Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 212337

Area Forecast Discussion
National Weather Service Burlington VT
737 PM EDT Fri Oct 21 2016

An upper level trough of low pressure is starting become
negatively tilted and will continue into early Saturday, keeping
frontal boundary and associated surface low pressure system over
the region through Saturday night/early Sunday. As the system
moves past, colder air will filter in bringing with it northwest
flow producing upslope rain and snow showers in higher elevations
from early Sunday morning through Tuesday. Another surface low
pressure system looks to affect the Northeast Thursday and into


As of 737 PM EDT Friday...Overall forecast in good shape with
just some minor tweaks to precipitation chances in the next few
hours. One area of rain is across Vermont with a break over
northeast New York and a much larger area of rain across the Saint
Lawrence Valley and points south. This is the area that will
eventually increase in areal coverage overnight and overspread our
entire area on Saturday. Have tweaked grids to fit this scenario.

Previous Discussion...
Saturday morning, models find more consensus on surface low
positioning along the southern New England coast, traversing
northward during the day. This means deep NW flow over the North
Country will bring in colder air and widespread terrain enhanced
rain showers throughout the day and into the early evening.


As of 312 PM EDT Friday...Extremely challenging and complex forecast
associated with low level thermal profiles and associated snow
levels. GFS/NAM/ECMWF and GEM are all in excellent with evolution
of negatively tilted and closing off 7h/5h circulation across
eastern NY by 21z Saturday and coastal low pressure being captured
and becoming vertically stacked over southern Canada by 06z
Sunday. This synoptic scale pattern will support a 12 to 18 hour
period of favorable deformation and upslope focused precip on
Saturday Night into midday Sunday.

The difficult question to answer is how cold are the thermal
profiles...and associated snow levels. NAM is much colder and faster
bring in the colder air...on brisk northwest winds...while GFS/ECMWF
are slower and much warmer in the boundary layer to 925mb layer.
This makes for a huge difference on impacts associated with snow
levels between 1200 and 1500 feet as supported by the NAM or 2000
feet or higher with limited impacts per the GFS/ECMWF solutions.
Interesting to note NAM BUFKIT data showing 1.20" qpf falling as
all snow...while GFS BUFKIT shows only 0.32". Given the lack of cold
air upstream and very warm ground temps from recent warmth...thinking
snow will initially have difficulties the mountains
towns on Saturday evening.

Have noted progged 850mb temp near 0C from KMSS to KBTV to KVSF at
00Z...while the 925mb 0C isotherm is still back northwest of our cwa
thru 06z Sunday...supporting snow levels above 2000 feet. Thinking
during heavier precip rates...especially on Saturday Night...column
will cool enough to support some wet accumulating snow in the
mountains towns of the Dacks and portions of the green mountains in
VT...mainly above 1500 feet. Have in the grids a dusting to several
inches of wet snow for Lake Placid...Tri Lakes Area...Newcomb...and
across the higher terrain of central Clinton and Franklin Counties.
Given the complex thermal profiles expecting a sharp snow gradient
as you climb in elevation with 8 inches or more possible above 4000
feet across the northern Dacks.

For the Green Mountains in Vt the colder air arrives first in the
central/southern portion...then advects north as closed cyclonic
circulations moves northeast. Thinking a similar type scenario plays
out with a dusting to several inches of wet snow likely between 1500
and 2000 feet...for towns like Killington...Bread
Loaf...Walden...Belvidere...and base area of local ski resorts.
Also...expecting a sharp snowfall gradient based on elevation with
Mansfield to Jay Peak having the potential of 6 or more inches by
Midday Sunday. A few wet snowflakes could mix in at times in the
valleys late Saturday Night...but no accumulation is expected.

Upslope parameters look very favorable with northwest 925mb to 850mb
winds of 35 to 45 knots...strong uvv`s signature across the
northern Dacks and western slopes...and plenty of 850 to 700mb
backside moisture. Thinking additional qpf values will range from
0.05 to 0.15 Saint Lawrence and Lower CT River Valley with near 0.50
eastern CPV/parts of the Northeast...and up to 1.0 northern Dacks
and parts of the central/northern Green Mountains from Mansfield to
Jay Peak.

Next concern will be developing winds associated with tightening
pressure gradient with 979mb low pres. Soundings show strong low
level wind fields of 40 to 50 knots...with bottom of the mixed layer
winds around 35 to 40 knots Sat Night into Sunday. Still some
questions how much mixing occurs with precip and clouds...but
thinking as drier air develops on Sunday...surface wind gusts
between 35 and 45 mph will be possible...with even higher gusts on
exposed mountain summits. For people adventuring out to experience
the first snow of the season in the mountains...please be prepared
for mid winter conditions on Sunday...especially near the summits.

Temps will range from the mid/upper 20s mountains to upper 30s/lower
40s warmer valley locations near Lake Champlain on Saturday night
and only warm 3 to 5 degrees on Sunday with brisk northwest winds.


As of 315 PM EDT Friday...After a prolonged warmer than normal period
through the summer and into the first 20 days of October...a
prolonged period of BELOW normal temperatures are in store for the
north country...Fall is here!

This weekends cut-off low will still strongly influence our
weather through Tuesday. The cut-off lifts into Quebec but the
cool, cyclonic and marginally unstable flow will persist through
Tuesday with the typical climo-favored upslope rain/snow showers
but less than the weekend.

Still cyclonic flow in trof pattern but lessening influence by
Wednesday with Surface high moving into region for dry day but
still cool. Another shortwave moves into the trof with a surface
low and warm front for Thu and possibly Fri which brings back more
inclement weather but not like this last system. Some mountain
rain/snow but timing later thursday should keep just rain for

Highs in the 40s through Thursday and back to seasonable L50s by


.AVIATION /00Z Saturday THROUGH Wednesday/...
Through 00z Sunday...Significant impact to aviation through the
TAF period mainly from OVC MVFR to LIFR ceilings and related
mountain obscuration. Ceilings likely to oscillate between these
three flight categories but will predominate as IFR. Rain showers
interspersed with a 1-3 SM mist continue to spread northward
affecting SLK, MSS, and MPV, with a break in steadier rainfall in
the Champlain Valley. These trends likely to continue through most
of the overnight. However, axis of steadiest rain axis then begins
to pivot from southwest to northeast during the day on Saturday,
resulting in a more widespread 4-6 SM rain across the TAFs.
Northwest winds 4-8 kts tonight, increasing in speeds to 8-12 kts
on Saturday.

Outlook 00z Sunday through Tuesday...

00z Sunday through 00z Monday: A prolonged period of unsettled
conditions is expected with widespread MVFR/IFR in periods of

00z Monday through 00z Wednesday: A mix of VFR/MVFR with upslope
showers in NW flow.


As of 315 PM EDT Friday...Through early this afternoon the heaviest
rainfall has been located over the Saint Lawrence Valley and
Northern Dacks (1 to 3.5 inches)...the amounts tapering off
sharply with < 0.75 inch across the central/southern Champlain
Valley and most of VT. Moving forward thinking additional rainfall
will range between 1 and 2 inches with some isolated higher
amounts along the western dacks possible. Several periods of rain
still anticipated for system tracks along the coast and
Atlantic moisture is advected back into the region tonight. Given
the long duration and breaks in the precip intensity/coverage...we
are not anticipating any hydro related issues associated with this
event. However...some minor urban and street flooding is possible
associated with the heavier rainfall rates through Saturday with
leaves clogging storm drains. Otherwise...some modest rises in
local rivers and streams are likely this weekend...but no
widespread flooding is anticipated.




NEAR TERM...Evenson/MV
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