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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
643 PM EST Tue Jan 15 2019

Synopsis...
high pressure slides southeast tonight. A cold front sweeps
through late Wednesday, but conditions remain dry. One weather
system slips past southern New England Friday, bringing a period
of snow changing to rain Thursday night and Friday. All eyes are
on a second storm that brings a mix of wintry weather for
Saturday night and Sunday. Arctic air returns for early next
week.

&&

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

645 PM update...

High clouds moving offshore through the cape and islands. Low
clouds moving in from New York state into western mass and northwest
CT. These lower clouds were showing signs of diminishing as the
moved down the Western Hills at 6 PM, but satellite showed
another push of low clouds moving over the existing clouds in
central NY, so the sky cover in our western areas may linger a
little longer than expected tonight. We adjusted sky cover
forecasts a little, but trended toward the clearing of the
previous forecast later tonight.

Temperatures cooled faster this evening over the cape and
islands, and slower in the CT valley. We adjusted temperatures
closer to trends and lowered min temps in southeast mass, which will
need to be monitored tonight.

No other changes.

Previous discussion...

Band of cirrus associated with a very weak and sheared vort-Max is
currently shifting east across the region at this time, such that
it should be mainly offshore a couple hours after sunset.
Meanwhile modest upslope into the Berkshires with some lower
clouds only spilling as far east as the Berkshires. Trend toward
clearing expected except where west component maintains the lower
clouds into the overnight.

Ridge shifts southeast overnight. Noting light return flow developing.
Although light, with some clearing radiational cooling will be
somewhat limited as warm advection begins aloft. Still, expect
morning mins mainly in the upper teens to mid 20s. Blended
coldest of available guidance to start, but then lowered temps a
bit based on expected trends and radiational setup.

&&

Short term /6 am Wednesday morning through Wednesday night/...
Wednesday...
acute shortwave rotates quickly through the mean west-east flow
during the day on Wed. At the sfc, this is reflected as a
clipper. The passage should remain well to the N, across central
on and qc before moving into Labrador. Climatologically, this
puts S New England on the drier, S quadrants of the clipper.
Therefore expecting little fanfare with its movement to the N,
and the attendant cold frontal passage late in the
afternoon/evening. Some low-mid clouds expected, but with
breaks. Low risk for a shsn mainly in the Berkshires. Breeze
conditions out of the west-southwest ahead of the front, stronger still out
of the northwest in the cold air advection during the evening, gusts 25-30 kt at
times cannot be ruled out. Highs milder than previous days
thanks to the modest low-mid lvl warm advection. Mainly mid 30s
in colder northwest areas, with some low 40s possible in the coastal
plain.

Wednesday night...
cold advection following the evening cold fropa. Dry air filters
through the low-mid lvls leading to clearing. Noting that 1030mb
high pres rapidly fills in from the west by around 12z. Therefore,
could actually see some early morning radiational processes
following The Drop in temps thanks to cold advection. Early
morning mins therefore in the low teens to low 20s are likely as
a result. Winds drop off toward morning, but early overnight
wind chills could approach the upper single digits in spots
before diminishing

&&

Long term /Thursday through Tuesday/...
big picture...

Longwave scale continues to show ridge-west/trough-east through the
long-term period, suggesting warm west and cold east. Shortwave
scale shows seperate northern and southern streams Thursday-Friday
with a weak shortwave racing through New England Friday. The streams
then merge Saturday with a deep trough forming over the Mississippi
Valley and then swinging east, crossing New England Sunday night or
Monday. The GFS and European model (ecmwf) maintain cyclonic flow over the region
through Tuesday.

Contour fields are near normal and trending milder through Saturday,
then sharply colder early next week. Temperatures at 850 mb are
below normal Thursday, trend milder over the weekend, then trend
brutally colder Sunday night Onward.

Model fields are in general agreement Thursday through Saturday, and
clearly divergent Sunday night through Tuesday. Confidence is high
Thursday, moderate Friday to Sunday, low Monday-Tuesday.

Concerns...

Two periods of unsettled weather, Friday and Saturday night-Sunday.
Cold/dry or seasonably cold/dry the remainder of the period. Strong
northwest wind gusts possible Sunday night and Monday.

Friday...

Shallow northern stream shortwave in a split-stream flow suggests
only limited moisture amounts. GFS cross sections show deep
moisture, but precipitable water values only a little above normal.
Consensus mslp fields show a surface low tracking just south of our
South Coast, suggesting thermal fields will support mainly snow
north and snow changing to rain south. Uncertainty regarding the
storm track leads to uncertainty regarding amounts at this time.

Saturday night and Sunday...

Phased-stream flow brings higher moisture amounts to this system.
Cross sections again show deep moisture, but this time precipitable water values are
forecast above an inch which is well above normal in mid January.
Surface low moves off The Rockies Friday and up the Ohio Valley
Saturday night. GFS brings the surface low across northern mass into
northern New England, while the European model (ecmwf) brings it along or just off
the South Coast. The former favors rain, the latter favors snow.
High pressure over Quebec tries to function as a surrogate maritime
high with a suggestion of cold air damming over interior New
England. We will continue to expect an initial period of snow,
changing to rain or ice. Confidence is between low and moderate.

&&

Aviation /00z Wednesday through Sunday/...
forecaster confidence levels...

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

Short term /through Wednesday night/...high confidence.

Tonight... VFR most places along with light winds. Clouds in New York
state moving east with cigs at 2000 feet...some of these may
move into the CT valley early tonight. The question will be how
long it takes for the downslope flow off the Berkshires to thin
out the clouds...probably 06-08z.

Wed and Wed night...
cold frontal passage with little impact except a deck of 030-050
clouds across interior S New England during the day. Some MVFR
ceilings possible ack/hya/chh/pvc. SW winds with gust to about 25 kt
during the afternoon, then shifting back to the northwest during the
evening and overnight with gusts continuing.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Thursday through Sunday/...

Thursday: VFR. Breezy.

Thursday night: mainly VFR, with areas IFR possible late.
Chance snow.

Friday: mainly IFR, with areas MVFR possible. Breezy. Snow
likely, rain likely. East wind, turning from south in the
afternoon.

Friday night: VFR. Breezy.

Saturday: VFR.

Saturday night: mainly IFR. Windy with gusts to 35 kt. Snow
likely, rain likely, freezing rain likely.

Sunday: MVFR/IFR. Strong winds with local gusts to 40 kt. Rain
likely, freezing rain likely, pl likely.

&&

Marine...
forecaster confidence levels...

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

Short term /through Wednesday night/...

A brief lull in activity, except for 5 ft seas on the eastern
outer waters is expected tonight.

Wednesday and wednedsay night, winds will reach around 30 kt at
times first out of the west-southwest, then northwest
evening/overnight. Seas will build to 5-8 ft offshore as a
result.

Some light freezing spray is possible, especially over the
eastern outer waters tomorrow night.

Outlook /Thursday through Sunday/...

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

Thursday night: winds less than 25 kt. Chance of snow late.
Visibility 1 to 3 nm.

Friday: low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas locally approaching 5 ft. Rain likely, chance of
snow. Local visibility 1 to 3 nm.

Friday night: winds less than 25 kt. Areas of seas approaching
5 ft.

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

Saturday night: low risk for gale force winds with gusts up to
35 kt. Areas of rough seas. Rain likely, chance of snow. Areas
of visibility 1 to 3 nm.

Sunday: moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 15 ft. Rain likely, freezing rain
likely. Some light freezing spray is possible Sunday night.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 7 am Wednesday to 4 am EST Thursday
for anz231>235-237.
Small Craft Advisory until 7 am EST Thursday for anz250-254.
Small Craft Advisory until 7 PM EST Wednesday for anz251.
Small Craft Advisory from 7 am Wednesday to 7 am EST Thursday
for anz255-256.

&&

$$
Synopsis...wtb/doody
near term...wtb/doody

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