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Area forecast discussion
National Weather Service Taunton Massachusetts
351 am EST Sat Nov 18 2017

Synopsis...
high pressure over New England this morning provides dry weather
much of the day. A warm front approaches the region late in the day
with rain overspreading the area toward sunset and into the evening.
Strengthening low pressure moves across western New York state tonight
with warm air, strong winds and showers impacting southern New
England. The attending cold front sweeps across the area midday
Sunday with strong winds ahead and behind the front. Dry and cold
weather overspreads the region Sunday night and Monday. Mild and dry
weather Tue ahead of a cold front which brings a risk for showers
Wed followed by dry and colder weather Thanksgiving day.

&&

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

4 am update...

Cold Ridge of high pressure over the region this morning with
predawn temps in the 20s, 30s along the coast including Cape Cod,
islands and city of Boston. Sunshine to begin the day but clouds
quickly on the increase as mid level ridge axis moves offshore and
flow becomes cyclonic with warm air advection pattern developing. Despite
increasing clouds today low level warm air advection will provide a nice temp
recovery from this morning/S cold with highs this afternoon 50-55
along the coastal plain, extending into the Boston-Providence-
Hartford corridor. Farther inland shallow cool air will be more
stubborn to modify and highs there rising into the mid and upper
40s.

Area of rain showers early this morning over eastern Ohio will weaken
and mainly pass northwest of our region as this lead short wave
trough deamplifies as it runs into the ridge over New England. Thus
expecting mainly dry weather today with any rain holding off until
21z-00z and mainly confined to CT and western-central Massachusetts. Therefore
expecting dry weather to prevail today across Rhode Island and eastern Massachusetts.



&&

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

4 am update...

*** strong south-southwest winds south coastal Massachusetts and Rhode Island ***
*** possible fine line of low convection ***

Tonight and Sunday morning...

Wind threat...

Strong low level jet overspreads the South Coast tonight. Good
agreement from models on magnitude of jet 55-60 kt at 925 mb. Model
soundings initially Show Low level lapse rates favorable this
evening for 35-40 kt Transfer in gusts. But as the night progresses
strong warm air advection steepens low level inversion with gusts becoming less of
a factor and sustained winds becoming the main issue with speeds 25-
30 kt. This will be sufficient to support Wind Advisory for the
entire South Coast including Cape Cod and the islands.

Rain and convective threat...

Periods of rain with warm front lifting north across the region.
Impressive warm sector for mid to late Nov overspreads the area late
tonight with dew pts surging to 55-60! Thus temps will rise
overnight toward 60 in the coastal plain. This will set the stage
for a potential fine line of low top convection toward daybreak
Sunday from west to east. Short wave energy merges into a negative
tilt trough approaching the region toward 12z Sunday. Also a strong
upper level jet streak of 130 kt approaches from the southwest 12z
Sunday with lfq over southern New England, enhancing qg forcing. In
addition very strong frontal convergence as low pres deepens to 982
mb over New York state. Thus very strong forcing for ascent in a high
shear environment. With dew pts rising to near 60 a few hundred
joules of cape advects across the area, anomalous for mid to late
Nov. However limiting factor for convection is marginal low and mid
level lapse rates. So question becomes will there be sufficient
instability to yield a strong response at the surface in the form of
fine low of convection. Unfortunately model timeframe here is 36-42
hrs, on edge of our mesoscale guid with hrefv2 only out to 36 hrs.
However the ncar ensembles, arw and nmmb all hint at possible fine
line in simulated reflectivity fields. For now will insert slight
chance thunder with gusty winds and heavy downpours as pwats climb
to 1.5 inches (+2 Standard deviations).

Sunday afternoon...

*** strong Post frontal winds likely ***

979 mb low traversing northern New England/St Lawrence River valley
will yield strong pres gradient, Post frontal cold air advection and impressive
pres rise-fall couplet for a period of strong west-northwest Post frontal winds
Sunday afternoon with speeds of 40 to 50 mph possible. Later shifts
will have to evaluate model trends for possible issuance of wind
advisories for Sunday afternoon. Drying trend behind the fropa along
with temps falling from morning highs near 60.

&&

Long term /Sunday night through Friday/...

*/ highlights...

- widespread strong to damaging west/northwest winds Sunday into Sunday night
- winds will linger into Monday with perhaps less of an impact
- Ebb and flow pattern of S warm air surges followed by north cold air
continues

*/ overview...

Ebb and flow continues. Without greater amplification and subsequent
Transfer between the Equator and the poles, the flow remains mainly
zonal. The mjo largely subdued as the present -wpo/-epo/-pna all
trend positive into December per ensemble means, counter-productive,
lending seemingly to a signal of persistent west to east flow. The only
consistencies it seems from analysis of potential vorticity and
isentropic surfaces is a persistence of poleward Transfer in the North
Pacific and the NE Atlantic that lends to S shearing of energy into
the Gulf of Alaska and S across Europe, respectively. Otherwise it's
zonal in-between. Any energy across the Continental U.S. Is quick moving with
little residency such that forces acting to invoke greater curvature
and storm development reach their full potential east of our region
across east/southeast Canada into the northwest Atlantic. With racing storm systems
over our region, warm-surge in advance followed by cool shots, both
with attributes of gusty winds, a chance of wet-weather in-between
along associated frontal boundaries, cold fronts with more emphasis.
Attendant threats and impacts highlighted in the discussion below.

*/ Discussion...

Sunday night into Monday...

Winds shifting northwest ushering colder air, remaining blustery. Consensus
of forecast guidance, cold front sweeping 7-10a Sunday, undercutting
colder air, lapse rates mixing out to h85, dry adiabatic, the top of
the mix layer averaging west/northwest 45 to 50 mph (40 to 45 kts). Widespread
Wind Advisory likely, more likely daytime Sunday with boundary layer
heating and contributions from quick pressure rises. Ensemble means,
strongest winds likely across the high terrain and along the coast
adjacent to 50-degree ocean waters, also North/East closer to the storm
center, so North/East portions of Massachusetts at greatest threat. Cips analogs with
a high probability of at least 35 mph widespread gusts, lower for 40
to 45 mph, however considering local climatology, believe there is
the potential to see 40+ mph gusts.

Headlines may drop overnight with the potential of a shallow surface
inversion, however model guidance consensus signals continued cold
air advection right into Monday with additional energy rotating
through the cyclonic flow across the region lending to an enhancement
of the wind profile. Something will have to watch closely.

Will highlight the wind threat in the hazardous weather outlook. No
headlines at this time associated with west/northwest winds.

Otherwise, drier air surging in aloft behind the quick sweeping cold
front, will see an end to shower activity rather abruptly during the
morning hours. With the blustery west/northwest winds and cold airmass pouring
in, across the warmer Great Lakes, the lake effect machine will be
ongoing and could see some of that energy reach far enough east with
the mean flow to impact north/west portions of Massachusetts and CT with some snow
and/or rain shower activity, 2m temperature dependent. If snow, some
light accumulations are possible, mainly for the high terrain.

All threat diminish into Monday night, and it is during this period
that winds may become light enough beneath clear conditions to allow
for radiational cooling and night-time lows to drop well down into
the 20s, possibly teens. Hard to say right now. If west/northwest winds remain
brisk, then the mechanical mixing in the environment would limit the
radiational cooling potential. Will hold it conservative with wide-
spread lows in the 20s. Highs Sunday into the upper 40s to low 50s
right before the cold front punches through in the morning, upper
30s to low 40s for Monday.

Tuesday...

Quiet. High pressure. Return S flow. Turning mild. Still believe
that warmest temperatures will be over interior southeast New England, that
an onshore will develop in response to an approaching storm system
for the overnight period into Wednesday. Highs in the upper 40s to
low 50s.

Tuesday night into Wednesday...

Possible showers. Keeping with slight chance pops, the ec on again
off again on the north and S stream energy beginning to interact with a
draw of S-moisture ahead of the north sweeping cold front before both
systems push offshore quickly and phase downstream. The GFS keeps
the dry-streak going. Given the emphasis on the cold front agreed
upon within all available guidance, would expect that the associated
lift on available moisture should get something going, especially
considering the warmer ocean waters that line the shore.

Thanksgiving Onward...

Thanksgiving looks quiet and seasonable, while into the weekend it
appears another quick moving storm system sweeps the region. The Ebb
and flow pattern continues with milder shots and SW flow ahead of
any disturbance followed by colder air and northwest winds as the system
departs.

&&

Aviation /09z Saturday through Wednesday/...
forecaster confidence levels...

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

Short term /through Sunday/...

06z...

Today...high confidence.

VFR and dry weather prevails. The exception will be across
western Massachusetts/CT where MVFR conditions in light rain will increase
between 21z-00z. Light winds this morning increase sharply this
afternoon from the south-southwest with gusts approaching 30 kts by sunset
(22z). Elsewhere south-southwest winds increasing to 10-15 kt.

Tonight...high confidence.

VFR-MVFR to start in areas of light rain but trending toward
IFR-MVFR after midnight. Strong south-southwest 20-25g45kt winds along the
South Coast including Cape Cod and islands. Elsewhere winds not
as strong and this results in low level wind shear across much of Rhode Island and eastern
Massachusetts including bos.

Sunday...moderate confidence. Some uncertainty on probability of
-tsra and areal coverage along and ahead of strong cold front.

MVFR-IFR with widespread showers, some with locally heavy
downpours. Could be a fine line of low top showers with embedded
thunder roughly 15z-18z. Gusty winds may accompany this
convection. Strong south-southwest winds 20-25g45kt continue along the South
Coast including Cape Cod and islands. Away from the South Coast
expect low level wind shear to continue thru the morning across Rhode Island and eastern
Massachusetts. Then a strong cold front sweeps across the area late morning
into the early afternoon, with improving conditions behind the
front and a wind shift to the west.

Kbos terminal...high confidence in taf thru 00z then some timing
issues on arrival of lower cigs and rain. Then period of rough
weather tonight into midday Sunday with low level wind shear along with heavy
showers. Isolated thunder possible 15z-18z Sunday, then strong
cold front sweeps across the area with improving conditions
after 18z Sunday.

Kbdl terminal...high confidence in taf thru 21z then some timing
issues on arrival of lower cigs and rain. Unsettled weather
tonight into Sunday morning with heavy showers and possible
isolated thunder.

Outlook /Sunday night through Wednesday/...

Sunday night: VFR. Windy with gusts to 35 kt.

Monday: VFR. Strong winds with local gusts to 40 kt.

Monday night: VFR. Windy with gusts to 35 kt.

Tuesday: VFR. Breezy.

Tuesday night: VFR. Windy with local gusts to 30 kt. Slight
chance shra, slight chance freezing rain.

Wednesday: VFR. Breezy. Slight chance rain showers.

&&

Marine...
forecaster confidence levels...

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

Short term /through Sunday/...high confidence.

Today...

Light winds this morning with high pressure overhead. However south-southwest
winds increase this afternoon and approach gale force by late in the
day.

Tonight...

South-southwest gales along with poor vsby in showers and fog.

Sunday...

Strong cold front sweeps across the region late morning and early
afternoon, with south-southwest prefrontal gales and west-northwest gales behind the front.
Showers, locally heavy in the morning along with fog but improving
with the wind shift.

Outlook /Sunday night through Wednesday/...

Sunday night: strong winds with gusts up to 35 kt. Rough seas
up to 14 ft.

Monday: moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft.

Monday night: low risk for gale force winds with gusts up to
35 kt. Areas of rough seas.

Tuesday: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

Tuesday night: moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Slight chance of
rain showers.

Wednesday: winds less than 25 kt. Local rough seas. Slight
chance of rain showers.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for
maz020>024.
Rhode Island...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for
riz005>008.
Marine...Gale Warning from 5 PM this afternoon to 6 am EST Monday for
anz232>234.
Gale Warning from noon to 11 PM EST Sunday for anz230.
Gale Warning from 1 am Sunday to 6 am EST Monday for anz231-
251.
Gale Warning from 7 PM this evening to 7 PM EST Sunday for
anz236.
Gale Warning from 3 PM this afternoon to 6 am EST Monday for
anz235-237-255-256.
Gale Warning from 7 PM this evening to 6 am EST Monday for
anz250-254.

&&

$$
Synopsis...nocera

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