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  Wednesday December 13, 2017

 

NWS Area Forecast Discussion


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000
FXUS61 KBTV 112013
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
313 PM EST Mon Dec 11 2017

.SYNOPSIS...
Weak high pressure will bridge across the area today with any
morning light snow showers ending and seasonably cold
temperatures expected. Low pressure will track across the region
on Tuesday and intensify to our northeast on Wednesday bringing
a widespread moderate to locally heavy snowfall to our region.
Behind this system, a cold airmass settles into the region into
Friday before temperatures moderate by next weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As of 204 PM EST Monday... The forecast through this afternoon
is playing out largely as expected with just some light flurries
across parts of southern Vermont. We`ve even got a few breaks in
the cloud cover with some sunshine peaking in here at the
office. Cold air advection remains in place with cold air
draining in across the valleys. I dont anticipate temps to warm
very much and so I`ve adjusted the temps down to max current
temps.

Prior discussion...
The forecast remains largely on track for today as a moderating
polar front drops through the area this morning. Scattered
light snows/snow showers will continue across mainly central and
northern counties in association with the boundary this
morning, though accumulations should be light and generally less
than an inch. Mainly dry weather is expected across the south.
High temperatures will average about 4 to 8 degrees colder than
yesterday across the north under light west to northwesterly
flow - mainly upper teens to mid 20s, though a few low 30s will
be possible far south where the front will arrive later in the
morning.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
As of 330 AM EST Monday...The most active period in the 7-day
forecast then takes shape later tonight into the
Tuesday/Wednesday time frame as a well-advertised clipper low
takes a favorable track across our area with a moderate to
locally heavy snowfall expected for our area. The low will then
deepen rapidly into Wednesday night as it moves to our north and
east. As discussed last night, the storm will have two phases,
the first occurring Tuesday morning and afternoon as favorable
warm advective processes interact with deeper moisture and
isentropic lift to produce a widespread moderate snowfall as the
low approaches. Southeasterly flow in the lower to mid-levels
should allow precipitation to take on a typical orographic
character with the heaviest totals along the eastern slopes of
the southern Greens where low-end Winter Storm Warnings have
been issued for a general 4- 8 inch snowfall. As a side note
I`ve leaned on the cooler side of guidance in regard to mid-
level warming in these areas and have leaned away from the
milder NAM output anticipating wet-bulb processes should be
enough to cool the column sufficiently to keep p- type all
frozen. Elsewhere, we`re mainly looking at advisory-level totals
ranging from 3-7 inches with the highest amounts along the
eastern slopes of the northern Greens/Dacks and in the St.
Lawrence Valley where where northeasterly channeling may enhance
QPF slightly. Lows tonight should generally range through the
teens with corresponding highs on Tuesday in the upper 20s to
lower 30s.

By Tuesday evening the low will track to our north and east dragging
the northern nose of a distinct dry-slot into central and southern
areas. During this period steadier snows should taper off in
intensity and lessen in overall coverage, especially across the
south where some patchy freezing drizzle may be possible. At this
point it appears there will be a saturated layer above the dry-slot
such that light seeder-feeder processes should keep any light pcpn
still mainly in the form of light snows/flurries.

As we progress into Wednesday phase two of the system affects our
area as deeper moisture wraps back into the region on increasingly
gusty northwest flow. During this period snows should reblossom in
earnest across the northern mountains with some backbuilding into
eastern portions of the northern Champlain Valley as flow trends
blocked over time. While additional accumulations should be light in
most spots, favorable upslope areas of the western slopes and
northern Greens stand to pick up several more inches where longer-
term Winter Storm Warnings will be in effect, both for the front-end
warm advective snows, and the back side upslope phase. Here two-day
totals will likely range in the 6-10 inch range with localized
heavier totals. With the pressure gradient tightening over time
areas of blowing snow will also be possible as northwesterly winds
gust into the 15 to 30 mph range. This will put quite the chill in
the air and with temperatures holding nearly steady from the mid
teens to lower 20s apparent T values will range through the positive
and negative single digits.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
As of 312 PM EST Monday...Large scale picture shows mid/upper prevailing
across the eastern Conus...while energy and moisture from the
Pacific NW ejects into the fast flow aloft every couple of days.
Latest guidance supports the northern tier staying on the cool
side of these systems with several mainly light snow events
possible. Below are my daily thoughts for days 4 thru 7.

Thursday...deep mid/upper level trof across eastern canada will
result in favorable upslope flow...but 1000 to 700mb rh is progged
to slowly dissipate by 18z Thurs. Based on llvl caa and upslope
flow...expecting remaining moisture to be squeezed out across the
northern dacks/greens. Have mentioned chc pops with only minor
accumulation expected. Temps aft a chilly start will only warm into
the single digits/lower teens mountains to mid/upper teens warmer
valleys with brisk northwest winds. Weak surface high pres builds
into our cwa on Thurs Night into Friday...but next system approaches
our western fa by Friday. Very difficult to determine cloud
cover/winds and impacts of fresh snow pack will have on temps Thurs
Night/Friday Morning. Have trended toward cooler side of guidance
envelope with -10f slk/nek to single digits to around 10f cpv. Some
lake effect clouds could impact temps near BTV.

Friday/Saturday...Both gfs/ecmwf show next weak clipper like system
and associated boundary crossing our cwa. This system will be weaker
and have less impact than current clipper. Still anticipating a
period of light snow activity...given good 5h energy... some enhanced
mid level moisture...and great lake moisture interaction with
approaching boundary. Several inches likely in the mountains with a
dusting to an inch possible in the valleys. Have continued to
mention high chc pops in the grids. Progged 850mb temps btwn
-14c and -16c...support highs mainly single digits summits to teens
mtn towns to l/m 20s valleys.

Sunday...weak mid/upper level ridge and associated 1025mb high pres
builds into the ne conus. Extremely difficult to time systems in
fast flow aloft...along with amount of clearing and impacts on
temps. Have lowered pops slightly for Sunday...but timing of next
system could arrive quicker based on fast flow aloft. Clouds and
winds will impact temps...but have kept temps close to superblend
with lows mainly in the teens and highs in the 20s.

Monday...Guidance supports another clipper like system moving thru
the Ohio Valley into the NE CONUS. Have noted lots of uncertainty
in ensemble data and between 00z/12z guidance in track of surface
low...with some to our northwest... while others show a system to our
south. Have continued on the cooler side of guidance...thinking
surface pres will keep system suppressed to our south...with another
light snow event possible. 12z ECMWF shows favorable mid level
moisture...good deep layer lift... and moderate llvl waa signature.
Plenty of time and uncertainty...so have mention chc pops for
now...with temps holding in the 20s. Overall pattern will support a
net increase in snow pack across the mountains with cool temps
keeping snowpack fresh.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Through 18Z Tuesday...Widespread ifr to lifr develops at all
sites by early Tuesday Morning...with significant impacts
anticipated to the aviation community. Meanwhile...VFR
conditions at mss/slk/pbg with intervals of mvfr cigs/vis thru
20z at rut/btv and mpv in light snow. Expecting mainly vfr
conditions from 20z- 04z this evening...before widespread snow
developed from southwest to northeast across our taf sites btwn
04z-08z. Vis will quickly drop to ifr with locally vifr in
bursts of heavier snow expected at all sites btwn 09z-15z
Tuesday. Cigs will vary from ifr at mss to mvfr at
rut/mpv/btv/slk/pbg...with periods of ifr possible. Localized
southeast winds gusts to 25 to 30 knots at Rutland on Tuesday
morning with breezy northeast winds at MSS. Vis/cigs will slowly
improve at rutland aft 15z. Some low level wind shear likely at
MSS/MPV with change of speed and direction...causing increased
shear and turbulence on Tuesday Morning.

Outlook...

Monday Night: Mainly VFR, with local MVFR possible. Chance SN.
Tuesday: Mainly MVFR, with local IFR possible. Definite SN.
Tuesday Night: Mainly MVFR, with areas IFR possible. Chance SHSN.
Wednesday: Mainly MVFR, with areas VFR possible. Chance SHSN.
Wednesday Night: Mainly VFR, with areas MVFR possible. Slight
chance SHSN.
Thursday: Mainly VFR, with local MVFR possible. NO SIG WX.
Thursday Night: Mainly VFR, with local MVFR possible. Slight
chance SHSN.
Friday: Mainly VFR, with local MVFR possible. Chance SHSN.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...Winter Weather Advisory from 1 AM Tuesday to 7 PM EST
     Wednesday for VTZ001-002-005-009-011.
     Winter Storm Warning from 1 AM Tuesday to 7 PM EST Wednesday
     for VTZ003-004-006>008-016>018.
     Winter Storm Warning from 1 AM Tuesday to 7 AM EST Wednesday
     for VTZ010-012-019.
NY...Winter Weather Advisory from 1 AM Tuesday to 7 PM EST
     Wednesday for NYZ027>031-034-035.
     Winter Storm Warning from 1 AM Tuesday to 7 PM EST Wednesday
     for NYZ026-087.

&&

$$

SYNOPSIS...Deal
NEAR TERM...Deal
SHORT TERM...JMG
LONG TERM...Taber
AVIATION...Taber


 
 
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