Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 190303

Area Forecast Discussion
National Weather Service Taunton MA
1003 PM EST Sat Nov 18 2017

Strengthening low pressure sweeps NE across the E Great Lakes
ahead of which widespread rain with embedded heavier showers,
possible thunderstorms, along with strong to damaging winds are
forecast through Sunday night. Breezy and cooler conditions for
Monday. Moderate temperatures on Tuesday followed by scattered
showers on Wednesday as a cold front sweeps through. Chilly but
dry for Thanksgiving and into Friday.



10 PM update...

*** Strong Winds, Heavy Downpours & Localized Wind Damage
 Possible Late Tonight & Sunday ***

Quick review of new 00z mesoscale guidance including the 00z
NAM/ARW/NMMB/HRRR AND HREFv2 continue to support current
forecast and headlines. Weather theme remains the same, periods
of rain overnight with heavier convective elements developing
ahead approaching cold front roughly 10z/5am thru 16z/11am.

May not be enough instability for thunder during this period
but all mesoscale guid has convective elements in the form of
heavier showers. This may be when our greatest wind threat
occurs as shallow but robust updrafts and precip drag transport
strong winds aloft to the surface.

Low level jet /925 mb/ peaks at 65 kt around 12z over south
coast including Cape Cod and the Islands. This also coincides
with the period of strongest forcing for ascent as LFQ of 130 kt
enhances QG forcing and combines with frontal convergence from
approaching cold front. Dew pts rising to 55-60 during this time
provides a few hundred joules of CAPE. Low and mid level lapse
rates are marginal at 6C/KM but this maybe sufficient to support
a broken fine line of low top convection 10z-16z. Nonetheless,
headlines remain unchanged.

Previous discussion...


*/ Highlights...

 - Strong to damaging SSW winds along S-coastal RI and MA
 - Widespread rain with embedded heavier downpours, possible t`storms
 - With t`storms, potential aid in mixing down strong, damaging winds

*/ Overview...

Strong to damaging southwesterly wind threats developing out ahead
of a deepening area of low pressure across the E Great Lakes, the
trailing cold front of which sweeping S New England is the focus of
widespread rain with embedded heavier showers and the possibility of

Nose of strong warm air advection (WAA) within atmospheric sounding
profiles around H8, ushered by a well agreed upon 60-80 mph SW jet
around H925-8, lending to a steep lapse rates aloft. Attendant low-
level frontal convergence / lift with the cold front beneath robust
mid-level vortmax, QG-forcing, and the left front quadrant H3 jet,
subsequent deep layer ascent in an area of high shear as the H5 trof
undergoes its negative tilt obtaining cyclonic curvature. Conveyor
belt motions enhancing with the ingest of a continental sub-tropical
airmass (cT) with H925-7 thetaE exceeding 320K, surface dewpoints
surging to around 50 to 60F, especially SE New England, as drier
air impinges from the SW, sharpening, as the cold conveyor belt
drives in from the NW behind the H85-7 mid-level low, descending.
Thermal gradients tightening with time.

All coming together shortly after midnight towards the early morning
hours Sunday, both strong SW jet and strong dynamics. Out ahead of a
continued deepening surface low down to 980 mb across the E Great
Lakes towards NE VT.

*/ Threats and Impacts (in priority)...


Strong to damaging wind threat is apparent. However the challenge is
in regards to WAA around H8, increasing the low-level inversion more
so over the Labrador Current looping down off the coast of the Outer
Cape and Nantucket. It`s here that mixing may not be as prevalent. A
lot of model consensus sounding profiles and ensemble means capture
this fairly well. However, elsewhere mean surface to H9 lapse rates
average around 6-7 C/km with consensus model forecast winds at
roughly 0.5 km agl (just above 1500 feet) 60 to 70 mph.

Based on wind climatology for SW cases along with forecast rules of
thumb, HIGH WIND WARNINGS have been posted along the S-coast of RI
and MA including the Islands for tonight with WIND ADVISORIES just
to the adjacent N. STORM WARNINGS over the southern waters including
Narragansett Bay.

There are moderate probability signals from NCAR and EC ensembles
that do justify such headlines, including the HREF signaling a
decent probability of sustained 10m 35 mph winds along the S coast.

Strong to damaging SW wind gusts forecast, considering that some
trees still have leaves, there are potential impacts of downed trees
and/or tree limbs possibly resulting in power outages, structural
damage, and/or blocked roadways.

Rain and Convection...

Undoubtedly widespread rain with embedded heavy downpours. Initially
with the warm front this evening, perhaps a lull around midnight,
then along the cold front thereafter into Sunday morning.

Depending on parcel lift, there is potential at various levels for
some weak instability up to 100-200 J/kg, especially within the low
levels as colder air undercuts right along the cold front. Given the
shear and strong lifting and ascent mechanisms, with any instability
can see storm organize. And along focusing boundaries, particularly
the cold front and leading nose of the dry slot, could see fine-line
convection develop. Rumbles of thunder along with heavier rain, can
not rule out mechanical mix-down/precipitation drag of faster winds
aloft to the surface aiding in the potential for strong to damaging
wind gusts. No particular area of focus, however if the 3 km NAM is
correct, could see clusters of heavier downpours, possible thunder-
storms, with perhaps a fine-line along the nose of the dry slot
going into Sunday.

Fast moving, precipitable waters nudging towards 1.5 inches, around
+2 standard deviations per NCEP SREF, looking at a widespread 0.5 to
0.75 inches of rain with possible amounts up around an inch in spots
(higher confidence along the S/W slopes of high terrain). Most of it
falling around midnight into the morning hours, should be able to
handle it, do not perceive a flooding threat.


Late Morning Sunday into Sunday night...

*/ Highlights...

 - Post-frontal winds W/NW driving in colder air
 - Widespread 40 to 45 mph wind gusts forecast
 - Areas of greatest impact across the high terrain, along the coast

*/ Discussion...

Strong post-frontal W/NW winds across S New England with widespread
40 to 45 mph gusts. In response to the surface low deepening down to
975 mb into SE Canada, well agreed upon by the consensus of forecast
guidance, expecting a strong pressure couplet especially across N
New England with pressure rises on the backside of the system up as
high as 3 mb / hr. Gradient / isallobaric wind response drives W/NW
flow with the top of the mix layer up to H85, dry adiabatic lapse
rates, subsequent of under-cutting colder air, where the top of the
mixed layer winds average 45 to 55 mph (40 to 50 kts), a few of the
models suggesting upwards of 60 mph.

Based on wind climatology for NW cases along with forecast rules of
thumb, WIND ADVISORIES have been posted for all of S New England
except where a HIGH WIND WARNING is in place along the S-coast.
Corroborating signals from both CIPS analogs and EC ensemble that
support such headlines.

Again, with strong to damaging NW wind gusts forecast, considering
that some trees still have leaves, there are potential impacts of
downed trees and/or tree limbs possibly resulting in power outages,
structural damage, and/or blocked roadways.

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 W/NW 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 E with
the mean flow to impact N/W portions of MA 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 of
the Berkshires.


*/ Highlights...

 - Blustery NW winds on Monday
 - Moderating temperatures Tuesday and into Wednesday
 - Dry but chilly for Thanksgiving day

*/ Pattern Overview/Confidence...

Building high pressure over Baja California will push the ridge into
the Western CONUS for next week. This will keep the eastern half of
the country in a trough til perhaps the end of the week. Overall 12z
guidance is in general agreement, however guidance spread occurs
with each potential shortwave and frontal systems that pass through.
Trended the forecast towards the UKMET and 12z EC as the GFS appears
to be too robust within this pattern set-up, esp towards the end of
the week.


Monday into Tuesday...High confidence.

Cold airmass continues to remain overhead on Monday as high pressure
to the south begins to build. Departing shortwave from the north
will increase the pressure gradient and thus anticipate gusty
westerly winds to continue during the day. Anticipate a mostly dry
day, but could see ocean effect showers across the open waters.

Mid-level ridge will pass through overnight into Tuesday as winds
switch and WAA returns across southern New England. Temperatures
will warm within this southwesterly wind as 50`s return back to the
region. Approaching cold front from the west will result in a
developing LLJ with 925mb winds reaching 40-45 kts. Once again
another windy evening for Tuesday.

Wednesday into Thanksgiving...Moderate confidence.

Approaching cold front from the Great Lakes will result in a mild
start on Wednesday. Depending on the available moisture, scattered
showers may develop along and/or ahead of the front. Both the EC and
CMC have continued to show precip chances while the GFS keeps the
bulk of the moisture offshore. Will continue to ride with chc pops
until guidance spread decreases.

Behind the front, CAA will move back into the region for Thursday.
Continue with the dry trend for the later half of the day as the 12z
GFS deterministic run appears to be an outlier with its developing
coastal low. Most of the GEFS also keeps the developing low well
offshore like the CMC. This developing low is also not supported by
the UKMET or the EC. Therefore anticipate a chilly day as temps fall
below seasonable with dry conditions.

Friday and beyond...Increasing confidence.

Increasing cloud cover overnight as secondary cold front swings
through. However, temps will begin to moderate as pattern turns a
bit more zonal.  A few shortwaves will push through next weekend but
timing and exact impact remain uncertain at this time.


Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...

10 Update...

No major changes to previous forecast.

Through early Sunday morning...

CIGs lowering towards MVFR / IFR as -RA/RA becomes widespread
with embedded +RA and possible TSRA towards morning. VSBY impacts
likely with RA/+RA, MVFR/IFR respectively. Increasing SW winds
throughout, strongest 3z-15z Sunday especially across SE New
England with sustained around 20-25 kts and gusts 40-50 kts..
Concurrent threat of LLWS as winds 2 kft agl will be around
55-65 kts, especially over the Cape and Islands.

Late Sunday morning through Sunday night...

Quickly improving VFR. Winds turning W/NW abruptly and immediately
gusting upwards of 40 to 45 kts, sustained around 15 to 20 kts.
Possible -RA/-SN impacts to high terrain terminals along the

KBOS Terminal...
Increasing SW winds overnight through roughly 16z Sunday. Gusts
up to 30 kts. Potential LLWS concurrent with strongest gusts as
winds aloft at 2 kft agl could be potentially as high as 50
kts. Lastly, despite -RA/RA with embedded +RA threats,
impacting the AM push for sure, can not rule out -TSRA closer to
12z Sunday. Wind shift after 16z Sunday, W/NW, will see a
greater risk of gusts exceeding 40 kts.

KBDL Terminal...
Stronger SW flow will lie S/E of the terminal closer to the coast.
Expect -RA/RA with embedded +RA increasing during the overnight
hours and going into morning with a wind shift out of the W/NW
around 15z Sunday. Thereafter there is an increased risk of wind
gusts of around 40 kts.

Outlook /Monday through Thursday/...

Monday: VFR. Windy with gusts to 35 kt.

Monday Night through Tuesday: VFR. Windy with gusts to 30 kt.

Tuesday Night: Mainly VFR, with local MVFR possible. Windy with
gusts to 30 kt. Chance SHRA.

Wednesday: Mainly VFR, with areas MVFR possible. Chance SHRA.

Wednesday Night: VFR. Breezy.

Thanksgiving Day: VFR.


Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...

*/ Highlights...

 - STORM WARNINGS posted for the S waters
 - GALES all other waters

*/ Discussion...

Storm center deepening across the E Great Lakes with a central
pressure down to 975 mb as it lifts into SE Canada Sunday night.

A trailing cold front ahead of which widespread rain is expected
with embedded heavier showers, possible thunderstorms, resulting
in visibility restrictions on the waters, as well as strong SW
flow with sustained winds around 20 to 30 kts, as high as 35 kts,
with gusts potentially exceeding 50 kts, again the greater threat
over the S waters.

Behind the cold front, winds shifting W/NW and remaining blustery.
Gusts 40 to 45 kts possible.

Waves building 10 to 15 feet throughout, can not rule out as high
as 20 feet during the height of the winds along the S/SE waters.

Outlook /Monday through Thursday/...

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: Low risk for gale force winds with gusts up to 35 kt.
Local rough seas.

Tuesday Night: Low risk for gale force winds with gusts up to
30 kt. Areas of rough seas. Chance of rain showers.

Wednesday: Winds less than 25 kt. Local rough seas. Chance of
rain showers.

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft.

Thanksgiving Day: Winds less than 25 kt.


CT...Wind Advisory from noon to 10 PM EST Sunday for CTZ002>004.
MA...High Wind Warning from 1 AM to 10 AM EST Sunday for MAZ020>024.
     Wind Advisory until 10 AM EST Sunday for MAZ017>019.
     Wind Advisory from noon to 10 PM EST Sunday for MAZ002>019-026.
RI...High Wind Warning from 1 AM to 10 AM EST Sunday for RIZ006>008.
     Wind Advisory until 10 AM EST Sunday for RIZ002>005.
     Wind Advisory from noon to 10 PM EST Sunday for RIZ001>005.
MARINE...Storm Warning from 1 AM to 10 AM EST Sunday for ANZ232>234-236.
     Gale Warning from 1 AM to 10 PM EST Sunday for ANZ230.
     Gale Warning until 6 AM EST Monday for ANZ231-250-251-254.
     Storm Warning until noon EST Sunday for ANZ235-237.
     Storm Warning until noon EST Sunday for ANZ255-256.


NEAR TERM...Nocera/Sipprell/Correia
SHORT TERM...Sipprell
MARINE...Sipprell/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.