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Area forecast discussion
National Weather Service Taunton Massachusetts
1018 PM EDT Wed may 24 2017

Synopsis...
dry weather will continue most of the night. Then later tonight
an approaching low pressure center delivers increasing clouds,
followed by a widespread rain Thu into Fri. The rain will likely
be heavy at times Thu night into Friday morning as the low
intensifies. Weak high pressure will bring a dry start to the
Holiday weekend Saturday and early Sunday. Another coastal low
brings more wet weather late Sunday into early Monday. Leftover
showers are possible through the first half of the week.

&&

Near term /until 6 am Thursday morning/...

Overall trend in the forecast remains on track for tonight. High
level clouds will continue to stream into the region ahead of
the next system approaching from the west. Easterly flow
remains persistent keeping dewpoints in the 50s. Temperatures
are slowly starting to fall, but should remain steady as
radiational cooling will be limited as clouds thicken up.
Overnight lows will remain in the low to mid 50s.

Dry weather will prevail overnight with a few showers moving up
from the south near dawn tomorrow morning.

Previous discussion...

High pressure over Newfoundland will maintain dry weather for
much of the night. Meanwhile the right entrance region of the
upper jet is moving over a portion of the Midwest/midsouth cold
front near its triple point, where the pressure pattern is
baggy. Expect a secondary low to develop in this area as it
swings east. This will bring increasing clouds, with a low
chance for measurable rainfall toward daybreak associated with
the warm front. We have slowed the onset of pcpn a couple of
hours toward daybreak, with slight chance to low chance pops
mainly south of the Mass Pike while areas to the north remain
dry. Overnight lows on the mild side, mainly low to mid 50s,
but some locales may manage to dip into the upper 40s.

&&

Short term /6 am Thursday morning through Thursday night/...
Thursday...

Anomalously large and deep upper trough makes its way eastward to
the eastern Great Lakes and mid Atlantic region. Surface low is
initially in the vicinity of Ohio early Thu, however developing
surface low over the triple point develops closer to the mid
Atlantic coastline during the day and eventually becomes the primary
surface low.

For our area, precipitation ahead of and associated with the warm
front will move in during the morning. Low level moisture
convergence sets up in our vicinity. Most robust moisture looks to
remain offshore, with pwats increasing to 1 to 1.5 inches over
southern New England. Could see quantitative precipitation forecast ranging from few tenths of an
inch to near an inch during the day. GFS and NAM indicate dry slot
reaches our area during the afternoon, which could allow for a break
in more widespread precipitation until evening.

Between the clouds, the rain, and an onshore steady east wind, temps
are not expected to climb very much. Daytime highs mainly in the
50s. A few sites may hit 60 but Thursday will have the feel of a raw
springtime day. Some areas of drizzle and fog may develop in the
afternoon.

Thu night...

Continued good agreement in models that a potent short wave rounding
the base of the high amplitude East Coast trough, prompting a
negative tilt. Surface low makes its way into southern New England,
and lingers in our vicinity for much of the night.

While periods of rain are anticipated in southern New England,
models indicate the plume of deeper moisture is further offshore.
GFS and NAM showing pwats of 1 to 1.5 inches in our area for Thu
night. Accompanied by decent lift, anticipating a night of soaking
rainfall for our area. Thinking quantitative precipitation forecast for Thu night would be around an
inch or so. NAM model soundings supportive of elevated instability,
so embedded thunder is possible. That could produce allow for some
localized higher precip totals. Low level inversion supports
formation of low clouds and areas of fog.

&&

Long term /Friday through Wednesday/...
big picture...

The longwave scale retains cyclonic flow over the eastern USA much
of the period while a ridge builds over the western USA.

Vertically stacked closed low over southern New England moves
northeast toward the Maritimes Friday. Upper flow evolves northern
and southern jets over the weekend. Closed low moved along by the
northern flow will dip south from Canada into the Great Lakes over
the weekend. Shortwave in the southern flow ejects northeast from
the southwest USA. Eventually the southern jet forms a coastal low
off the mid Atlantic states Sunday night that moves up the coast
past our area Monday. The northern stream closed low remains stalled
over the Great Lakes through the early week, with signs of it
breaking down around Wednesday.

Upper mass fields remain similar through Saturday. The models agree
on the concept of a closed low moving from Canada Sunday to the
Great Lakes Monday and Tuesday, but differ on placement. This in
turn brings differences with the interaction with the southern jet
and shortwave. All of this leads to higher confidence for the late
week and lower confidence for early next week.

Details...

Friday...

Low pressure over our area, surface and aloft. Forecast details will
depend on the small scale placement of the surface low, but expect
lingering wet weather leading the low, but diminishing as the low
lifts northeast into the Gulf of Maine. Cold conveyor moisture will
wrap around the low, and the cyclonic curvature will support lift.
We will keep showers in the forecast much of the day, although
less coverage than at the start of the day.

The low moves off Friday night. Expect any remaining showers to
taper off and partial clearing.

Saturday-Sunday...

High pressure brings a day of rest from the wet weather. Temps of 8c
to 9c at 850 mb suggests Max temps in the low to mid 70s. A light
southeast surface flow will occur along the east mass coast and hold
Max temps there in the 60s.

Weak high pressure lingers through Sunday morning, but moves
offshore during the day. Expect increased mid and high clouds
Saturday night and Sunday. Dew points Saturday night will be around
50...so expect min temps near 50. Temperatures aloft warm a degree
by Sunday, but also expect more clouds than on Saturday. Will
forecast Max temps similar to Saturday.

Sunday night through Wednesday...

Low pressure in the Great Lakes. Southern stream shortwave reaches
the mid Atlantic coast and generates surface low. Models show a 30-
35 knot south low level jet feeding up into our area Sunday night,
then sweeping east on Monday. Expect a period of rain showers Sunday
night and Monday. Low pressure lifting into Canada swings a cold
front through the region Monday night or early Tuesday, maintaining
a chance of showers until passage. Upper trough swings across about
Wednesday bringing a chance of showers, mainly in northwest mass.

&&

Aviation /02z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Thursday night/...high confidence.

Tonight...high confidence mainly VFR with easterly winds. We are
monitoring Nantucket for potential low ceilings in stratus
overnight. Fog patches could develop across low lying
terminals.

Thursday...deteriorating to MVFR/IFR cigs and vsbys in rain and fog
with east wind gusting to 20 knots along the shoreline.

Thursday night... IFR cigs and vsbys in rain and fog with east wind
gusting to 20 knots along the shoreline. Rain moderate to heavy at
times.

Kbos taf...high confidence in taf. Easterly flow through the
period.

Kbdl taf...high confidence in taf.

Outlook /Friday through Monday/...moderate to high forecast
confidence.

Friday... initial conditions IFR, but improve to mixed VFR/MVFR in
showers as winds shift from the northwest.

Saturday and Sunday daytime... VFR. Diminishing northwest wind
Saturday. Light wind Sunday.

Sunday night-Monday... cigs/vsbys trending to MVFR/IFR in showers
and fog. Winds from the southeast, trending from the south Monday.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday night/...high confidence.

Tonight...steady easterly flow, departure of offshore low and
approach of next system will allow for building seas. Small Craft Advisory headlines
for tonight primarily for seas, on southern outer coastal waters, bi
and Rhode Island sounds.

Thursday...approaching system from the west will help increase winds
and seas through the day. Small Craft Advisory continues where confidence is highest
for seas above 5 feet. Good inversion through the day will limit
gusts but anticipating some gusts around 25 kts where Small Craft Advisory headlines
are hoisted.

Thursday night...

Poor vsbys in rain and fog Thursday night. Easterly winds around 25
knots eastern coastal waters with strongest winds during the
evening. These winds then lift north of the waters. Seas building up
to 10 feet across the Cape Ann waters, less elsewhere. Small Craft Advisory headlines
have been extended thru Thu night for outer coastal waters.

Outlook /Friday through Monday/...

Friday...

Low pressure center crosses coastal Massachusetts Friday morning and
then exits northeast through the Gulf of Maine. Easterly winds
around 25 knots leading the low center will shift from the north
behind the low with gusts near 25 knots possible. Seas will maintain
at 5-7 feet on the outer waters and the exposed nearshore waters
such as mass Bay and Rhode Island Sound. Small craft advisories will continue
in these areas.

Winds and seas diminish Friday night.

Saturday-Sunday...

Weak high pressure moves over the waters. Winds will be 20 knots or
less both days. Seas on the outer waters will be around 5 feet on
Saturday, otherwise seas should be less than 5 feet both days.

Sunday night-Monday...

Increasing southeast winds Sunday night become east on Monday.
Speeds should remain less than 20 knots. Seas on the outer waters
may approach 5 feet but otherwise expect seas to remain less than 5
feet. An area of rain moves across the waters during this time with
locally poor vsbys in rain and fog.

&&

Tides/coastal flooding...
725 PM update...
noted that we have a tide departure of about three quarters of
a foot at most tide gages. This will result in the total water
level topping out just below the still water flood stage at most
points along both our east and south coasts. This is close
enough to probably result in isolated minor coastal flooding of
most vulnerable shore roads. For example the tide could be high
enough to impact a portion of Morrisey Boulevard in Boston.
Inundation depths should be generally under a half foot.

Prior discussion...
combination of a very high astronomical tide and a .7 to 1.0 foot
storm surge will likely result in minor coastal flooding about the
time of the late Thursday night high tide along much of the Massachusetts East
Coast. The Salisbury to Cape Ann stretch of coastline is most at
risk, since the low level east-northeast jet would have just moved north of
this area just a couple of hours before high tide with a moderately
significant NE wave fetch in place at the time of the high tide.
However, the expected total water level looks to be high enough to
support areas of minor coastal flooding along the coastline south of
Gloucester to Plymouth as well as Provincetown. Thus, it may become
necessary to issue a coastal advisory for much of the Massachusetts East Coast
including Cape Cod for the late Thursday night high tide. Waves will
not be a major factor but looks like enough low level wind gradient
to produce 6 to 9 foot waves in the near shore waters in Ipswich
Bay, about Cape Ann, and Massachusetts Bay. Some erosion is likely
along the Salisbury and Plum Island shorelines where wave action
will be somewhat more significant.

Elsewhere along the Massachusetts and Rhode Island coasts, the combination of the high
astronomical tide and a tidal departure near a half foot may be
enough to cause pockets of minor coastal flooding as has become more
common during such King tide cycles. A statement may be needed for
the islands, and both the Massachusetts and Rhode Island south coasts, including
Narragansett and Buzzards Bay shorelines, for the Thursday
evening/night high tides.

Given that we are expecting no impacts beyond minor coastal
flooding, we will hold off on headlines for now and let the next
shift have the opportunity to refine the coastal impact forecast and
issue any advisories/statements deemed necessary.

Little to no storm surge is expected at the time of the late Friday
night or very early Saturday am high tide, but that astronomical
tide by itself may be sufficiently high to produce isolated pockets
of minor coastal flooding and possibly worthy of a statement as that
time draws nearer.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 6 PM EDT Friday for anz235-237-
254>256.
Small Craft Advisory from 6 am Thursday to 6 PM EDT Friday for
anz250.
Small Craft Advisory from noon Thursday to 6 PM EDT Friday for
anz251.

&&

$$
Synopsis...wtb/nmb
near term...dunten/nmb

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