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Area forecast discussion
National Weather Service Taunton Massachusetts
1017 PM EDT Fri Jun 23 2017

Synopsis...

Mild, humid air continues tonight out ahead of the remnants of
Cindy which will bring the threat of showers and thunderstorms
through the first part of Saturday. 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 weather and few diurnally driven showers
and isolated thunderstorms Monday and Tuesday. A return to
Summer-like warmth and humidity is expected by the end of the
work week.

&&

Near term /until midnight tonight/...

10 PM update...

Let's keep it simple. Scattered showers and thunderstorms are
expected tonight given the broad tropical moist plume and the
remnants of Cindy out ahead of a mean long-wave mid-level trough
axis digging S/east out of the Great Lakes.

Particular focus is on any forcing mechanisms, and broadly
speaking there are two of focus: 1.) Convergent forcing and
subsequent frontogenesis along the nose of h925-85 winds which
are starting to amplify with the tightening baroclinic zone
between the aforementioned trough axis and northwest Atlantic high
pressure. Exacerbating Theta-east convergence beneath favorable
venting per westerly flow aloft. And 2.) Along and ahead of a
cold front sweeping S/east parent with the mid-level trough.

Expecting overnight clusters of showers and thunderstorms that
may pose an isolated, localized threat of nuisance flooding.
None of the forecast guidance is quite locking in on one area.
Just given the general synoptics and the sultry, tropical air-
mass, the upstream activity over the mid-Atlantic should
maintain with the absence of dry air downstream as observed from
latest 0z soundings. There aren't any clear indications as to
why scattered wet-weather will not continue overnight. So chance
pops continue accordingly.

The much stronger signal comes towards morning as the remnants
of Cindy are flushed North/East with the mean wind out ahead of the
sweeping cold front which acts as a lifting mechanism. Ahead of
the mid-level long-wave trough axis and parent elongated vortmax,
indications of forced ascent beneath falling heights and an
accompanying jet streak. It is these sharp dynamics that strongly
indicate the likelihood of outcomes, especially across southeast New
England where there is the possibility of marginal instability
that could aid in the invigoration of Cindy remnants, yielding
the development of strong thunderstorms, possible severe weather.
Then again it may just end up benign.

The simple takeaway here is to keep our heads up, maintain an
awareness, and be prepared for anything. As the previous
forecaster alluded to, this is a tricky forecast. We are dealing
with an anomalous setup with very high atmospheric moisture
content, and with any squeeze put on the environment, and the
stronger that squeeze, there is the threat for some measure of
flooding, and incorporating instability and shear as was observed
upstream earlier today, there is the possibility of strong to
severe weather.

We'll keep analyzing it through the night, especially as 0z
guidance becomes available. Please see the discussion below
which touches on Saturday interpreted from earlier forecast
guidance.

Previous discussion...

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. Muggy to start the day tomorrow
but depending on how quickly the front will push through, the
later half of the day will begin to improve. Highs will remain
in the mid 80s.

&&

Short term /midnight through 6 PM Saturday/...

Into Saturday...

* increasing localized flood potential for southern New England

Active weather pattern for the first half of tomorrow as S New
England remains in the warm sector for this period.

Biggest focus is on precipitation potential. Increasing moisture
will begin to pool into the area with precipitable water values increasing to
above 2.5 inches. This tropical moisture from Cindy will push
precipitable water near 3 to 4 Standard above normal. In fact, moisture flux values
at 850 mb during this timeframe is nearing 4 Standard above normal.
Thus within any heavy shower, the potential for flooding rains
is possible.

Models are struggling with the placement of the axis of heavy
precipitation as there are several features to note. Aside from the
approaching front out west, a low level jet will be across the southeast
portion of the area. However, the higher instability values will be
across the western half of the area. If both of these features
could align, then we could see widespread heavy rainfall. But
because there is some separation, believe that the threat is more
localized. Again the heavy rainfall axis amongst the guidance are
in different locations, but there is agreement that the potential
for over 1.5 to 2 inches. If this falls in just a few hours, then
flash flooding is possible. Best timing for this to occur will be
after midnight and into Saturday morning.

Cold front sweeping through the region. Latest hi-res guidance
is a lot slower than synoptic models so have blended the forecast
down the middle. More widespread showers and embedded thunderstorms
are anticipated along the frontal passage. Convection along the
front continues to be a possibility especially across eastern
Massachusetts during the late morning hours. Cape values will be near 1000
j/kg with 40 kts of 0-6 km shear. The caveat is poor lapse rates
and the bulk of the moisture will be pushing offshore. One thing
to watch for is the potential for a quick spin-up thanks to the
tropical airmass with 70f dewpoints and lower LCLs. Hodographs
do show some helicity and good 0-1 km shear. The frontal system
should push through by Saturday afternoon resulting in clearing
skies and drier air as westerly flow takes hold.

&&

Long term /Saturday night through Friday/...

Highlights...

* seasonably warm/less humid sun with a spot shower/T-storm possible
* a few showers/iso T-storms possible Mon/Tue but not a washout
* below normal temps Mon/Tue return to summerlike readings Thu/Fri

Details...

Saturday night...

Dry and a much less humid airmass will work into the region behind
saturday's cold front. This should allow low temps to fall into the
50s across many outlying locations, to between 60 and 65 in some of
the urban centers.

Sunday...

A pleasant end to the weekend across southern New England. 850
temps between +11c and +12c should allow highs to reach into the
lower to middle 80s in most locations, but dewpoints in the 50s will
make it feel much more comfortable outside. Mainly dry weather
anticipated, but approaching shortwave may be enough to trigger an
isolated shower/T-storm or two by mid-late afternoon across the
interior.

Monday and Tuesday...

Anomalous upper trough/cold pool aloft overhead will result in
slightly below normal temps with comfortable humidity levels. Highs
mainly in the 70s are expected both days. The cold pool
aloft/shortwave energy will result in the potential for a few
diurnally driven showers/isolated thunderstorms both days. Not
expecting a washout by any means, but the main risk for any activity
will be during the afternoon/early evening hours.

Wednesday...

Axis of upper trough will probably have shifted to the east of the
region. This should allow temps to warm a few more degrees than
Mon/Tue and result in mainly dry weather. However, if trough ends up
slower than currently expected there would be the risk for a few
more showers/isolated thunderstorms.

Thursday and Friday...

Upper trough lifts northeast of the region with rising height
fields. This should result in a warming trend for the end of the
work week, with a return to summerlike warmth and humidity.

&&

Aviation /02z Saturday through Wednesday/...

Forecaster confidence levels...

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

Short term /through Saturday/...moderate confidence.

Tonight into Saturday...moderate confidence. MVFR to IFR
conditions expected into Saturday morning. Low risk of LIFR
conditions along the South Coast in fog, but enough wind in the
boundary layer may prevent that from happening. Scattered
showers and thunderstorms mainly late tonight into early Sat
afternoon.

Improving conditions from west to east around midday ahead of a
wind shift from the west. Prior to the wind shift will continue to
see SW winds with potential gusts 25 to 30 kts low level wind shear possible
for southeast coastal terminals as is some embedded thunderstorms and rain. Improving
towards evening to low-end VFR / sky clear.

Kbos terminal...moderate confidence in taf.

Kbdl terminal...moderate confidence in taf.

Outlook /Saturday night through Wednesday/...

Saturday night through Wednesday...high confidence. Mainly VFR
conditions other than a few mainly diurnally driven showers and
perhaps isolated T-storms.

&&

Marine...

SW gusts up to 25 to 30 kts through early Saturday prior to a
cool frontal passage. Waves building around 5 to 7 feet on the
waters as the remnants of Cindy are advected NE around the
morning hours. Prior to and during this time, likely to be
either a combination of low clouds and fog with visibility down
to a quarter mile or less possible. Improving late Saturday as
winds become westerly while beginning to diminish along with
wave action.

Outlook /Saturday night through Wednesday/...

Saturday night through Wednesday...moderate to high confidence.
Quiet boating weather overall during this time period. A few hours
of near shore lower 20 knot wind gusts and choppy seas are possible
on both sun and Mon afternoons.

&&

Tides/coastal flooding...

Astronomical tides are rather high through this weekend during the
night time cycles. Boston has a 12.3 ft tide just before midnight
tonight, a 12.4 ft high tide around midnight Sat night and 12.2 ft
just after midnight Sun night. While, offshore winds are forecast
current conditions suggest a 0.4 surge which will result in
minor splashover. Thus will go ahead an issue a coastal flood
statement for tonight's high tide.

&&

Box watches/warnings/advisories...

CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 3 PM EDT Saturday for anz231>234.
Small Craft Advisory until 8 PM EDT Saturday for anz235-237-
250-254>256.

&&

$$

Synopsis...Frank/dunten
near term...Frank/sipprell

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