Area Forecast Discussion
Issued by NWS Boston, MA

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266
FXUS61 KBOX 230205
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1005 PM EDT Thu Jun 22 2017

.SYNOPSIS...

Warm frontal passage tonight will result in mild, humid air to
build into the region on through Friday along with the threat
of heavy rain associated with the remnants of Cindy more towards
Friday night into Saturday morning. A cold front will sweep the
remnants of Cindy out to sea through late Saturday, improving
overnight. Seasonably warm and less humid conditions Sunday
will be followed by cooler and somewhat unsettled weather for
early next week.

&&

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

10 pm update...

Overall a mild, quiet night. As discerned from radar mosaic and
latest high-res guidance there are a series of warm frontal waves
through the mid-latitude flow, round the Bermuda high situated
off the SE CONUS shoreline. Localized convergence of sub-tropical
air with parent instability, seeing areas of showers and thunder-
storms that are moving with the mean flow.

Specifically focused on a warm-frontal wave stretched from the
Lake Ontario region into S New England. Weak height rises ongoing
within the mid-levels per the intrusion of warm air over the NE
CONUS as the baroclinic zone sharpens out ahead of the mid-
latitude trough into the E Great Lakes ahead of which low-level
winds enhance. A combination of low-level inflow along the warm-
frontal wave of sub-tropical air as mid to upper level westerly
flow enhances, likely activity to our N/W, but suppressed S/E
owing to less favorable dynamics and weak height rises. Can`t
rule out a chance of a shower over the high terrain as energy
emerging along the aforementioned warm frontal boundary meanders
S/E with the mean flow, but think for the most part it`ll remain
dry.

S flow increasing, surge of humidity towards the N/E, the high
dewpoint air into the 60s, increasing overnight, will keep over-
night temperatures mild. Looking at lows in the 60s with urban
centers close to 70. Over the cooler ocean waters along the S-
coast, more than likely will see the redevelopment of fog as we
saw earlier in the week. If not, low clouds just off the ground.
Increasing clouds otherwise.

&&

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

Friday into Friday night...

Active weather pattern for tomorrow as southern New England remains
in the warm sector for this period. Warm and muggy to start the day
as temperatures will quickly rise into the mid to upper 80s. If
breaks of sun can occur, then a few spots may be in the low 90s,
especially across the CT and Merrimack Valleys. Continuous
southwesterly flow will keep both clouds and tropical moisture
around Friday night. Overnight lows will remain mild, dropping into
the upper 60s to low 70s. This may result in another round of fog
and stratus with the highest confidence across the south coast once
again. May need to watch for a possible dense fog advisory as
surface dewpoints will be in the 70s.

Biggest focus is on precipitation potential for this time period.
Increasing moisture will begin to pool into the area with PWAT
values increasing to above 2 inches by Friday evening/overnight.
This Gulf moisture will eventually be tapped into the tropical
moisture from Cindy which will push PWAT near 3 STD above normal. In
fact, moisture flux values at 850 mb during this timeframe is
nearing 4 STD above normal. Thus within any heavy shower, the
potential for flooding rains is possible.

Through this period, an upper level trough over the Great Lakes will
begin to approach from the west. A few weak waves will move along
the flow as the ridge begins to push eastward. Southwesterly flow
aloft will remain overhead as a few isolated showers move into
southern New England from the Mid-Atlantic. Believe these showers
will maintain themselves as they ride along a theta-e boundary and
increasing LLJ. Best timing will be during the morning hours. Models
continue to indicate a dry slot moving aloft as 700 mb RH values dry
up and K values decrease. Thus the remainder of the day could be on
the dry side, however, cannot rule out a few hit or miss showers
developing since we are in the warm sector. The risk for thunder
continues to be a possibility as surface CAPE will be near 1000
J/kg. Not expecting anything severe as lapse rates are quiet poor,
less than 5.5 C/km.

A merger between the upper level system and leftovers from Cindy
will approach the area late Friday night. Slow moving cold front
will be the focus on heavy rainfall development as higher PWATs and
moisture transport moves into southern New England. Concerned that
models are underdoing the potential rain set-up for Friday night
into Saturday morning as mass fields would indicate widespread heavy
rainfall. One possibility is that our moisture could get robbed by
the surface low associated with Cindy`s remnants. Something to watch
in the upcoming forecasts. Right now have trended the forecast
towards the EC and Canadian which highlights the heavy rainfall
potential across southern New England.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

Highlights...

* Scattered showers and isolated t-storms Sat, especially in the
  morning
* Seasonably warm and less humid on Sunday
* Unsettled pattern for early next week, cooler with showers possible

Overview...

Longwave trough evolving over the Great Lakes Region continues to be
the main feature for the extended forecast. A series of weak
shortwaves move through the flow as the trough gradually moves
towards New England early next week. This will bring a somewhat
unsettled pattern into the middle of next week with cooler temps
trending near or below normal.

Details...

Saturday...

Cold front stalled out across southern New England for at least a
portion of the day, while remnant low and shortwave from Cindy
trek near or south of our area. 12Z NAM/12Z ECMWF indicate drier
air pushing into our area during the afternoon while front
makes an eastward push towards the southern New England
coastline. Meanwhile the 09Z SREF and 12Z GFS/GFS ensemble mean
are more pessimistic, keeping some of the deeper moisture and
associated precip chances lingering over southern New England
into the afternoon.

During at least the morning hours, a plume of tropical moisture is
in place with precipitable water values of 1.5 to 2+ inches, highest
east of the stalled front. The shortwave will likely bring showers
with the chance for thunderstorms, especially eastern MA and south
of the Mass Pike. Brief locally heavy rainfall is possible.

For this forecast package will include likely pops Saturday morning
except chance north central/western MA, then chance pops continuing
into Saturday afternoon due to uncertainty of arrival time of drier
airmass. Could still see developing sunshine at Saturday afternoon,
especially over the interior.

Forecast highs continue to be a challenge, especially south of the
Pike where showers may hold temps down. Will go with continuity
here, highs mostly in the 70s to near 80 but low to mid 80s
northeast MA.  However if we clear out in the afternoon the highs
could be several degrees higher.

Saturday night drier air moves into southern New England in earnest,
with PWATs plummeting to 0.75 inch. A dry night is forecast.
Shortwave energy mainly crossing thru northern New England, may
brush our area but only anticipating some clouds at best with this
feature.

Sunday...

Quite dry airmass in place. Mid level shortwave moving into our area
should mainly bring diurnal clouds, can`t rule out a brief shower or
two across the interior. Seasonable temps in the upper 70s and lower
80s with low humidity as dewpoints will be in the 50s.

Monday into Wednesday...

Unsettled weather pattern anticipated for the first half of the next
workweek, as trough over the Great Lakes Region gradually shifts
eastward towards southern New England. Series of short waves
rotating thru the base of the trough will bring chances for showers
and some afternoon/evening thunderstorms at times. Monday looks to
have low moisture, so will limit pops to slight chance/low chance
pops. Otherwise, being several days out, still have low confidence
on timing. Temps averaging near or a bit below seasonable normals.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...

Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Tonight...Moderate confidence. BKN CIGs lowering towards 4-6
kft, with scattered SHRA moving in, the better chance of which
will be north of the Pike with a low risk SHRA. Increasing S
winds with the threat of MVFR/IFR VSBYs along the S coast from
stratus and fog.

Friday...Moderate confidence. IFR/MVFR stratus and fog across
the south coast should improve to VFR during the day. Much of
the region will remain VFR with hit or miss showers. Heavy
downpours possible with the potential for iso thunder. Stratus
will fill back in late afternoon esp across Cape and Islands.
Gusty southwesterly winds during the day with gusts near 25 kts.

Friday night...Moderate confidence. VFR conditions north of the
Pike through the night, with MVFR/IFR south of the Pike in
another round of stratus and fog. Better chance for showers and
iso thunder during the overnight hours. Heavy rainfall could
limit vsbys.

KBOS Terminal...High confidence. VFR, except MVFR CIGS possible
Fri evening. Gusty SW winds near 25kts Friday afternoon.

KBDL Terminal...High confidence. VFR, except MVFR CIGS possible
late Fri afternoon/Fri evening. Gusty SW winds near 25kts
during Friday afternoon.


Outlook /Saturday through Tuesday/...

Saturday...Moderate confidence.
MVFR and localized IFR in sct showers and isold t-storms, especially
south of the Pike. During the afternoon improvement to mainly VFR
north of the Pike.

Saturday night and Sunday...High confidence. Mainly VFR.

Monday...High confidence.
Mainly VFR cigs with a few showers possible in the interior.

Tuesday...Moderate confidence.
Mainly VFR cigs with some scattered showers/isolated t-storms
possible.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

S winds overnight as warm front passes. Wind will increase into
Friday with sustained winds 10 to 15 kts and gusts up to 25
kts. This will push seas 5 to 6 feet and will continue with the
SCA. With a cold front sweeping down from the N as the remnants
of TS Cindy advect N from the S, will likely see the return of
mist / fog conditions resulting in low visibility down to a
quarter mile or less at times.

Outlook /Saturday through Tuesday/...

Saturday...Moderate confidence.
Some gusty W/SW winds early Sat, possibly near SCA thresholds in the
morning, should be diminishing in the afternoon. Seas up to 5-6 ft
over the south coastal waters.

Sunday and Monday...High confidence.
Winds should generally remain below small craft advisory thresholds
through the period. Lingering 5 ft seas over the south coastal
waters will subside Sun night.

Tuesday...Moderate confidence. There is a chance that winds and seas
may increase to small craft advisory thresholds.

&&

.TIDES/COASTAL FLOODING...

Astronomical tides are rather high this weekend during the night time
cycles. Boston has a 12.4 ft high tide around midnight Sat night and
12.2 ft just after midnight Sun night. Fortunately, offshore winds
are forecast with minimal or no surge so do not anticipate any
issues.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM Friday to 8 AM EDT Saturday for
     ANZ231>235-237.
     Small Craft Advisory from 2 PM Friday to 8 AM EDT Saturday for
     ANZ250-254>256.

&&

$$

SYNOPSIS...Dunten/NMB
NEAR TERM...Dunten/Sipprell/NMB
SHORT TERM...Dunten
LONG TERM...NMB
AVIATION...Dunten/NMB
MARINE...Dunten/NMB
TIDES/COASTAL FLOODING...WFO BOX Staff



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