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Area forecast discussion
National Weather Service Taunton Massachusetts
339 am EDT Tue Oct 24 2017

Synopsis...

Scattered showers developing ahead of the main line of moderate
to heavy rain expected this afternoon continuing into Wednesday
ahead of a sweeping cold front. Potential hazards of flooding,
strong to damaging winds, and severe weather included. Expect a
few lingering showers Thursday. High pressure then builds up
from the southern USA to bring fair dry weather Friday and the
weekend. Another deep weather system approaches from the west
late Sunday and Monday.

&&

Near term /until 4 PM this afternoon/...

330 am update...

Increasing areal coverage of wet weather. The combination of
upslope ascent of sub-tropical air lifted by the presence of
increasing convergent low-level winds (via latest near-term
guidance and vertical wind profilers) is resulting in the
development of widespread rain with embedded heavier showers.

Expect the wet weather trends to continue over the course morning
into afternoon hours especially over the west half of the forecast
region. Could see some lowered visibility with the locally heavy
downpours in addition to increasing S winds. Temperatures remaining
steady or slowly rising beneath the thick deck of clouds.

&&

Short term /4 PM this afternoon through 6 PM Wednesday/...

*/ highlights...

- trailing cold front to an occlusion across the Great Lakes Ushers
threats of flooding, strong to damaging winds, and perhaps
some severe weather

- impacts mainly late Tuesday into Wednesday

*/ overview...

An anomalous, stormy setup Tuesday into Wednesday. Contributions via
teleconnections, the mjo in particular, yields a greater Transfer of
energy between the Equator and poles subsequent of an high-amplified
pattern of troughs and ridges with the east Continental U.S. Beneath preferred
troughing. The buckled, slow pattern lends to a more active weather
given strong synoptics and Transfer of energy as well as increased
residency time of threats and impacts.

In our case, a presently ongoing deep low occlusion across the Great
Lakes region beneath a closed 500 mb low will undergo a dying transition
as it becomes stacked and fills. Behind this feature, drier air
wraps within pushing a surface cold front east which stalls against 500 mb
ridging over the northwest Atlantic late today into Wednesday. It isn't
until additional Pacific-energy rounds the base of the occlusion
that cyclogenesis is promoted immediately offshore of New England in
regions of greater baroclinicity kicking out to sea the cold front
and sub-tropical air as the now open-wave 500 mb trough takes on a
negative tilt.

Synoptics and cyclogenesis throughout the forecast period, winds
strengthen aloft drawing north sub-tropical, unstable air. Potential
threats emerge of flooding rains, severe weather, and damaging
winds. Will hit on the specifics in the details below focusing on
the what, where and when.

*/ Details...

Flooding rains...

- initially this afternoon into evening
- secondary threat area possible over east/southeast New England late Wed
- 1"/hr rainfall rates with storm totals in excess of 3"
- Flash Flood Watch maintained and not expanded at this time

Excessive rain threat, especially with heavier showers and thunder-
storms. Precipitable waters in excess of 1.5 inches, +3-4 Standard
deviations above normal. Conditionally unstable column moistening
throughout, especially along the lifting cold front as freezing
levels hover around 13 kft signaling potential warm-rain processes.
A mean S to north fetch of sub-tropical air yielding likely training of
rain. Coupled instability plume up as high as around 1k j/kg. And
considering flash flood guidance values ranging around 2-3 inches
for both the 1-hour and 3-hour timeframe.

Altogether, rainfall rates in excess of 1-inch/hour and storm-total
rainfall in excess of 3-inches not out of the question. Developing
by late afternoon Tuesday, gradually sweeping east but slowing against
high pressure over the northwest Atlantic. Absent forcing but better lift
associated with the cold front and support via low-level jet (llj)
and surface upslope east-component of wind along the high terrain.

Cips/ncar/sref ping on flood threats from Scranton PA into Albany New York
towards west Massachusetts and west CT early on. But threats may continue east given the
slow progression of the cold front along which S to north training
continues. This prior to renewed cyclogenesis off the coast prior to
a secondary vortmax kicking everything offshore late Wednesday into
Wednesday night.

Agree with the prior shift on the initial Flash Flood Watch to the west
noting area river basins can get flashier given the high terrain and
narrow channels, runoff can be exacerbated. However, this Flash
Flood Watch may need to be extended E, especially east/southeast New England
around the I-95 corridor with particular attention to the Wednesday
PM timeframe into Wednesday night. Will allow later shifts to take
another look.

Strong to damaging winds...

- S gusts in excess of 40 to 45 mph
- late Tuesday through Wednesday
- Wind Advisory continues for all of S New England

S winds 2-3 kft agl up around 60 as high as 70 mph, +3-4 Standard
deviations above normal. Increasing this morning, the height later
Tuesday diminishing into Wednesday as the low level jet sweeps east along and
ahead of the surface cold front. Conditionally unstable profile
merited with instability as high as around 1k j/kg, the potential
for thunderstorms along with the threat of heavy rain given both the
lift and forcing mechanisms in place, can't rule out mechanical mix-
down (precipitation drag) of faster momentum just immediately aloft
to the surface even in the nocturnal timeframe in addition to
boundary layer mixing with the conditionally unstable profile.

Widespread wind gusts in excess of 40 mph, in excess of 45 mph for
some locations, again late Tuesday into Wednesday, west to east. Trees
either leafed or losing leaves, aside, not getting too cute, will
keep the Wind Advisory for all of S New England. Forecast may be far
from perfect, perhaps not reaching criteria within out forecast
grids. However, given the time of year and winds forecast, there is
the expectation of downed trees, tree limbs, and power lines.

Severe weather...

- potential for a brief spin-up of a tornado
- non zero threat, at minimum damaging wind potential
- agree with Storm Prediction Center outlook, impacts late Tuesday into Wednesday morn

Conditionally unstable profiles yield stretched elevated instability
throughout the column as high as around 1k j/kg. In areas of strong
0-1/0-3 km shear and helicity well above thresholds of consideration
there is definitely the threat that with any thunderstorm there is
the possibility of a brief spin-up of a tornado.

Agree with the Storm Prediction Center convective outlook noting its collocation with
the sref significant tornado parameter. Mainly focused out west around
the Albany area during the later half of Tuesday as the cold front
approaches the high terrain ahead of which S winds usher a sub-
tropical, unstable profile north. The region on the leading edge of the
mid level dry slot wrapping around the Great Lakes occlusion. This
activity likely to drift into west Massachusetts and CT towards the evening hours
with the sweeping cold front, the severe threat still needed to be
monitored along the cold front overnight as it continues east.

Some uncertainty. Perhaps a more robust, supportive environment for
convection per cips / ncar over the coastal mid-Atlantic and across
the adjacent waters east that could rob the sub-tropical, unstable
environment from filtering north.

Aside, believe there is a non-zero threat for severe weather over
all S New England beginning late Tuesday continuing into Wednesday
morning ahead of the sweeping cold front. Given the wind orientation
and nature of the cold front, at a minimum will likely be dealing
with either embedded cores and/or a fine-line of convection that can
manifest faster, damaging winds to the surface requiring short-fused
headlines.

&&

Long term /Wednesday night through Monday/...

Big picture...

The longwave scale features a persistent ridge along and offshore of
the Pacific coast, while a broad trough covers the USA east of The
Rockies including New England. Shortwave scale shows a plains upper
low sweeping east as a deep shortwave this week and crossing New
England around Thursday. A second shortwave moves east from the Gulf
of Alaska into Canada, then digs over the plains late in the week
and sweeps across the eastern USA early next week. This suggests two
stormy periods midweek and early next week with a period of dry
weather in between. Height fields build above average over the
weekend, so the dry period looks to be milder than normal.

Mass fields are similar through Friday. Differences in details pop
up over the Midwest during the weekend, but all indicate an
upper trough over that area that moves east and draws surface
low pressure up the East Coast Sunday and Monday. High
confidence in the forecast late this week, trending to moderate
early next week.

Details...

Wednesday night-Thursday... moderate-high confidence.

Drier air moves in from the west Wednesday night especially
after midnight. Expect diminishing pops during the night as
the cold front moves farther offshore.

Upper low and its -26c cold pool will move across New England
Thursday with totals of 50-55 from late morning through afternoon.
Lifted indices are marginal but lightly subzero. Meanwhile, moisture
fields are briefly favorable in the afternoon. This suggests a
period of convection, showers and possibly some thunder. We will
increase pops to chance levels with highest values in western and
central mass.

Friday through Sunday morning... high confidence.

High pressure builds up from the southern USA. Mixing looks to reach
900-925 mb, where temps are supportive of Max sfc temps in the 60s.
Light wind Friday night and dew points 35 to 45 suggest min sfc
temps in the mid 30s to the 40s. Then as the high shifts east
Sat and sun, the overall flow will become southeast to east.

Sunday afternoon through Monday...moderate confidence.

Increasing moisture Sunday with increasing low level south to
southeast jet of 50-60 knots moving up the mid Atlantic coast. The
strong low level jet crosses New England Sunday night and Monday
morning. The upper jet remains over New York Sunday, then crosses
New England Sunday night and Monday. Precipitable water values are
forecast to reach between 1.5 and 2.0 inches. A 1.5 inch value would
be near the Max for the day, suggesting a slug of water. Timing on
these features will likely shift over the next few forecast
packages, but the overall pattern suggests a period of showers and
strong gusty wind.

&&

Aviation /07z Tuesday through Saturday/...

Forecaster confidence levels...

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

Short term /through Wednesday/...moderate confidence.

Into this morning...
trend low-end VFR to MVFR with ceilings. Highest confidence of any
IFR across the high terrain. Similar trends with respect to fog.
Increasing S winds with gusts up to 25 kts along the S coast.
Risk of low level wind shear as winds 2 kft agl will be 35 to 40 kts, especially
in sheltered interior valleys. Increasing coverage of -ra/rain may
result in some tempo visibility restrictions.

Tuesday through Wednesday...
continued low-end VFR and MVFR ceilings with IFR confined to high
terrain. However, areas of -ra/rain expand slowly across the region
today. +Ra closer to 20z across west areas with a risk of thunderstorms and rain. Tempo
IFR with such conditions. S/southeast winds gusting as high as 40 to 45
kts. Areas of low level wind shear with S winds 2 kft agl around 50 kts. Rain
threats linger into Wednesday, however winds diminish.

Kbos terminal...moderate confidence in taf.
Will keep low-end VFR to MVFR cigs with increasing winds. Rain/+ra
threat into the terminal after the PM push.

Kbdl terminal...moderate confidence in taf.
Will keep low-end VFR to MVFR cigs with increasing winds. Rain/+ra
threat into the terminal closer to 21z.

Outlook /Wednesday night through Saturday/...

Wednesday night...
MVFR/IFR conditions possible. Rain showers likely, patchy br.

Thursday...
MVFR/IFR conditions possible. Breezy. Chance shra, slight
chance thunderstorms and rain.

Thursday night...
mainly VFR, with local IFR possible. Patchy br.

Friday...
VFR.

Friday night...
mainly VFR, with local IFR possible. Areas br.

Saturday...
VFR.

&&

Marine...

Forecaster confidence levels...

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

Short term /through Wednesday/...

Ahead of a cold front sweeping the waters late tonight and on
through Wednesday expect increasing areal coverage of moderate
to heavy rain along with strengthening S winds exceeding gale
force with the potential of gusts up around 45 kts. Can't rule
out a thunderstorm as well. Temporary reductions in visibility
due to rain. Seas building 10 to 12 feet on the outer waters.

Will see wind and wave action diminish slowly through Wednesday.

Outlook /Wednesday night through Saturday/...

Wednesday night...
winds less than 25 kt. Rough seas up to 11 ft. Rain showers.
Local visibility 1 to 3 nm.

Thursday...
winds less than 25 kt. Rough seas up to 10 ft. Chance of rain
showers.

Thursday night...
winds less than 25 kt. Areas of rough seas.

Friday...
winds less than 25 kt. Local rough seas.

Friday night...
winds less than 25 kt. Areas of seas near 5 ft on the outer
waters. Local visibility 1 to 3 nm in fog.

Saturday...
winds less than 25 kt. Seas 4 feet or less.

&&

Box watches/warnings/advisories...

CT...Wind Advisory from noon today to 11 am EDT Wednesday for
ctz002>004.
Flash Flood Watch from this evening through Wednesday morning
for ctz002.
Massachusetts...Wind Advisory from noon today to 11 am EDT Wednesday for
maz002>024-026.
Flash Flood Watch from this evening through Wednesday morning
for maz002-003-008>011.
Rhode Island...Wind Advisory from noon today to 11 am EDT Wednesday for
riz001>008.
Marine...Gale Warning from 11 am this morning to 2 PM EDT Wednesday for
anz231>234-250-254>256.
Gale Warning from 2 PM this afternoon to 8 am EDT Wednesday
for anz230-236-251.
Gale Warning from 11 am this morning to 8 am EDT Wednesday for
anz235-237.

&&

$$

Synopsis...wtb/sipprell
near term...sipprell

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