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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1255 PM EST Fri Nov 22 2019

Synopsis...
a cold front crosses the region this afternoon, bringing a
period of light showers, along with falling temperatures and
gusty northwest winds late this afternoon through tonight. Weak
high pressure then settles over the area for Saturday. Low
pressure will bring rain Saturday night and Sunday as it passes
near Nantucket, with the possibility of light icing across the
interior due to a wintry mix at the start. Dry weather follows
Monday and Tuesday as high pressure builds into the region. A
cold front may bring showers Wednesday followed by dry but
breezy weather on Thanksgiving.

&&

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

Cold front was on the doorstep of western Massachusetts at 1 PM. This front
will sweep from west to east across southern New England this afternoon, with winds
becoming northwest and gusty. Pops increased to likely/categorical for
rain showers based on radar and observations. Looking at a
window of 2-4 hrs of -shra activity for any given locale. It is
possible across the highest terrain of the east slopes of the
Berkshires, that precipitation may mix with or change over to
-shsn before ending. Little to no accumulation is expected.

Early morning discussion follows...

Though 850 mb temperatures are expected to climb into the +4 to
+6c range, expecting mostly overcast skies to continue even in
the wake of the warm frontal passage early this morning. That
really cuts into the extent to which insolational heating can
boost daytime temperatures before the front gets here. Will be
looking for daytime highs in the upper 40s to mid 50s, warmest
towards the South Shore, Cape Cod and the islands. Will see SW
winds increase with sustained winds 10-20 mph and around 20-30
mph in gusts. Pre-frontal mixing of stronger gusts doesn't look
likely given shallow mixing depth capped by an inversion.

Guidance continues to show generally good agreement on a cold
frontal passage between 17-23z cwa-wide. Have again noticed an
uptick in model-derived qpf, especially near/south of the Mass
Pike. Did opt to raise pops in this window towards solid
likely/low categorical levels, but even with the increase in
qpf, most areas will only see quantitative precipitation forecast amounts a few hundredths to a
tenth of an inch, and the best chance of seeing rain amounts
near that high is mainly southeast of a bvy-orh-hfd line.

The bigger concern is the resulting marked increase in
northwesterly gusts both over the land and the waters, this
roughly occurring in lockstep with quickly falling surface
temperatures. Once the 850 mb front clears the forecast area,
850 mb temps crash to -5 to -8c by 00z Saturday. This
contributes to very steep low-level lapse rates primed to tap
into the 45-50 kt nwly low-level flow. Should see northwest gusts of 30
to near 40 mph developing by late afternoon-early tonight
across most of the land areas. However, wind gusts towards Cape
Cod and the islands may approach Wind Advisory levels with gusts
40-45 mph. Entertained the idea of a short- fused Wind Advisory
for Cape Cod but thought it was somewhat marginal especially in
the absence of leafed trees, and that could mitigate adverse
impact. Temperatures are likely to fall back into the 30s to
low 40s by early Friday evening, and with the winds we'll be
looking at wind chills in the low 20s f across the Berkshires
into northern Worcester County and into the hills in northern
CT.

&&

Short term /6 PM this evening through 6 PM Saturday/...
tonight:

Surface ridge builds into southern New England affording
clearing skies and dry weather, but continued very steep low-
level lapse rates even overnight will still support a
continuation of gusty winds well into the nighttime hrs. When
factoring in that temperatures will continue sharply falling to
lows in the 20s to near freezing, hard-pressed to really call it
a good night to be outdoors as wind chills in the teens to low
20s. Will continue with gusts 30 to near 40 mph away from Cape
Cod/islands (and closer to 40-42 mph for there), and while wind
gusts do subside slightly after midnight, they're still 20-30
mph and in the mid-upper 30s mph towards the cape and islands.

Saturday:

A pretty chilly start to Saturday, but will have plenty of sun
Saturday to help boost temperatures. Mid to high clouds build in
late in the day from SW to NE but overall a clear to mostly
sunny day with highs upper 30s to mid 40s. Will see winds/gusts
continue to lighten considerably through the day and eventually
become S/SW 5-10 mph late as high pressure builds off the mid-
Atlantic coast.

&&

Long term /Saturday night through Thursday/...
highlights...

* rain Saturday night and Sunday, starting as a wintry mix interior.
* Dry and seasonable Monday and Tuesday
* milder with scattered showers Wednesday
* dry but breezy and cooler Thanksgiving day

Active pattern continues over weekend as southern stream trough
rotates through New England, as its associated surface low tracks
near Sunday. Broad trough then develops across much of Continental U.S. Next
week as a northern stream short wave rotates through region
Wednesday.

We continue to focus on the potential hazards associated with low
pressure which is forecast to pass near Nantucket sunday:

Interior light icing:

Confidence is increasing for the potential of slippery travel across
interior southern New England Saturday night into Sunday morning due to a wintry mix.

Keep in mind there is still a good amount of spread among ec/GFS
ensemble members, showing low tracks anywhere from southeast New England to
a little farther east of Nantucket. Despite the range in track of
surface low, which is tied to it being captured by 500 mb low,
overall pattern still looks on the warm side with a lack of high
pressure and cold air to the north, so the potential for any
significant snow or ice across the interior remains low.

Temperature profiles and ensemble probabilities suggest rain for
most of southern New England for this event. That said, there are hints of cold air
damming across western and north central Massachusetts, and possibly
into northwest Hartford County, CT which is where models show possibility
of light icing Saturday night into Sunday morning, before milder air
eventually scours out the low level cold and causes a change to
rain. Even in a "worst case" scenario, snowfall amounts would only
be around an inch with around 0.10" of icing.

Minor coastal flood potential for Sunday/Sunday night...see
tides/coastal flooding section of this discussion.

Beyond the weekend, high pressure will bring dry and seasonable
weather Monday and Tuesday. An approaching cold front will
bring a milder day Wednesday along with scattered showers.
Behind the front, dry but breezy and cooler weather is forecast
for Thanksgiving day.

&&

Aviation /18z Friday through Wednesday/...
forecaster confidence levels...

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

Short term /through Saturday/...high confidence.

18z taf update:

Thru 00z this evening...

VFR ceilings for most of the period. However, as showers along/ahead
of a cold front sweep from west to east from 18-23z, an brief period
of MVFR cigs is possible. With west-northwest-northwest wind shift, wind gusts
increase to 25 to near 35 kt, except 35 to near 40 kt gusts on
Cape Cod/Nantucket.

Tonight...

VFR with continued strong northwest gusts for most of the night as
much colder air filters in. Early wind gusts 25-35 kt in the
interior, to 30-near 40 kt for coastal Massachusetts/Cape Cod and
Nantucket and the waters. Wind gusts gradually decrease after
04z.

Saturday...

VFR under high pressure. High clouds approach late. Northwest winds
become west-west-southwest and subside through the day, though will still gust
to 20-25 kt into mid-morning hrs especially eastern Massachusetts and the
cape/islands.

Kbos taf...high confidence in taf.

Kbdl taf...high confidence in taf.

Outlook /Saturday night through Tuesday/...

Saturday: VFR. Windy with local gusts up to 35 kt.

Saturday night: mainly VFR, with local MVFR possible. Chance
ra, chance freezing rain.

Sunday: MVFR/IFR conditions possible. Windy with local gusts up
to 30 kt. Rain likely, freezing rain.

Sunday night: VFR. Windy with local gusts up to 30 kt. Chance
rain showers.

Monday: VFR. Breezy.

Monday night through tuesday: VFR.

&&

Marine...
short term /through Saturday/...moderate confidence.

Gale warnings remain in effect for all waters. Winds will
continue to increase on all waters to Small Craft Advisory levels through the
morning, with building seas especially southern waters. Looking
at gale-force west to northwest winds then settling in around mid-
afternoon and continuing into the overnight hours as colder air
races across the waters behind a strong cold front. Subsiding
trends to winds and seas into Saturday.

Today: SW winds 10-15 mph with gusts to 25 kts thru early
afternoon. Wind shift to northwest and increase to 15-25 kt with gusts
35-40 kt on most waters toward late afternoon into early
evening, on the lower end of that range in Boston Harbor and
Narragansett Bay. Seas build to 4-8 ft southern waters early,
building late to 4-6 ft for eastern waters.

Tonight: nwly gale conditions on all waters tonight. General
improvement trend after midnight, with winds/gusts diminishing
to Small Craft Advisory levels earliest on the bays/harbors, longer in eastern
offshore waters. Seas 5-10 ft, highest southeastern waters,
with lowering trend to seas nearest the coasts.

Saturday: early-day Small Craft Advisory winds/gusts on eastern offshore waters,
diminishing to 10-15 kts with gusts to 20 kts late Saturday.
Steady fall in seas to 3-5 ft.

Outlook /Saturday night through Tuesday/...

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

Saturday night: winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain.

Sunday: low risk for Small Craft Advisory winds with local
gusts up to 30 kt. Local rough seas. Rain likely.

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

Monday: winds less than 25 kt. Local rough seas.

Monday night: winds less than 25 kt. Seas locally approaching
5 ft.

Tuesday: winds less than 25 kt.

&&

Tides/coastal flooding...

Sunday/Sunday night...

Even though we are coming into a period of high astronomical tides,
we are a bit less confident with the potential for coastal flooding
due to uncertainty in the track of the surface low, which will
determine wind direction/fetch as well as timing of the highest
surge (does it coincide with high tide)?

For the Sunday morning high tide, it looks like both the east and
south coasts may be impacted due to expected east/southeast flow. Expecting a
storm surge of 1.0 to 1.5 ft which may cause minor flooding (less
than one foot inundation agl) of vulnerable shore roads.

Times and heights of high tide (mllw) Sunday morning are:
* providence: 541 am, 5.7 ft
* boston: 845 am, 11.2 ft
* nantucket: 946 am, 3.9 ft

For the Sunday evening high tide, the focus will be on the eastern
Massachusetts coast including Martha's Vineyard and Nantucket as winds are from
the N/NE. Storm surge will be a little higher than the morning high
tide, on the order of around 2.0 ft, which would bring another
possible round of minor flooding (up to one foot inundation agl) to
vulnerable shore roads. Wind direction favors the coastline from
Plymouth County to the Bay Side of Cape Cod as well as the islands.

Seas offshore are forecast to reach 4 to 7 feet, so beach erosion
would be minimal.

Times and heights of high tide (mllw) Sunday evening are:
* boston: 917 PM, 10.5 ft
* nantucket: 1024 PM, 3.2 ft



&&

Equipment...

Kbox WSR-88D has returned to service.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Gale Warning until 3 am EST Saturday for anz231>235-237-251.
Gale Warning until midnight EST tonight for anz230-236.
Gale Warning until 6 am EST Saturday for anz250-254.
Gale Warning until 4 am EST Saturday for anz255-256.

&&

$$
Synopsis...loconto/jwd
near term...loconto/nmb/jwd
short term...loconto
long term...jwd

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