Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 182008
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
408 PM EDT Fri Aug 18 2017

.SYNOPSIS...

Widespread rain with embedded heavier showers and thunderstorms
overnight. Main threat of localized flooding. Drying out into
Saturday ahead of a cold front which moves offshore Saturday
night. High pressure returns with fair seasonable weather Sunday
through Tuesday. Another cold front sweeps through New England
Wednesday, followed by more dry weather Thursday into Friday.

&&

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

*/ Highlights (4p update)...

 - SEVERE THUNDERSTORM WATCH 455 in effect over far W MA and CT.

 - Flooding concerns across the area, especially with regards to
   the present train of rain with embedded heavier showers over
   N/W MA, and over S/E New England as convection out of SE PA,
   NJ and the Delmarva region advects N/E fueled and maintained
   by a convergent low level jet of tropical moisture. Rainfall
   rates of around 1-2 inches with storm total rainfall amounts
   up to around 3 inches.

 - Lower risk with respect to strong to severe storms. Greater
   focus over S/W portions of MA and CT. Potential of gusty
   winds. Lower risk of a brief tornado given the environment.

 - No headlines will be issued with this update.

*/ Overview (4p update)...

Ahead of a deep low center over the N Great Lakes Region, tropical
moisture is surging N, converging and over-running along a warm
frontal boundary presently hung up along the E-waters immediately
offshore of New England as discerned via H925-85 SPC mesoanalysis.
Meanwhile the atmosphere has destabilized ahead of a pre-frontal
trough where partial clearing has occurred in an area of higher
surface dewpoints, deeper tropical moisture, and modest shear. All
of this activity is forecast to converge over S/E New England and
adjacent waters this evening and overnight out ahead of a surface
cold front and attendant H5-7 mid-level dry punch. The H925-85 low
level jet intensifying per isallobaric response out ahead of the mid-
upper level ridge over the NW Atlantic, maintaining convection and
heavy rain threats N/E through Saturday morning prior to cold frontal
passage. Drying out NW to SE as winds turn westerly. Any dense fog
that develops during the overnight period as it is expected with
the very humid, muggy, moist airmass, eroding. Lows around the low
70s.

*/ Discussion (4p update)...

Straight-forward, continued forecast of widespread rain with embedded
heavier downpours and thunderstorms. Two things of continued concern:
1.) localized flooding, and 2.) strong to severe storms.

Near-term high-res guidance such as the HRRR is preferred. However,
18.12z NAM and WRF models are only partially considered given poor
initialization.

Localized flooding...

While all of S New England remains under threat, higher confidence
of potential impacts is across S/E MA and CT and all of RI. Expect
thunderstorms over SW PA into NJ to advect N/E with the mean wind,
maintained and fueled by an inflow of low-level H925-85 tropical
moisture. A measure of instability running up against the warm
front still lingering along the immediate E waters as discerned
via H925-85 SPC mesoanalysis, and out ahead of a sweeping cold
front and mid-level dry punch, a convergence of moisture leading
to heavy rain is progged. This along with high freezing level
heights and H85 dewpoints well in excess of +12C (up to +17C),
efficient warm-rain processes signaled .As we have seen already
with prior heavy showers / storms, rainfall rates of around 1-2
inches per hours are easily possible. Quick dousing amounts up
to 3 inches within 2 hours is not out of the question. The
propensity of flash flooding is there but thinking isolated,
localized rather than widespread. Will forego any headlines at
this point and focus on short-fused products.

Strong to severe storms...

Watching closely as to whether we need to coordinate with SPC on
a SEVERE THUNDERSTORM WATCH for portions of S New England. Per
SPC mesoanalysis, instability has manifested across S/W MA and
CT. However marginal, the better environment still resides well
S/W where partial clearing has allowed for better low-level lapse
rates and subsequent destabilization which has resulted in present,
ongoing convection. Still convinced that ongoing activity presently
is going to rob the environment to the N. However, not ignoring the
low LCLs, high shear, and measure of instability across the region.
While the greater threat is S/W, still need to maintain a watch for
portions of our area. Somewhat banking on the convection over SE
PA and NJ as it advects N/E across SE areas of New England to have
some strong, possibly severe elements. Heavy rain and frequent
lightning the main threats, but gusty winds also possible.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...

Not a washout. Widespread rain with embedded heavier showers,
possibly a thunderstorms, lingers over SE MA during the morning
period, eroding with the influx of mid-level H5-7 drier air
despite the cold front lagged. This is easily evident within the
K-indices, contrary to other convective parameters such as CAPE.
Should see gradual clearing across the region for a brief period
before comma-wrap moisture sweeps through the region behind the
low filling back in the dry punch. Some scattered cumulus through
the day, however the cyclonic flow and some favorable mid level
lapse rates with a weak cold pool aloft, could see some shower
activity over the N/W MA and CT overnight by which point the
surface cold front will have pushed offshore, winds becoming W
and turning light as high pressure builds into the region of the
OH River Valley behind the low.

Lower surface dewpoints during the day but still out ahead of
the cold front, should turn a bit more comfortable with highs
into the mid to upper 80s. Clouds

lingering overnight with the
comma-low, may hamper what might be a favorable night of
radiational cooling given light winds. Lows back down into the
60s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

Much of this period will feature a mid level ridge across the
southern USA. For our neck of the woods, nearly zonal flow should
prevail through early next week. Expecting a potent mid level trough
to get close to our region towards late next week.

Latest guidance suite is in reasonable agreement with the overall
pattern, with the typical detail differences. Favoring a consensus
blend to smooth over the less predictable details.

Thinking heat and humidity slowly builds from Sunday on, peaking
Wednesday before a cold front moves through our region. Temperatures
during this period should be near to above normal. Once this cold
front passes Wednesday night, expecting near to below normal
temperatures which much more comfortable humidity levels.

Increasing risk for showers and possible thunderstorms Tuesday night
into Wednesday, otherwise mainly dry much of next week.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...

Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...

Through tonight...Moderate confidence.

Conditions lowering IFR-LIFR over much of the region as the
threat of RA/+RA with embedded TSRA becomes more widespread.
TEMPO VSBY impacts with RA/+RA and expected dense fog, more
likely over S/E coastal terminals. Clearing out towards morning.
Blustery S/SW winds with gusts up to 30 kts, strongest over the
S/SE coastal terminals. Despite, LLWS impacts possible especially
over far SE MA with 40 kts SW at 2 kft AGL. Gradual improvement
NW to SE across the interior towards Saturday morning.

Saturday into Saturday night...Moderate confidence.

Gradual improvement for S/E terminals, lowest IFR-LIFR conditions
with -RA/RA lingering for SE coastal terminals. SW winds prevailing
turning W towards evening. SCT 4-5 kft low-end VFR CIGs during
the day, becoming BKN-OVC N/W overnight with the low risk of -RA.

KBOS Terminal...Moderate confidence.
MVFR-IFR mix through morning. Feel there is a threat of RA/+RA
this evening around 0-6z. TEMPO IFR VSBY impacts possible. Low
risk of TSRA, feel that may stay S of the terminal, so will
continue VCTS mention.

KBDL Terminal...Moderate confidence.
MVFR-IFR through the overnight period with the threat of RA/+RA
along with TSRA roughly 21-03z. Should see improvement after
midnight into the Saturday morning hours with CIGs lifting.

Outlook /Sunday through Wednesday/...

Sunday through Tuesday...High confidence.
VFR. Areas of IFR in early morning fog/low clouds, then improving
to VFR by mid morning each day.

Tuesday night and Wednesday...Moderate confidence.
Mainly VFR. Scattered MVFR is SHRA/TSRA, especially Wednesday.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...Moderate confidence.

S winds will be on the increase, gusting up to 25 kts across
the S/SE waters. SMALL CRAFT ADVISORY remain as winds will
result in heightened seas up around 5 feet.

Heavy rain with embedded thunderstorms sweeping SW to NE across
the waters today and tonight. Will likely be some visibility
restrictions.

Conditions improving Saturday into Saturday night as a cold
front sweeps the waters late in the period with winds turning W.

Outlook /Sunday through Wednesday/...

Sunday through Tuesday...High confidence.

High pressure builds over the waters. Winds remain 20 knots or less,
and seas 4 feet or less.

Tuesday night and Wednesday...Moderate confidence. Increasing
southwest winds ahead of a cold front should lead to rough seas
across the outer coastal waters, especially on the southern coastal
waters. There may be a few gusts to 25 kt on Wednesday, too.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 9 AM EDT Saturday for ANZ231>234.
     Small Craft Advisory until 2 AM EDT Saturday for ANZ236.
     Small Craft Advisory until 11 AM EDT Saturday for ANZ235-237.
     Small Craft Advisory from 8 PM this evening to 5 PM EDT
     Saturday for ANZ250-254.
     Small Craft Advisory until 8 PM EDT Saturday for ANZ255-256.

&&

$$

SYNOPSIS...Belk/Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...Belk
AVIATION...Belk/Sipprell
MARINE...Belk/Sipprell



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