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  Monday January 21, 2019

 

NWS Area Forecast Discussion


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FXUS61 KBTV 200808
AFDBTV

AREA FORECAST DISCUSSION
National Weather Service Burlington VT
308 AM EST Sun Jan 20 2019

.SYNOPSIS...
A dangerous winter storm is on tap for the North Country
tonight through Sunday as low pressure over the Ohio Valley
tracks to near New York City by Sunday morning, and into the
Gulf of Maine by Sunday evening. Steady snow will develop across
the region through this evening, and become heavy at times
overnight into Sunday morning. Snowfall rates of 1 to 2 inches
per hour are expected, making for hazardous travel. As low
pressure departs to our east across the Gulf of Maine and
Canadian Maritimes, blowing snow will increase throughout the
day Sunday as north winds increase. Frigid temperatures and very
low wind chills are also expected tonight through Monday night,
adding to the hazardous conditions. Temperatures will moderate
back into the teens on Tuesday, and into the low to mid 30s for
Wednesday of next week.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 913 PM EST Saturday...Overall, the forecast for the
upcoming winter storm remains on track. Many locations have seen
a lull or lighter snowfall over the past few hours but the
leading edge of the system is just beginning to enter our
forecast area. The heaviest snowfall is still expected to fall
after midnight through the early morning hours on Sunday. Latest
hi-res guidance continues to show an impressive mesoscale
banding feature developing in response to impressive 850 mb and
700 mb frontogenesis coupling. Wherever this band sets-up over
and pivots over will likely be the winner in terms of total snow
accumulations. All guidance is suggesting this will occur
somewhere over central Vermont between 08Z and 14Z.

Temperatures continue to remain tricky as the low levels
continue with a stout northerly wind which is advecting colder
air down from Canada. However, at higher elevations, southerly
winds are advecting warmer air into the region. Surface
temperatures will be slow to respond as scouring out this colder
air will take some time while higher elevations will likely
continue to see gradual warming through the overnight period.
These temperatures will likely influence our snow ratios as
temps at or below zero don`t typically support high snow to
liquid ratios. This could end up leading to slightly less snow
in some locations but be a higher density snowfall.

Previous Discussion...Have upgraded the St. Lawrence
Valley to a Winter Storm Warning, and Winter Storm Warnings
remain on track elsewhere.

Rather amazing snow event underway across the North Country,
even as low-level arctic continues to filter southward out of
sern Ontario and srn Quebec. Temperatures at BTV have been
slowly falling all day, from about 1F at 13Z to -4F at 2030Z, on
northerly winds around 10 mph. True arctic air mass with
dewpoints -10F to -20F across northern sections of the CWA. Also
seeing wind chill values of 10 to 20 below, making for
dangerous outdoor conditions with frostbite risk.

Even as this has occurred, we`ve seen 850-700mb frontogenesis
forcing in the frontal zone result in developing light to
moderate snowfall well in advance of the sfc low across the
lower Ohio Valley during the early afternoon hours. Snow
crystals are very fine - mainly needles - likely a combination
of being colder than favorable dendrite growth zone and
sublimation processes ongoing in the sub-cloud layer. We`ll be
"fighting" the replenishment of shallow arctic air throughout
this major snowfall, as north winds increase, especially Sunday
and with observed dewpoints in the -20F to -30F range around
Montreal and points north! As a result of the crystal type
(small columns and needles), visibilities will likely be very
low throughout the event, with 1/4SM to 1/2SM vsby common during
the next 24hrs due to snow and blowing snow across the forecast
area.

The 12Z NWP guidance suite indicated a northward shift in the
sfc low track and best 850-700mb frontogenesis forcing. This is
unusual given the ongoing low-level CAA (short wavelength
between systems), and thinking is that the above freezing layer
in the 850-750mb layer and potential p-type mixing is too
extreme in the 12Z NAM. The rest of the guidance is less NAM-y,
but could see a S/IP mix from 13-18Z Sunday across
Rutland/Windsor counties, and changed the forecast in s-central
VT with this package accordingly. Otherwise, looking at an all
snow event central VT and points north and west.

Overall QPF has also increased, especially across northern VT
and northern NY. As a result snowfall has been increased for
northern zones, and decreased slightly for Rutland/Windsor
counties as S/IP mix reduces frozen ratio. Have upgraded the St.
Lawrence Valley to a Winter Storm Warning with this package,
with 8-12" generally expected there. The remainder of the CWA is
in the 12-18" range, and good potential for 1-2" snowfall rates
9-15Z Sunday morning with best frontogenesis forcing as low-
level wind fields increase around deepening low pressure passing
to our south. Have lowered snow ratios down as low as 12:1, but
not expecting the snow to be clinging to trees and powerlines
given ambient surface temperatures. Thus, power outage threat is
low with this event.

For impacts, mainly looking at significant to dangerous travel
conditions CWA-wide late tonight through Sunday morning. This
will be because of 1-2"/hr snowfall rates and wind chills of 10
to 20 below, especially across northern sections. Blowing and
drifting will also increase on Sunday morning. Travel is
generally not advised due to the dangerous cold and poor road
conditions/low visibility overnight into Sunday morning.

Snowfall will gradually taper off Sunday afternoon into Sunday
night, but will see continued orographic snow, and even some
lake effect off Lake Champlain continuing. North winds 15-25
mph, with gusts to 35 mph, will maintain blowing and drifting
snow through Sunday night as low pressure moves into the
Canadian Maritimes. Frigid temperatures across the region Sunday
night, generally 5 to 15 below zero, with wind chills 20-35
below.

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 307 AM EST Sunday...Dangerously cold wind chill continue
Monday night with values between -20 to -40 below, with coldest
values across the northern Dacks into the central and northern
Greens.

Deep closed mid/upper level circulation and associated sfc low pres
will be lifting across eastern Canada while 1039mb high pres builds
into our cwa. This continues to support brisk northwest winds thru
06z at 10 to 15 knots with localized gusts. However, expect winds to
slowly weaken toward sunrise, as high pres builds into the cwa,
especially over northern NY and temps will fall quickly btwn 09z-
12z. Expecting lows btwn -5f and -25f with coldest values over the
northern dacks/parts of the SLV. Any lingering snow shower activity
will quickly end over the northern Greens by 03z, soundings show
limited moisture.

By Tuesday, low level waa develops as winds shift to the southwest
and sfc high pres moves into eastern New England. Tues will be dry
with temps warming back into the teens. The warming trend continues
overnight Tuesday Night with strong low level waa on breezy 850mb
winds of 30 to 40 knots. Progged 925mb temps warm btwn -4c and -10c
by 12z Weds with increasing clouds and snow showers developing by
sunrise. NEK will see temps fall quickly in the evening, before all
areas warm into the mid teens to mid 20s with likely pops entering
the slv by 12z Weds.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 307 AM EST Sunday...The combination of waa lift/moisture will
produce a period of mainly snow showers on Weds morning.
Initially thermal profiles support all snow with 925mb to 850mb
temps <0c thru 18z, before 925mb temps warm above 0c across the
slv/western dacks. Thinking as temps warm enough to support a
mix, best lift/moisture will be exiting our cwa, with just some
lingering light rain/snow showers possible. In addition, gusty
850mb southwest winds of 40 to 50 knots will create some
shadowing across the cpv on Weds, which will limit qpf/snowfall.
A general 1 to 3 inches likely with localized 4 inch amounts
for high peaks and northern Greens. Temps warming mainly 20s
east of the greens to mid 30s.

Uncertainty develops associated with potential wave along the
boundary on Weds Night into Thurs. In addition, plenty of
uncertainty with regards to low level thermal profiles and
associated ptype. Have kept things simple for now with high chc/low
likely pops and rain or snow showers. System sweeps off the east
coast overnight Thursday with much cooler air arriving for Friday
into the weekend. Some lingering mountain snow showers are possible
with temps falling back to below normal values.

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
Through 06Z Monday...IFR/LIFR vsby and IFR/MVFR cigs will
continue through the next 24 hours as a winter storm affects
the area with widespread moderate to locally heavy snow. The
heaviest snow with vsby 1/2SM or less will affect terminals in
the 08-16Z time frame in general with some sleet possibly mixing
in at KRUT from 12-00Z. Snow lightens in intensity beyond
16-18Z as storm starts to pull away but northerly winds gusting
>20kts will keep vsby IFR or below at many sites in BLSN through
the remainder of the period. Exception on winds will be KRUT
where strong east-southeasterlies will develop shortly and last
through the daylight hours. Some LLWS is possible at KRUT as
well during the overnight as a low level jet traverses southern
Vermont.

Outlook...

Martin Luther King Jr Day: Mainly VFR, with local MVFR possible.
Chance SHSN.
Monday Night: VFR. NO SIG WX.
Tuesday: VFR. NO SIG WX.
Tuesday Night: VFR. Chance SHSN.
Wednesday: Mainly MVFR and IFR, with areas VFR possible. Likely
SHSN, Likely SHRA.
Wednesday Night: Mainly MVFR, with local VFR possible. Chance
SHSN.
Thursday: Mainly MVFR, with local IFR possible. Likely SHSN.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...Winter Storm Warning until 4 PM EST this afternoon for
     VTZ001>012-016>019.
NY...Winter Storm Warning until 4 PM EST this afternoon for
     NYZ026>031-034-035-087.

&&

$$
SYNOPSIS...Banacos
NEAR TERM...Banacos/Clay
SHORT TERM...Taber
LONG TERM...Taber
AVIATION...Lahiff


 
 
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