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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
648 PM EST Wed Dec 11 2019

Synopsis...
a reinforcing cold front will slide in tonight bringing dry, gusty
and cold weather tonight. High pressure will bring cold conditions
with light winds on Thursday and Friday. Temperatures warm up
Saturday, but a developing low across the southeast U.S. Will shift
up the coast with the potential for periods of heavy rain on
Saturday, possibly beginning as a period of light snow across
interior Massachusetts late on Friday. Drier and cooler weather
returns Sunday into Monday. Another storm may impact the region
Monday night into Tuesday with snow or rain.

&&

Near term /until 6 am Thursday morning/...
645 PM update:

Still have weak ridging in place over much of southern New
England early this evening, with mainly light S/SW winds with
temperatures in the 20s to lower 30s. We continue to await the
cold front associated with a shot of much colder air and a wind
shift to northwest. This synoptic feature is now working its way
through eastern New York into the eastern slopes of the Adirondacks.
Temperatures should fall quickly now that the sun has set, at
least until the front clears our area late tonight into early
overnight. A dry frontal passage, but winds will likely increase
Post-frontal with gusts to 20-25 mph with cold advection into
the overnight. The winds may otherwise slow radiational cooling
somewhat but we still expect a steady fall in temperatures to
lows in the low teens to low 20s. Single- digit readings could
be possible in northwest Massachusetts with existing snowpack.

Because temperatures are expected to be in the teens to low 20s,
any wet pavement or slushy snow on the roads or sidewalks
should freeze, with untreated surfaces becoming slippery. Some
wet spots should evaporate given the dry advection and the winds
overnight, but expect there to still be some icy patches. A Special Weather Statement
for black ice was issued.

Previous discussion follows...

Reinforcing cold front sliding in tonight. This will advect -10 to
-15 degree celsius air at 850 hpa. This will usher in gusty winds
and cold air. Nudged temperatures down toward to the coldest
guidance due to the fresh snowpack across the interior and
relatively clear skies. Am somewhat uncertain how much the strong
winds will offset the radiational cooling, but there will be strong
cold air advection.

&&

Short term /6 am Thursday morning through Thursday night/...
Thursday...

Mid level ridge slides across the eastern Great Lakes and New
England. At the surface high pressure will build into the region
bringing mostly sunny skies. This will relax the pressure gradient
over the region and should bring an end to the gusty winds early on
Thursday. Could see a few gusts up to 30 to 35 mph tonight. It may
not be until mid to late in the morning that gusty winds subside
over the cape and islands. Northwest winds will gradually shift to a
southwesterly direction as the day progresses. Warmer air will begin
advecting in aloft as winds shift to a southwesterly direction at
850 hpa with temperatures warming to -5 to 0 degrees celsius. High
confidence in the forecast.

Thursday night into early Friday...

A negatively tilted shortwave trough will lift from the central
Great Lakes into southwesterly Quebec overnight. During this same
timeframe high pressure will shift off to the northeast into Nova
Scotia. The shortwave trough lifting off to the northwest of the
region will bring increasing cloudiness. Nudged temperatures toward
the 25th percentile of guidance to account for light winds and
relatively clear skies. If clouds take longer to enter then am not
cold enough, but if clouds move in quicker than currently forecast
then temperatures are too cold.

&&

Long term /Friday through Wednesday/...
highlights...

* temperatures moderate to near seasonal levels on Friday as next
weather system brings the potential for heavy rain & localized
flooding on Saturday. Flooding risks may be exacerbated by
snowmelt.

* Leftover rain/snow showers linger into early Sunday. Strong
northwest winds on Sunday, especially in coastal areas with gusts
up to 40 kts. Gale headlines might be needed for Sunday over the
waters.

* Drying out and seasonably cold on Monday before the next system
arrives on Tuesday, giving US another round of snow before mixing
with or changing to rain. Confidence is very low this far out.

Details...

Friday through Saturday night...

***heavy rain and flood potential on saturday***

By Friday morning, the 1040mb high moves east into the Canadian
Maritimes. This will allow a potent low pressure system to move
north. A southeasterly flow should see high temperatures moderate to
the upper 30s to low 40s in the interior and mid to upper 40s near
the coast. These values are near seasonal levels for mid December.
Friday looks to be mostly dry except for isolated showers for parts
of the interior late Friday afternoon. This is ahead of the main
system that arrives on Saturday. Enough shallow cold air could
linger in interior Massachusetts and CT for the precipitation to start off as
snow or mixed precipitation. Surface maps also have signatures of
cold air drainage from the northeast so there may be a few hours of
light snow or mixed precipitation. But as warmer air lifts north,
any precipitation should change over to plain rain showers by Friday
evening.

By Friday night, the 500mb long wave trough starts digging out of
the Midwest into the southeast states. This will set up a strong
southwest mid level wind flow allowing the deep layer moisture to
work into the northeast late Friday into early Saturday morning,
with pwats up to 2-3 Standard deviations above normal and pockets of
3-4 Standard deviations along the eastern Massachusetts coast. Looking at the
guidances from wpc and using model blend, it looks like we can
expect between 1 to 1.5 inches of quantitative precipitation forecast for western Massachusetts and 1.5 to 2
inches of quantitative precipitation forecast near the coast. There is still some uncertainty with
regards to the total qpf, because the European model (ecmwf) ensemble shows a 50%
chance of 24-hour quantitative precipitation forecast exceeding 1 inch. But with the precipitable water anomalies
being so anomalous and a 60 to 70 kt 850mb jet in the vicinity, I
would lean towards the system being an impressive rain maker. While
this is a relatively progressive system, heavy rainfall rates at
times on Saturday morning and afternoon could lead to localized
flooding. With the low level jet kicking in late Fri night,
overnight temperatures on Friday will exhibit a non-diurnal trend.
There would also be a significant uptick in dew points, with 40s in
western Massachusetts/CT and 50s elsewhere. High temperatures on Saturday look
to be in the mid to upper 40s across the interior and mid to upper
50s near the coast.

An issue of concern for Saturday is the flooding potential.
According to co-op observers' reports this morning, we have about a
6 inch snow depth with just under 2 inches snow water equivalent in
northwest Massachusetts. That gives US about 30% snow density, which indicates
a ripe snowpack. So the combination of snowmelt runoff and heavy
rain may result in localized urban and poor drainage flooding. We
will continue to monitor river levels for any rises.

Sunday...

** strong winds on Sunday **

Low pressure will shift northeast into Maine and the Maritimes by
Sunday morning, though there may be scattered showers on Sunday
morning. It could be cold enough in the east slopes of the
Berkshires and the northern Worcester Hills for snow showers but any
accumulations would be very light. The main concern on Sunday,
however, is the wind. In wake of the departing low, northwest winds
increase, possibly gusting up to 25 kts in the interior higher
elevations and up to 40 kts over the waters. This is supported by
BUFKIT soundings show a momentum Transfer of 34 kts at the bottom of
the boundary layer and 45 kts at the top of the boundary layer in
Boston early Sunday afternoon. As a result, we will likely need gale
headlines for Sunday.



Next Monday through Wednesday...

** potential for snow mixing with/changing to rain on Tuesday but
low confidence this far out**

Another cold front will sweep across the region by Monday morning,
bringing gusty northwest winds and colder air across the region.
Expect highs on Monday to be in the 30s and 40s. Another low
pressure system looks to impact the area by early Tuesday. With the
cold air in place, it should start off as snow before changing to
mixed precipitation or rain away from the interior. But since we are
still 6 days away, confidence is extremely low in any specific
solution. The precipitation comes to an end late Tuesday along with
a cold front passage. Wednesday looks dry but with below average
high temperatures in the 20s in the interior and 30s near the coast.

&&

Aviation /23z Wednesday 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/...high confidence.

00z taf update:

Tonight: VFR. A cold front will result in initial S/SW 4-6 kt
winds to become northwest and increase with gusts to 18-25 kts
starting around 03-06z.

Thursday: VFR under high pressure. Northwest winds around 7-10 kts with
gusts to 20 kts early, though will begin to subside into the
late-aftn hrs toward light/variable.

Thursday night: VFR for the first half of the evening, then
beginning to trend to VFR/MVFR late. Leading edge of a warm
front then encourages an increasing clouds from the S/se,
perhaps to MVFR levels early Friday morning towards CT/Rhode Island into
the South Coast/cape. Winds become east-southeast/se, with possible low level wind shear by
daybreak as swly low-level jet of 35-40 kts brushes areas near/west
of orh.

Kbos terminal...high confidence. Northwest gusts 20-25 kts developing
after 05z with dry frontal passage.

Kbdl terminal...high confidence.

Outlook /Friday through Monday/...

Friday: MVFR/IFR conditions possible. Breezy. Chance ra, patchy
br.

Friday night: mainly IFR, with areas MVFR possible. Breezy.
Chance ra, patchy br, chance freezing rain.

Saturday: mainly IFR, with areas MVFR possible. Windy with
local gusts up to 30 kt. Rain likely, patchy br.

Saturday night: mainly MVFR, with local IFR possible. Breezy.
Chance ra, patchy br.

Sunday: mainly VFR, with local MVFR possible. Windy with gusts
up to 35 kt. Slight chance rain.

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

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

&&

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

Tonight...a reinforcing cold front will slide through and bring
strong mixing over the relatively mild ocean. Westerly wind
gusts of 30 to gales of 35 knots will develop over the open
waters. Best opportunity for gales is across eastern
Massachusetts waters.

Thursday...wind speeds and gusts diminishing early as high
pressure builds in. Winds 10 to 20 knots during the morning with
gusts of 15 to 25 knots. During the afternoon speeds decrease
to 5 to 15 knots with gusts diminishing to 10 to 15 knots.

Thursday night...high pressure shifting to the northeast of the
region. Wind speeds of 5 to 10 knots. Speeds increasing after
midnight. Gusts increasing to 15 to 20 knots after midnight.

Outlook /Friday through Monday/...

Friday: winds less than 25 kt. Seas locally approaching 5 ft.
Patchy fog.

Friday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Chance of rain, patchy fog.
Local visibility 1 to 3 nm.

Saturday: moderate risk for Small Craft Advisory winds with
areas of gusts up to 30 kt. Rough seas up to 11 ft. Rain,
patchy fog. Areas of visibility 1 to 3 nm.

Saturday night: moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain,
patchy fog.

Sunday: moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft. Slight chance of rain.

Sunday night: moderate risk for gale force winds with gusts up
to 40 kt. Rough seas up to 13 ft.

Monday: moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 11 ft.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 10 am EST Thursday for anz231>234.
Small Craft Advisory until 4 am EST Thursday for anz230-236.
Small Craft Advisory until 11 am EST Thursday for anz235-237-
255-256.
Gale Warning from 11 PM this evening to 7 am EST Thursday for
anz250-251-254.

&&

$$
Synopsis...bl/chai
near term...bl/loconto

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