

Dry weather with above normal afternoon highs are anticipated
away from the immediate coast. Dewpoints mainly from 55 to 60, so
it will still feel humid.

Thursday into Saturday...

Forecast confidence remains rather low for this time frame.
Increasing humidity with south winds ahead of an approaching cold
front, especially Friday into Saturday. Timing this front remains
one of the biggest challenges. Showers, and perhaps a few
thunderstorms, possible. Way too early to try to narrow down the
timing, although afternoon and evening hours would be
climatologically favored. Kept just a chance for measurable
rainfall in the forecast, until the details become more clear.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

Short Term /through Monday Night/...

Overnight...Moderate to high confidence.  Mainly VFR but may see
some patchy fog resulting in some MVFR conditions after midnight.
Also, backdoor cold front may allow a deck of 1000 to 1500 foot
stratus and perhaps some fog to slide into Northeast MA including
Boston near 12z.

Sunday...Moderate Confidence.

Winds back NE across E/NE MA with MVFR/IFR cig and vsby impacts.
Towards the SW of this wind shift, SCT SHRA/TSRA expected with
TEMPO MVFR/IFR impacts and possible 20-40 kt gusts. Focus around
midday into afternoon. S winds overall for those areas S/SW of the
NE wind shift.

Sunday Night into Monday...Moderate Confidence.

MVFR/IFR as RA/+RA with possible TSRA sweep W to E through the
day. Cig and vsby impacts with strong S winds gusting 20 to 25
kts. For those areas W that clear out earlier, there is the
potential for additional SHRA/TSRA activity late in the day. Low
confidence with respect to impacts/outcomes.

KBOS TAF...Moderate to high confidence in TAF.  Mainly VFR overnight
but probably will see low end MVFR CIGS arrive near 12z behind the
backdoor cold front.

KBDL TAF...Moderate to high confidence in TAF.

Outlook /Tuesday through Thursday/...

High confidence VFR. Low probability for MVFR in isolated showers
or thunderstorms Thursday.

&&

.MARINE...
Forecaster Confidence Levels.

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

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

S/SW winds prevailing with gusts up around 20 kts and seas holding
below 5 feet through Sunday evening over the S waters. Some
visibility restrictions with mist or patchy fog not out of the
question with warm-moist air streaming over the cooler waters.
Whereas the E waters winds will shift out of the NE briefly
tonight and through Sunday before reverting back out of the S/SW.

Late Sunday night into Monday will see tropical moisture associated
with TD 2 yield rain, locally heavy at times, along with the
chance of thunderstorms across all waters. Likely visibility
restrictions in accordance with rain, along with mist or fog
forecast. Pronounced S/SW winds ahead of the front with gusts up
to 25 kts look to yield seas of 5 to 6 feet Monday into Monday
evening. Small Craft Advisories may be needed.

All this activity clears out late into Monday night as a cold
front sweeps the waters behind which winds back out of the W.

Outlook /Tuesday through Thursday/...

Tuesday...High confidence. Lingering 5 foot swell possible across
our southern waters , but should diminish by late Tuesday night.

Wednesday and Thursday...Moderate confidence. Relatively tranquil
boating weather. Increasing east winds Thursday could lead to some
rough seas across the outermost coastal waters Thursday afternoon
and evening.

&&

.CLIMATE...
Record highs for today, Saturday May 28...

Boston...          92 Tied (also set in 1931)
Providence...      91 set in 1931
Hartford...        93 Tied (also set in 1977)
Worcester...       89 New Record (previously 88 in 1929, 1911)
Milton/Blue Hill...91 New Record(previously 90 in 1929)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ004>007-
     009>018-026.
RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>005.
MARINE...None.

&&

$$
SYNOPSIS...Belk/Sipprell
NEAR TERM...Frank
SHORT TERM...Sipprell
LONG TERM...Belk
AVIATION...Frank/Belk/Sipprell
MARINE...Belk/Sipprell
CLIMATE...Staff



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