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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
711 am EST Tue Dec 10 2019

Synopsis...
unseasonably mild temperatures will continue today. Another
round of showers develop late this morning and especially this
afternoon. A strong cold front crosses the region this evening,
followed by much colder temperatures. Showers will change to a
period of accumulating snow later tonight into Wednesday
morning, especially south of the Massachusetts Turnpike where a
significant impact may occur during the morning commute. Snow
will end by Wednesday afternoon. Large high pressure will bring
cold and dry conditions Thursday and Friday. Developing low
pressure across the southeast U.S. Will shift up the coast with
the potential for periods of heavy rain late Friday into
Saturday, possibly beginning as a period of light snow across
interior Massachusetts. Light rain may linger into early Sunday,
then drier and cool weather returns late Sunday into Monday.

&&

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

650 am update...

Cloudy skies and mainly dry conditions in place early this
morning. Noting a few bands of showers lingering off the S
coast, which should pass across Nantucket and Martha's Vineyard
through mid to late morning. Another area of light precip
across central New York may push across western mass by mid morning or
so as it shifts east-NE, while one more well organized area across
central and SW PA is also moving toward the region. This area
should reach western Massachusetts into north central CT by around midday and
will linger into the afternoon as it moves into central and
northeastern areas.

The day is starting off quite mild with temperatures in the
upper 40s across portions of the CT valley ranging to the mid to
upper 50s from near kbvy-khfd southward to the cape and islands.
With the mainly dry conditions through at least mid morning,
temps should rise a bit further before the next area of rain
arrives.

Previous discussion...

Showers will begin to increase in areal coverage again later
this morning and early afternoon across portions of western
Massachusetts/CT as the forcing for ascent increases ahead of an
approaching cold front. These showers should arrive in eastern
Massachusetts/Rhode Island by mid to late afternoon. Despite an abundance of clouds
today temps are already very mild. Given southwest flow and 850t
near +8c expect highs to reach well into the 50s across the
interior and lower 60s across much of the coastal plain. It is
possible the record high of 64 in Boston could be challenged
today. It will remain breezy today, given modest low level jet so expect
southwest wind gusts of 20 to 30 mph strongest of which will be
across eastern New England.

&&

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

* accumulating snow expected late tonight into Wed am for much
of the region, but a sharp cutoff is possible in northern Massachusetts

* significant impact to Wed am rush hour possible, especially
across parts of northern CT, Rhode Island and southeast Massachusetts

Tonight and Wednesday...

Quite the challenging forecast tonight into Wednesday morning.
This revolves around snow amounts and a potentially significant
impact to the Wed morning commute. Greatest concern is across
parts of CT, Rhode Island/southeast Massachusetts with a possible sharp cutoff in
appreciable snow in northern Massachusetts.

A cold front will cross the region this evening with
temperatures quickly falling through the 40s and then into the
30s. However, an approaching shortwave will keep our region in
southwest flow aloft into Wed morning. Very strong jet dynamics
are in place as southern New England will be in the right rear
quad of a 180 knot jet streak. This leads to a narrow, but very
strong area of mid level frontogenesis in the 600 to 700 mb
layer. Snowgrowth also looks favorable especially across
portions of CT/Rhode Island and southeast MA, where NAM indicates 20+ units of
Omega in the snowgrowth region for a short time.

The problem is that the northern extent of the mid level
frontogenesis and sufficiently deep moisture remains uncertain.
It will also be short-lived and offset by low level drying with
west-northwest winds just off the deck. This is what makes these anafrontal
waves so difficult to forecast even inside 12 to 24 hours.
There often can be a narrow area of moderate to heavy snow and a
sharp cutoff to the north.

Based on model consensus and overall pattern recognition,
thinking a general 1 to 4 inches of snow across the region.
Greatest risk for the higher amounts will be south of the Pike
across portions of CT/Rhode Island and southeast Massachusetts. If the dry air ends up being
a little stronger than expected, snow accumulations will be on
the order of a coating to 2 inches with little if any
accumulations north of the Massachusetts Turnpike. On the flip side of the
Coin, if forcing/deeper moisture ends up a bit further north may
see a swath of 4 to 6 inches of snow.

After collaboration with surrounding offices, opted to go with
a Winter Weather Advisory late tonight into Wed am for northern
CT, RI, as well as south central and southeast Massachusetts. The biggest
concern is the strong mid level frontogenesis coinciding with
the Wed am rush hour. While amounts may not reach advisory
criteria across the entire area, moderate to briefly heavy snow
may impact the morning commute and that can often be quite
problematic. Therefore, felt the Winter Weather Advisory was
warranted. Fortunately, this system is progressive with the snow
pretty much over by afternoon. High temperatures will remain in
the 30s, which will be a big change after today/S anomalously
mild temps.

&&

Long term /Wednesday night through Monday/...

Highlights...

* dry conditions and colder than normal temperatures Wednesday
night and Thursday

* temperatures moderate as next weather system brings the
potential for heavy rain & localized flooding late Friday into
Saturday

* leftover rain/snow showers linger into early Sunday as a cold
front approaches

Details...

Wednesday night through Thursday night...

Large 1040 hpa high pressure will shift east across the Ohio
Valley and across the northeast during this timeframe.
Temperatures will run around 10 degrees below normal as 850 mb
temps run from -9c to -15c from S to north into Thursday. Will see
dry conditions thanks to the strong subsidence as the high
passes.

It will feel even colder on Wed night as west-northwest winds at 10-15 kt
with gusts up to 25 to 30 kt across the higher terrain and
along the immediate coast. Wind chill values will range from the
single digits to mid teens Wed night, then in the teens to
lower 20s during Thursday though the winds will diminish during
the afternoon.

The center of the high will allow winds to become light and
variable or calm winds by Thu night, though will become light
east-southeast along the coast as the high shifts east after midnight. Lows
will mainly be in the teens, ranging to the 20s along the
immediate coast. Clouds will start to increase across the
interior after midnight.

Friday through Saturday night...

***heavy rain and flood potential***

A developing mid level wave across the southern tier states
will start to take shape across the southeast U.S. Thu/Thu night, will
see southeast winds beginning to bring low level moisture into the
region. Noting 500 mb long wave trough digging out of the Midwest to
the southeast states, which will set up a strong SW mid level wind
flow allowing the deep layer moisture to work into the northeast
late Friday through Saturday.

With the cold air in place at the onset of the precip, will
start off as light snow across most areas except along the
immediate S coast where temps will be around or above freezing.
As winds shift to S-SW, the milder air may lift aloft, so could
see a thin band of mixed precipitation. However, the colder air
will continue to shift southeast overnight, so should see any mixed
precip change over to snow.

Gefs model ensemble signaling a deep moisture plume working up
the eastern Seaboard as well. This plume will shift NE across
the region late Fri or Fri night, with pwats up to 2-3 South Dakota above
normal (about 1.2 to 1.5 inches). However, the 500 mb steering
pattern, while amplified, remains progressive so should see the
precip to shift NE.

The heaviest rain is expected from late Friday night through
midday Saturday, with current forecast totals from 1.2 to 1.6
inches regionwide. However, these amounts may change depending
upon the timing and track of the area of heavier rain may fall.

These rainfall amounts may combine with any snow to cause
clogged catch basins which may result in localized urban and
poor drainage flooding. This heavy rain, along with the rainfall
from today's event, may cause an increase in river flooding
generally during Saturday. Will continue to monitor river levels
for any rises.

Expect temps on Saturday to be mild, ranging from 45-50 across
the higher terrain to the mid 50s across the coastal plain.

Sunday and Monday...

Low pressure will shift NE into Maine and the Maritimes by
Sunday morning, though light rain may linger especially near and
north of the Mass Pike. Another cold front may wrap around the
departing low late Sunday or Sunday night. May see the rain mix
with or change to snow, but looks like the best chance may occur
across the east slopes of the Berkshires and possibly the northern
Worcester Hills during the afternoon before tapering off.

Another cold front will sweep across the region by Monday
morning, bringing gusty west-northwest winds and colder air across the
region. Current forecast suggests highs will run close to
seasonal levels.

&&

Aviation /12z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Wednesday/...

Today...moderate to high confidence. Mainly MVFR-IFR conditions
with some local LIFR mainly across portions of Cape Cod at 12z.
Dry conditions through mid morning, then will see cluster of
showers develop late morning through the afternoon ahead of an
approaching cold front. MVFR conditions will dominate today with
IFR-LIFR levels on the cape/islands will continue at times. SW
wind gusts up to 30-35 knots across far southeast New England
will diminish somewhat through 14z. Otherwise, SW wind gusts of
20 to 30 knots still expected into the afternoon with the
strongest of those across eastern New England.

Tonight and Wednesday...moderate confidence. Winds shift to the
northwest behind a strong cold front. Lingering showers should change
to snow across portions of the interior during the evening and
then eastern Massachusetts/Rhode Island between 05z and 10z. Once the showers change
to snow, MVFR conditions should lower to IFR levels especially
near and south of the Massachusetts Turnpike where the highest risk exists
for a period of moderate to even briefly heavy snow. This may
result in even LIFR conditions for a time. Snowfall amounts look
to be on the order of 1 to 4 inches with the higher of those
totals most likely south of the Massachusetts Turnpike. There also might be
a sharp cutoff in appreciable snow across northern Massachusetts.

Bulk of the snow should exit the region from northwest to southeast between
14z and 18z. Dry weather follows with conditions improving to
MVFR and then VFR.

Kbos terminal...moderate to high confidence in taf. One to 3
inches of snow likely between 06z and 15z Wed with an impact to
the morning push.

Kbdl terminal...moderate to high confidence in taf. One to 4
inches of snow likely between 04z and 14z Wed with an impact to
the morning push.

Outlook /Wednesday night through Saturday/...

Wednesday night through thursday: VFR. Windy with local gusts
up to 30 kt.

Thursday night: VFR.

Friday: MVFR/IFR conditions possible. Breezy. Chance ra, slight
chance snow.

Friday night: mainly IFR, with areas MVFR possible. Breezy. Rain.

Saturday: mainly MVFR, with areas IFR possible. Breezy. Rain
likely.

&&

Marine...

Short term /through Wednesday/...

Today...high confidence. Borderline gale force SW wind gusts
continue across most of the waters, but will diminish by around
14z or so as the core of the low level jet shifts east of the region.
Otherwise, still expect SW wind gusts of 20 to 30 knots so will
need to replace with Small Craft Advisory headlines shortly. Long southwest fetch
will continue to result in 7 to 13 foot seas across the outer
waters. Showers become more numerous again late today.

Tonight and Wednesday...high confidence. Winds shift to the northwest
this evening behind a strong cold front. Northwest 20 to 25 knot gusts
expected into Wed. Showers tonight will change to snow for a
time Wed morning before things wind down by mid afternoon.
Reduced vsbys expected. Small Craft Advisory headlines will likely be needed for
seas and marginal 25 knot wind gusts across the open waters.

Outlook /Wednesday night through Saturday/...

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

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

Thursday night: winds less than 25 kt.

Friday: winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain. Local visibility 1 to 3 nm.

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

Saturday: moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 13 ft. Rain likely.

&&

Climate...

Record highs for today (12/10). Best chance to tie or break
record would likely be at Boston.

Bos ... 64/1907
pvd ... 67/1946
bdl ... 67/1946
orh ... 63/1946

&&

Box watches/warnings/advisories...
CT...Winter Weather Advisory from 10 PM this evening to noon EST
Wednesday for ctz002>004.
Massachusetts...Winter Weather Advisory from 10 PM this evening to noon EST
Wednesday for maz012-013-015>022.
Rhode Island...Winter Weather Advisory from 10 PM this evening to noon EST
Wednesday for riz001>007.
Marine...Gale Warning until 9 am EST this morning for anz231>235-237-
250-251-254>256.
Small Craft Advisory until 5 PM EST this afternoon for anz230-
236.

&&

$$
Synopsis...Frank/evt
near term...Frank/evt
short term...Frank

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