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FXUS61 KBOX 202101
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
401 PM EST Tue Nov 20 2018

.SYNOPSIS...
Low pressure exits tonight followed by weak high pressure. An
arctic cold front may bring a few snow showers/squalls late
Wednesday into Wednesday evening, followed by record cold and
bitter cold wind chills Wednesday night into Thursday. Dry
weather expected Friday into Saturday afternoon with moderating
temperatures. Periods of unsettled weather appear to be in the
cards at times Saturday night through next Monday although
specific timing is uncertain. Odds favor precipitation mainly
being in the form of rain...but some ice/snow is possible at the
onset across the interior.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Light precip exiting eastern MA, but another area of light snow
moving across western/central MA. These snow showers are assocd
with the mid level trough/shortwave which swings through SNE by
early evening. As it does, there may be a few rain/snow showers
late afternoon into early evening.

Otherwise, weak shortwave ridging moves into the region tonight
behind the departing mid level shortwave and sfc low pres. This
will lead to partial clearing. Brief gusty NW winds this evening
across Cape/Islands will diminish overnight. Lows will range
through the 20s.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday...
Next robust northern stream shortwave will be diving into the
eastern Lakes and northern New Eng. The attending sfc low pres
tracks along the international border dragging an arctic front
across SNE late in the day and early evening. Best low level
forcing and steep 0-3km lapse rates 7-8 C/km are confined to
northern New Eng where best chance for a few heavier snow
showers/squalls. This activity should weaken as it approaches
SNE where environment is a bit less favorable, but a few of
these snow showers or perhaps a brief heavier squall may
survive the trip into SNE late Wed/Wed evening, especially
northern and western MA. The other concern for late Wed is a
period of brief strong wind gusts behind the arctic front.
There is a risk for brief 35-45 mph wind gusts right behind the
front late Wed, especially western New Eng and higher terrain.
Highs will range from the mid 30s higher terrain to lower 40s
coastal plain.

Wednesday night...

* Near record cold & bitter cold wind chills late Wed night
  into Thanksgiving morning *

An isolated snow shower possible early in the evening,
otherwise strong cold advection will deliver an arctic blast
with near record cold. Highly anomalous airmass with 850 mb
temps down to -20 to -22C at 12z Thu which is about as cold as
it gets for this time of year. Lows will drop into the single
numbers central/W MA with teens elsewhere. Gusty NW winds will
result in wind chills zero to 10 below across much of SNE by Thu
morning. The arctic air over the still mild waters will result
in extreme instability developing late Wed night with delta T
from SST to top of the mixed layer around 30C with ocean induced
Capes approaching 1000 J/kg! Given trajectory around 320
degrees, the bulk of the ocean effect snow should remain
offshore, but snow showers may clip the outer Cape late Wed
night.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights...

* Cold Thursday. Some snow possible on the outer arm of Cape
  Cod.

* Dry with moderating temps Fri and especially Saturday
  afternoon

* Unsettled at times Sat night-Mon with periods of rain favored
  but some ice/snow possible at the onset mainly across the
  interior

Overview and model preferences...
Arctic influenced vortex spins S from roughly the Hudson's Bay
region late week. Given its origins, continue to anticipate
anomalous could along with given this vortex has H5 heights
nearly 3 std deviations below normal and H85 temps as much as
20C below normal. The influence is felt not only in the cold
ambient temperatures but with deepening low shifting into the
Maritimes, blustery winds for Thanksgiving Day. Do continue to
note some breakdown in the synoptic pattern however toward the
weekend as the strongly positive PNA tends toward neutral
allowing weakening ridge to eject eastward. This will help the
airmass moderate in preparation for another Aleutian wave moving
E early next week. Whether or not this phases with the N stream
and to what extent will have to be watched as it is expected to
extract more Gulf of Mexico moisture than previous waves. In
spite of the anomalies, guidance shows fair agreement through
late week, so feel that continuing with a consensus blend of
guidance will still work. There is still a fair amount of spread
early next week such that will add more weight to the ensembles
to account for most solutions across the envelope.

Details...

Thu and Thu night...
The coldest stretch. Any lingering SHSN/squalls should
dissipate with the intrusion of dry air into the lower lvls
overnight. However, strong pres rise/fall couplet is noted
yielding blustery cold advection which will continue into Thu.
H85 temps drop to nearly -25C through Thu night. Even with fully
effective mixing this will limit highs Thu to the upper teens
and low 20s. Combine this with the 45kt LLJ, expect gusty winds
which will force wind chills into the single digits through much
of the day. Ambient mins in the single digits to low teens can
be expected both Wed night and Thu night, Wed night due to
strong cold advection, Thu night with the assistance of some
radiational cooling. As previous forecaster noted, will need to
watch particularly from HYA eastward on the Cape, nearly 30C
delta-T values suggest a good risk for ocean effect snows
particularly late day and overnight as the wind takes on a more
northerly component. Accumulations are likely given the setup.

Fri into Sat...
Gradual moderation of the airmass as high pres above 1030mb
crests across the region. This will be a gradual process, and
highs/lows on Fri are still expected to remain below normal
(peaking in the 30s by day, low 20s by night). Slightly warmer
Sat as return flow develops. Looking at highs back into the 40s
in advance of cloud shield from the SW.

Sun through Tue...
Complex scenario here given two shortwaves indicated by models.
The first slides E out of the Aleutians and shifts toward the S
CONUS, meanwhile a second weaker northern stream wave may slide
ESE from BC and the Northwest Territories. Whether or not, and
how, these two phase is at question, and given neither are well
sampled models continue their usual longwave struggle. In any
case a risk for more unsettled and wet wx is expected with
potentially two rounds of precip. Moisture content will be high
enough given the tapping of Gulf of Mexico moisture from the
first wave. Agree that the Sun/Sun night wave may be one to
watch given that near sfc temps may still be cold enough for
some mix of ice before a change to all rain. The second will be
dependent on low pres track, as an inland track would favor a
warmer solution, while a coastal track may introduce enough cold
air for winter to be in play. More on these as we approach.

&&

.AVIATION /21Z TUESDAY 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.

Through tonight...
IFR/MVFR conditions will improve to VFR this evening. Rain/snow
ends late afternoon in eastern New Eng. NW gusts to 20-25 kt
developing tonight over the outer Cape/Islands.

Wednesday...
Mainly VFR with bkn cigs developing in the afternoon. Localized
brief MVFR possible in isolated snow showers in northern and W
MA in the afternoon. W/SW wind gusts to 25 kt developing in the
afternoon. Brief 35-40 kt gusts possible late day in the
interior behind arctic front.

Wednesday night...
Mainly VFR, but areas of MVFR cigs developing over the outer
Cape late along with risk of snow showers. NW gusts to 25-35 kt
at times.

KBOS Terminal... High confidence in trends.

KBDL Terminal... High confidence in trends.

Outlook /Thursday through Sunday/...

Thanksgiving Day and Night: VFR. Windy with local gusts to 30
kt.

Friday and Friday Night: VFR.

Saturday: Mainly VFR.

Saturday Night and Sunday: Mainly MVFR, with local IFR
possible. Breezy. Chance RA, chance FZRA.

&&

.MARINE...
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.

NW gusts to 25 kt expected at times tonight. Winds will
temporarily diminish Wed morning while shifting to W/SW, but
will increase Wed afternoon and especially Wed night while
shifting back to W then NW behind arctic front. Gusts to 35-40
kt likely, peaking Wed night. Gale warnings will be issued.
Vsbys may be reduced in snow showers late Wed night across
waters east and NE of Cape Cod.

Outlook /Thursday through Sunday/...

Thanksgiving Day: High risk for gale force winds with gusts up
to 35 kt. Rough seas up to 11 ft. Slight chance of snow. Areas
of visibility 1 to 3 nm.

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Slight chance of
snow. Areas of visibility 1 to 3 nm.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night: Winds less than 25 kt.

Saturday: Winds less than 25 kt. Chance of rain showers.

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas. Chance of rain.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Chance of rain.

&&

.CLIMATE...
With the potential of the coldest air of the season moving into
the region around Thanksgiving, here are the record temperatures
for November 22nd and the holiday of Thanksgiving.

November 22nd
Location / Record Low / Record Minimum High

Boston..........9 (1879) / 24 (1880)
Hartford.......14 (1969) / 27 (1978)
Providence.....16 (1987) / 30 (2008)
Worcester......11 (1987) / 24 (2008)

Thanksgiving
Location / Record Low / Record Minimum High

Boston.........11 (Nov 27, 1873) / 24 (Nov 28, 1901)
Hartford.......12 (Nov 28, 2002) / 27 (Nov 23, 1989)
Providence.....14 (Nov 23, 1972) / 30 (Nov 28, 1996)
Worcester.......9 (Nov 23, 1989) / 22 (Nov 23, 1989)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 4 PM Wednesday to 7 PM EST Thursday for
     ANZ231>235-237-250-251-254>256.
     Gale Watch from Wednesday evening through Thursday evening for
     ANZ230-236.
     Small Craft Advisory from noon to 7 PM EST Wednesday for
     ANZ230-236.

&&

$$
SYNOPSIS...KJC/Doody
NEAR TERM...KJC
SHORT TERM...KJC
LONG TERM...Doody
AVIATION...KJC/Doody
MARINE...KJC/Doody
CLIMATE...staff