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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
553 PM EST Wed Nov 20 2019

Synopsis...
low pressure exits bringing rain showers to and end from west
to east early tonight. High pressure then builds in from the
west Thursday yielding dry conditions and seasonable
temperatures. Mild and gusty Friday as a cold front swings
through New England. Colder air moves in behind the front, with
dry weather from Friday afternoon through Saturday. Low pressure
moving out of the southern Appalachians will bring rain and
snow to much of our area Saturday night and Sunday. High
pressure brings drier weather Sunday night through Tuesday. A
cold front may bring showers Wednesday afternoon and night.

&&

Near term /until 6 am Thursday morning/...

7 PM update...

Low pressure over the waters south of Nova Scotia continues to
circulate a moist flow through Maine and New Hampshire into southern New
England. Mainly just sprinkles/drizzle in the CT valley, snizzle
in the higher elevations of Worcester County, and light rain
showers in eastern MA/RI.

Satellite shows clouds lingering over New York and PA, although
clearing is working south from eastern Ontario to connect with
partial clearing over the lower Hudson Valley. Not much of a
clearing trend in southern New England as of 530 PM. The trend
is correct, but the timing is off. Based on this, we have
slowed the end of precipitation and clearing by 3-6 hours.
Temperatures have also been adjusted based on 5 PM values. No
changes to the overall forecast at this time.

Previous discussion...

Looking at the late afternoon into overnight period we'll see a
drastically improving weather pattern. This improvement comes
thanks to the exiting of the offshore surface low and mid level
disturbance. The mid level low center is currently spinning just
over Cape Cod as visible on water vapor satellite imagery. An
influx of mid and upper level moisture around this circulation
has increased the areal coverage of rain showers this afternoon,
but this will begin to exit from west to east this evening.
Currently expecting the last of the showers to exit the East
Coast by midnight. Beyond that we begin to make the transition
to drier weather as surface high pressure and mid level ridging
move in. Drier air returns first at the mid/upper levels,
working down and providing for clearing skies in the early
morning hours of Thursday. Potential for some radiational
cooling, especially out west, but will be contingent on how
quickly those clouds can clear out and if winds calm enough.
Winds will be coming down through the evening as the gradient
slackens. Low temperatures are likely to drop into the 20s and
low 30s inland, with upper 30s along the immediate coast.

&&

Short term /6 am Thursday morning through Thursday night/...
rising heights, building high pressure, and decreasing winds will be
the story of the day. Skies will be sunny under a neutral to slightly
warm air advection (moreso warm air advection later in the day) regime and this should
support temperatures well into the upper 40s. This would be
closer to (but still slightly cooler than) normal for late
November. After many days of gloomy wet weather Thursday will be
a nice change of pace.

By Thursday evening into Thursday night return flow commences on
the downstream side of the next shortwave. Warm air advection as the warm front
lifts north and a plume of moisture moves into the region. Along
with the moisture a low level jet associated with the low over the Great
Lakes reaches into southern New England. However, the best forcing and better
moisutre resides to our north, so any rain/high elevation snow
showers should be spotty. Increasing clouds and warmer southerly
flow will keep temperatures warmer than previous nights, in the
30s across the board.

&&

Long term /Friday through Wednesday/...
big picture...

Split flow in place with northern and southern streams over the
nation. This resolves to a single jet late in the weekend
through midweek next. Shortwave in the northern stream moves
across New England Friday. Closed low that initialized over the
Desert Southwest this morning will move to the Ohio Valley
Saturday and cross New England as a shortwave Sunday. A third
system crosses the Great Lakes Wednesday night.

Operational models show good agreement through Saturday, then
show differences with the Sunday shortwave, and then show
differences in timing and placement with the Wednesday upper
system. Forecast confidence is high through Saturday morning,
then diminishes to between low and moderate Sunday through next
Wednesday.

Daily concerns...

Friday...

Strong low-level winds associated with the Friday northern
shortwave. Winds forecast at 30-40 kt at 950 mb and 50 kt at 850
mb. Much of the former is withing the mixed layer. Expect gusty
winds in the warm advection leading the front, and a better
chance of strong gusts in the cold advection trailing the front.
The cloud layer is forecast to reach up through 700 mb and
shows limited lift. While not impressive, it should be enough
for scattered showers along/ahead of the cold front. The mixed
layer is forecast to reach near 900 mb, and temperatures in that
layer support Max sfc temps in the 50s...possibly near 60 in
spots.

Continued cold advection wind gusts of 30-40 kt supported
Friday night. Temperatures fall to 25-35.

Saturday...

Shortwave ridge builds over the region, and should provide
sufficient subsidence for a dry day. Increasing high level
moisture over CT and western Massachusetts during the afternoon suggests
increasing cirrus sky cover at that time, but dry weather at the
surface. Mixed layer reaches to about 950 mb, with temperatures
that support surface Max temps in the 40s.

Saturday night through Wednesday...

As noted above, confidence trends lower during this period. Low
pressure moves up from the Appalachians spreading precipitation
into southern New England. Temperature profiles on BUFKIT show
rain over most areas including Hartford-Springfield, with snow
or mixed pcpn over higher elevation areas, including Worcester.
Meanwhile the combined model data shows a little more cold air,
with a period of snow/ice in western/central Massachusetts and extreme nrn
CT/RI. We have used the latter, which gives US less than an inch
of snow and less than 0.1 inch ice potential. As usual,
changes in the storm track could change this...either for the
milder or the colder...so this will continue to be monitored.
The low center passes closely to Cape Cod Sunday afternoon with
rain tapering off Sunday evening.

High pressure brings dry weather Monday and Tuesday. Increasing
south-southwest flow Wednesday will bring milder temps. Cold
front approaches for Wednesday night, although timing on this
system remains variable and will be monitored.

&&

Aviation /00z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Thursday night/...moderate confidence.

Tonight...

The clearing trend is a little slower that previously expected.
This is based on trends from satellite and radar. Therefore we
have slowed the trend lifting ceilings and eventual clearing by
3-6 hours. Still expect IFR and low-end MVFR cigs to lift
between 00z and 06z, with clearing in the CT valley around 3
am/08z, in the central hills 4 am/09z, and bos-pvd 5 am/10z.

Thursday...

VFR under high pressure. Gradient slackens into Thursday with
decreasing north winds towards light/variable (tending west-SW late).

Thursday night...

VFR. Light SW winds.

Kbos taf...moderate confidence in taf.

Kbdl taf...high confidence in taf.

Outlook /Friday through Monday/...

Friday: mainly VFR, with areas MVFR possible early. Windy with
gusts up to 35 kt. Slight chance rain showers.

Friday night: VFR. Windy with gusts up to 35 kt.

Saturday: VFR. Windy with local gusts up to 30 kt.

Saturday night: mainly VFR, with areas IFR possible. Chance ra,
with a chance snow or freezing rain inland.

Sunday: mainly VFR, with local IFR possible. Breezy. Chance ra,
with a chance freezing rain or snow in northern and western Massachusetts early.

Sunday night: IFR early, then trending to VFR overnight.
Breezy.

Monday: VFR.

&&

Marine...
short term /through Thursday night/...moderate confidence.

Tonight: north winds remain 15-25 kts gusts to 25-30 kts. Building
seas eastern offshore waters to 6-9 ft, 3-6 ft in coastal waters
in eastern Massachusetts/Cape Cod Bay. Southern waters should see seas 4-6
ft.

Thursday: decreasing north winds to around 10 kts, gusts to 15-25
kts by afternoon. Seas 4-7 ft outer offshore waters.

Thursday night: light SW winds early will increase through the
night, gusting to 25-30kts by daybreak. Seas becoming 3-5 ft.

Outlook /Friday through Monday/...

Friday: low risk for gale force winds with gusts up to 35 kt.
Areas of rough seas. Slight chance of rain showers.

Friday night: moderate risk for gale force winds with gusts up
to 35 kt. Rough seas up to 11 ft.

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

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

Sunday: winds less than 25 kt. Local rough seas. Chance of
rain.

Sunday night: winds less than 25 kt. Areas of rough seas.
Slight chance of rain.

Monday: winds less than 25 kt. Seas up to 5 ft.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 9 PM this evening to 4 am EST
Thursday for anz232.
Small Craft Advisory until noon EST Thursday for anz231-255-
256.
Small Craft Advisory until 7 am EST Friday for anz250-254.
Small Craft Advisory until 6 PM EST Thursday for anz251.

&&

$$
Synopsis...wtb/bw

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