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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
115 PM EST Sat Dec 7 2019

Synopsis...
cold high pressure will move from the eastern Great Lakes to New
England by late tonight. Many areas will have sub-freezing high
temperatures today. It will be frigid tonight however winds will
be light. The high moves offshore Sunday, starting a return
flow of milder and moist air for the first half of the week.
This will bring off and on rain for Monday and Tuesday. A cold
front moves through Tuesday night, and there might be some back-
end snow on Wednesday although confidence is very low at this
point. We dry out by late Wednesday and return to colder than
normal temperatures for early December on Thursday and Friday.

&&

Near term /until 5 PM this afternoon/...

115 PM update ...

Previous forecast remains on track so no major changes with this
update. Earlier discussion below.

================================================================

Chilly day in progress with temperatures slowly rising thru the
20s into the low and mid 30s. However weak cold air advection
continues today with 850 mb temps lowering from -12c to about
-14c by days end. This will only support highs this afternoon of
30-35, mid to upper 20s in the hilly terrain. In addition with
1032 mb high still west of our region over OH, this will
support enough of a north-northwest gradient for a brisk wind 10-20 mph
providing wind chills in the 20s, teens higher terrain. Overall
a colder than normal day by early Dec standards when the normal
high temp should be 40-45.

Abundant sunshine today with some cloudiness from time to time
over western Massachusetts/CT in response to lake effect clouds from cold
air streaming across the relatively mild waters of Lake Ontario.
Also with cold air streaming on north-northwest winds ocean effect clouds
will continue to impact the Outer Cape and Nantucket from time
to time. Previous forecast captures these details nicely so no
major changes with this update.

&&

Short term /5 PM this afternoon through 6 PM Sunday/...
tonight, strong high pressure will be centered over southern New
England. The combination of light wind, clear skies, and snow
cover will allow temperatures to plummet rapidly. We went lower
than model guidance, with lows ranging from 3 to 10 above zero
in northern and western Massachusetts and from 10 to 15 elsewhere, except
upper teens to mid 20s on the immediate coast.

On Sunday, high pressure moves off the coast and winds turn to a
southwesterly direction. Temperatures will moderate a little,
with highs in the 30s, with lower 40s southeast Massachusetts. Skies will
start out sunny, but there will be an increase in mid level
cloudiness in the afternoon, due to warm advection aloft.

&&

Long term /Sunday night through Friday/...
big picture...

Broad upper trough digs over the plains and Great Lakes early in the
week while upper ridge builds over the western Atlantic. This
creates a southwest flow aloft across the eastern USA and brings
milder air into southern New England. The upper trough moves east
midweek, crossing New England Thursday, and eventually flattens into
a zonal flow for the end of the week.

Normal 500-mb heights in this area in mid December range from 545 to
555 dm. Forecast heights for Sunday night are in the 560s,
diminishing to 555 to 562 dm by Tuesday. So the deep layer is
expected to be mild during the early week with no low level features
to hold any surface cold air over our area. As the upper trough
swings east these values lower to between 535 and 540 dm, although
the European model (ecmwf) and ggem suggest values around 530 dm. So the trend will
be to colder than normal Wednesday-Thursday, then trend toward
normal Friday.

Mass fields and thermal fields in the models are similar through
Monday but then show some model-to-model differences midweek. This
means moderate to high confidence through the early week, with
diminishing confidence midweek.

Daily concerns...

Sunday night...

Surface high pressure offshore with a south-southwest flow over
southern New England. Surface winds coming off the ocean into srn
Rhode Island/southeast Massachusetts are around 30 kt/35 mph while winds 3000-5000 feet abv
surface show a 50 kt/55 mph southwest jet over the interior. If
winds from the base of this higher jet get mixed, it may also bring
gusts to 30 kt/35 mph farther inland.

Model soundings show a saturated layer developing below 3000 feet,
but dry air higher up in the nucleation layer through 6 am. This
shows some potential for spits and drizzle but otherwise a dry night.

Expect lowest temperature in the early night, with temperatures
rising overnight. Based on dew points, the expected evening
temperatures should be in the upper 20s to mid 30s. All temperatures
are above freezing by midnight, so any precip that does form
overnight should be rain.

Monday-Tuesday...

Mild south-southwest flow will continue to bring warmer air and
increased moisture. Saturation climbs through the -10c to -15c layer
around sunrise Monday, so expect rain/showers to develop around that
time and spread over all areas of southern New England. Precipitable water values
will climb to between 1 inch and 1.25 inches Monday, then hover
around 1 inch Tuesday. Relative humidity fields in cross-sections show a deep
column of moisture over US for Monday and Tuesday. Current guidance
favors a break in the rain Tuesday morning, then another burst late
in the day and Tuesday night...but this timing may shift in future
model runs. Currently, rain totals expected in the 1-2 inch range,
possibly higher in spots.

Low level jet segments move up the coast Monday and Tuesday. One jet
on Monday brings 60-70 kt winds at 2000 feet, which would support 35
kt wind gusts at the surface along the coast. A second jet
Tuesday shows 40-45 kt winds at 2000 feet and above. This shows
the potential for strong gusty south-southwest winds each day.

The upper trough pushes a cold front through our area late Tuesday
and early Tuesday night. Winds will shift out of the west Tuesday
night. At the same time, a strong upper jet will move over New
England with srn New England in the right entrance region, where
lift will be promoted. This may generate a few showers Tuesday night
even after the front moves through, especially over Rhode Island and southeast mass.

Wednesday-Thursday-Friday...

Upper trough and associated cold pool move across New England on
Wednesday. Cold pool instability may generate some clouds and
scattered rain/snow showers during Wednesday, with best potential in
western and central Massachusetts and areas north of the Massachusetts border.

High pressure then builds in with dry weather Thursday and Friday.
Temperatures remain cold Thursday, but then turn milder as the high
center moves offshore and a light south flow develops.

&&

Aviation /18z Saturday through Thursday/...
forecaster confidence levels...

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

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

1815z update ...

Thru 00z ... VFR with patchy cigs bkn035-045 across western-
central Massachusetts into northern CT/Rhode Island at times. Modest west-northwest winds
continue.

After 00z ...

High pres crests over the region resulting in winds becoming
calm. VFR and dry weather prevails.

Sunday...VFR. Light SW winds slowly increasing to 10-15 kt later
in the day.

Sunday night ... VFR to start becoming lowering to MVFR in
scattered showers after midnight. Low level wind shear possible late.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Monday through Wednesday/...

Monday: MVFR/IFR conditions possible. Windy with areas of gusts
up to 30 kt. Rain showers likely, patchy br.

Monday night: MVFR/IFR conditions possible. Windy with gusts up
to 35 kt. Rain showers likely.

Tuesday: MVFR/IFR conditions possible. Windy with gusts up to
30 kt. Rain showers likely.

Tuesday night: mainly VFR, with areas MVFR possible. Breezy.
Rain showers likely, chance shsn.

Wednesday: mainly VFR, with local MVFR possible. Breezy. Chance
shsn.

&&

Marine...
short term /through Sunday/...high confidence.

Today...small craft advisories remain in effect this morning
for all coastal waters except Boston Harbor and Narragansett
Bay. Winds will gradually diminish this morning and midday, such
that all advisories should be discontinued by 1 PM this
afternoon, even on the outer waters. Seas on the outer waters
will be subsiding from 4 to 7 feet this morning to 3 to 4 feet
by late this afternoon.

Tonight...northwest winds diminish to 10 to 15 kt and seas subside to
1 to 3 feet.

Sunday...as high pressure moves off the coast, light winds in
the morning will turn to the SW and increase, with gusts to 20
to 25 kt by late in the afternoon.

Outlook /Sunday night through Wednesday/...

Sunday night: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain
showers.

Monday: low risk for gale force winds with gusts up to 35 kt.
Local rough seas. Chance of rain showers.

Monday night: moderate risk for gale force winds with gusts up
to 35 kt. Rough seas up to 14 ft. Rain showers.

Tuesday: low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 14 ft. Rain showers likely.

Tuesday night: moderate risk for Small Craft Advisory winds
with gusts up to 25 kt. Areas of rough seas. Rain showers
likely.

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

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...wtb/gaf
near term...wtb/nocera/gaf

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