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Area forecast discussion
National Weather Service Taunton Massachusetts
1017 am EDT Thu may 25 2017

Synopsis...

Low pressure and its attending warm front will bring a steady rain
and drizzle to the region today along with cool northeast winds. The
rain will be heavy at times tonight into Fri as the low tracks along
the southern New England coast to just east of Cape Ann Friday. Big
improvement to start of the Holiday weekend as weak high pressure
delivers dry weather with mild days and cool nights both Sat and
sun. However by Sunday night and into Monday another coastal low may
bring the risk of showers, although a washout is not expected.

&&

Near term /until 6 PM this evening/...

10 am update...

*/ highlights...

- rain into the midday period, heaviest S/E, mainly offshore
- fog / drizzle developing 3 to 4 PM, roughly
- north/NE winds, potential for some rogue 30 to 40 mph wind gusts

*/ discussion...

A cool, wet day on tap. Mature low pressure beginning to undergo
occlusion as it becomes negatively tilted across the mid-Atlantic,
lifting over the NE Continental U.S. Overnight. Into this evening, ahead of
the low, will continue to see isentropic ascent of the warm-moist
conveyor belt with sub-tropical connections beneath broad diffluence
aloft. Widespread rain across the region with embedded heavier rain
showers aided by convergent low-level forcing of higher Theta-east air.
The deep column moisture present, as seen per morning 12z soundings
out of Upton New York and Chatham Massachusetts plus intrusion of warmer air that
contributes to higher freezing level heights with the southerly sub-
tropical connection sets up efficient warm-rain processes. But with
evolving dynamics and negative-tilting, would appear that the crux
of deeper moisture and strongest forcing stays S/east of S New England,
the heaviest rain into this evening confined over the Outer Cape and
islands from now till roughly 3-4 PM. It is then that dryer air aloft
associated with conveyor belt motions wrapping into the low, dries
the column within the mid to upper levels. Still a measure of lift
and forcing upon the moist boundary layer, expect areas of fog and
drizzle to encompass the region as the surface warm front slowly
drags N, stalling roughly SW to NE along the immediate S coast of
New England where moisture will pool.

Previous forecast fairly spot on and there were very little changes
other than accounting for the latest trends. Agree with what has
been said prior: cooler with greater rain intensity as the column
saturates towards the respective wet-bulb, winds out of the north/NE
with the surface warm front situated S slowly lifting north. A cool,
raw, damp day overall.

Still monitoring low level winds for the afternoon period as they
ramp up in response to the deepening low center with the negative-
tilt occlusion. Agree with the previous forecaster that a Stout
inversion north of the warm front should limiting the mixing of such
winds to the surface. Again, can't rule out some rogue wind gusts
but nothing to the effective of widespread warranting a Wind
Advisory.

&&

Short term /6 PM this evening through 6 PM Friday/...

Tonight into Friday...

Anomolous low pressure system will continue its eastward
progression during the overnight hours. A potent pv-anomaly will
round the base of the trough and develop a surface low over the
middle Atlantic region. This low will trek right over southern New
England bringing widespread rainfall. Some of it will be heavy at
times as the system becomes vertically stack resulting in deep
moisture through the entire column. Precipitable water values are also near an
inch to 1.5 inches with increasing low level jet aiding in moisture advection.
This will result in another chance for locations south and east of
the I-95 corridor of seeing an inch of rainfall. This much rain
combined with the King high tide may result in urban flooding as the
water will have trouble receding.

Guidance is also hinting as some elevated convection with a few
models having some surface cape late tonight into tomorrow morning.
Therefore continued with the mention of isolated thunder. This may
also enhance precipitation amounts.

Widespread heavy rainfall will begin to wind down by Friday morning
as the low begins to move offshore. However, pretty good comma head
signature with a deformation zone in the mid-levels could set up.
This will keep the dreary weather going through much of the day
before beginning to let up by nightfall.

&&

Long term /Friday night through Wednesday/...

*/ highlights...

* mainly dry weather W/mild days and cool nights Saturday and Sunday

* risk of rain returns Sun night into Monday

* seasonably warm Tue & Wed but low risk of scattered showers

*/ details...

Friday night...

Improving weather as closed mid level low over the Gulf of Maine
exits seaward and allows rising heights along with mid level
anticyclonic flow to move across New England. This will promote a
drying trend overnight. Temps will be seasonably cool with lows in
the upper 40s and low 50s.

Saturday...

West-northwest flow aloft with mid level mean ridge axis across the eastern
Great Lakes will provide dry weather and seasonable temps across
southern New England. Models do differ on cloud cover Sat with GFS
showing much more low level moisture. Given the west-northwest flow aloft
thinking at least partial sunshine will verify. This combined with
light winds will provide very pleasant conditions with highs 70-75
CT River Valley and 60s elsewhere. Weak pressure gradient will
support cooling afternoon seabreezes for the coastline.

Sunday...

Deep layer ridging lingers across the area promoting dry weather
much of the day. However model time sections and moisture profiles
indicate lots of clouds likely on Sunday. 00z deterministic and
ensemble guidance in good agreement that rain/showers associated
with upstream system should arrive into southern New England either
late in the day or after sunset. So as of now the bulk of the
daylight hours of Sunday appear dry, especially across Rhode Island and
eastern Massachusetts. Given the expected cloud cover highs should be a few
degs cooler than Saturday. Fortunately south-southeast winds should be fairly
light but will keep the immediate coastline cooler.

Monday...

Weak coastal low may pass south of New England along with its precip
shield impacting the area. Doesn't look like a washout but enough
support to keep showers in the forecast. Somewhat cool with highs in
the 60s given cloud cover and light onshore winds.

Tuesday/Wednesday...

Not as cool as parent low pressure over the Great Lakes provides
warmer southwest flow into New England. With the milder temperatures
comes the risk of scattered showers as mid level flow becomes
increasingly cyclonic.

&&

Aviation /14z Thursday through Monday/...

Forecaster confidence levels...

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

Short term /through Friday/...moderate confidence.

15z update...

7 am update...

Today...
into midday -ra/rain transitioning to dz with increasingly lower
visibility with fog. Lowering cigs S to north with time towards IFR.
North/NE winds becoming more east/NE, gusty at times potentially as
high as 30-35 kts though thinking isolated. Mainly 20-25 kts.

Tonight...
IFR-LIFR cigs and vsbys. Rain/+ra potential mainly from 6-12z with
embedded thunderstorms and rain possible. East winds continue with mainly 20-25 kt
gusts.

Friday...
IFR/MVFR cigs with scattered-broken -shra, on and off through the day.
Conditions may improve to VFR across western terminals in the
late afternoon. Northwest winds for all terminals.

Kbos taf...
lower conditions moving in as low pressure passes over the
terminal. So overnight winds are going to be a challenge, gusty
at first but becoming light while turning counter-clockwise NE
to northwest with low passage. Closely watching the 6-12z Friday time-
frame for +ra potential and possible embedded thunderstorms and rain. Whether
impacts to the am push.

Kbdl taf...
will hold winds more north funneling through the CT River Valley,
turning northwest with low pressure passage overnight. Fog/dz into this
afternoon, more rain and possible +ra/thunderstorms and rain overnight 6-12z time-
frame. Lowering IFR into midday, LIFR into evening and holding
so, especially with cigs through Friday morning.

Outlook /Friday night through Monday/...moderate confidence.

Friday night...
a mix of MVFR-VFR but a trend toward dry weather overnight as
surface winds become west-northwest.

Saturday and Sunday daytime...
VFR. Light winds Sat with seabreezes at the coast. Winds Sunday
more from S-southeast direction at fairly light speeds.

Sunday night-Monday...
cigs/vsbys trending to MVFR/IFR in showers. 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 Friday/...high confidence.

10 am update...no major changes.

Today...
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. Also went ahead an issued Small Craft Advisory for near
shore waters for the potential for gusts near 25 kts. Rain and
fog will limit vsbys through the day.

Tonight...
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 continue. Cannot rule out an iso thunderstorm
over the waters tonight.

Friday...
potent low pressure system still over the region will keep seas
and winds up through the period. Small Craft Advisory will be needed.

Outlook /Friday night through Monday/...

Friday night...

Low pressure over Gulf of Maine moves seaward with west-northwest winds
developing across the Massachusetts/Rhode Island waters resulting in a drying trend and
improving vsby.

Saturday and Sunday...

Fairly light winds and tranquil seas as high pres crest over the
area. Dry weather and good vsby too.

Sunday night and Monday...

A series of weak lows may develop and pass south of New England,
near the 40n/70w benchmark.

&&

Tides/coastal flooding...

Combination of a very high astronomical tide and onshore flow
will result in widespread minor coastal flooding tonight.

Persistent easterly flow yesterday and today will help aid in a
storm surge around .7 to 1.0 which will push many sites above
flood stage. The Salisbury to Cape Ann stretch of coastline
continues to be the 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. 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, went ahead an issued a coastal advisory for
much of the Massachusetts East Coast including Cape Cod and Nantucket for
tonight's high tide. Waves will not be a major factor but looks
like enough low level wind gradient to produce 6 to 10 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. Right now confidence is to low.

Little to no storm surge is expected at the time of the late Friday
night or very early Saturday am high tide. This as well as
offshore flow may limit the potential. Will have to continue to
watch as we cannot rule out the potential for isolated pockets
of minor coastal flooding.

&&

Box watches/warnings/advisories...

CT...none.
Massachusetts...coastal flood advisory from 10 PM this evening to 3 am EDT
Friday for maz007-015-016-019-022-024.
Rhode Island...none.
Marine...Small Craft Advisory until 4 PM EDT this afternoon for
anz231>234.
Small Craft Advisory until 6 PM EDT Friday for anz235-237-250-
251-254>256.

&&

$$

Synopsis...nocera/dunten
near term...nocera/dunten/sipprell
short term...dunten

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