Area Forecast Discussion
Issued by NWS Burlington, VT

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758
FXUS61 KBTV 241423
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
923 AM EST Fri Feb 24 2017

.SYNOPSIS...
A brief cool down today before abnormally warm temperatures
ranging 15 to 30 degrees above normal return for Saturday. A
stronger cold front moves through the North Country on Saturday
with the combination of very warm temperatures and widespread
rainfall around an inch leading to increased snowmelt and runoff
which in turn will increase the potential for ice jams and river
flooding. More seasonable weather returns Sunday with some light
snow in the higher elevations before a quieter period of weather
to start next week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
As of 923 AM EST Friday...Going forecast is in excellent shape
this morning with only some minor tweaks needed to sky cover and
hourly temps at this time. Regional radar shows some light
echoes working into the St. Lawrence Valley and should increase
in areal coverage across the rest of northern New York through
the morning and into central/northern Vermont from around the
noon hour onward. Previous forecast has all this covered very
well, so no changes there.

Previous Discussion...Friday sees cooler temperatures compared
to yesterday, but temps are still ranging 10-20 degrees above
normal. This will continue to lead to ice breakup in the rivers,
especially northeast New York and areas along and north of
Route 2 in Vermont where river ice still prevails. Adding to
this, a warm front associated with a low pressure system over
the Central US will push up into the region during the day
before lifting north of the border by early morning Saturday.
Models continue to show instability both surface based and aloft
which to lead to some thunderstorm activity to go along with
showers. QPF will generally be around two tenths to a quarter of
an inch, but some areas that see convection could see some
localized higher amounts. This extra water along with the
continued warm temps will lead to further rises for rivers
through Friday night, prepping things for the second round of
precipitation on Saturday.

High temperatures for today will range in the 40s up north to
upper 50s to 60 in the southern counties.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/...
As of 428 AM EST Friday...Very active and dynamic period of
weather expected with near record to record warmth, gusty south
winds, and potential river flooding concerns Sat/Sat Night.
We`ll actually see post-frontal precipitation end as snow
showers with some light accumulations expected across the higher
terrain Saturday night into Sunday morning. The most critical
aspect of the weekend weather though continues to be related to
potential flooding (please refer to hydrology section below for
full details relating to flooding potential). Flood watch
remains in effect 00Z Saturday thru 00Z Monday.

Deepening low pressure system near Georgian Bay at 12Z Saturday will
lift NNEWD into Ontario well to the west of the North Country.
Strengthening low-level southerly gradient flow east of the low will
allow surface warm front to lift north of the intl border early
Saturday morning, with very warm 850mb temperatures of +8 to +10C
moving into the region. Good low-level turbulent mixing should allow
low-clouds to scour out of the region, especially as 925mb winds
increase to 40-50kt. Should result in period of filtered sunshine
(thru high cloudiness) with sufficient heating to bring valley temps
to record levels in the low-mid 60s (record high for Saturday at BTV
is 56F). Even by 12Z Saturday, should see temps already into the
lower 50s across the Champlain and St. Lawrence Valleys as low-level
sly winds increase. Thru 18Z or so, looking at a partly sunny and
windy day, with south winds 15-25mph with gusts 35-45 mph, likely
strongest in the Champlain Valley during Saturday aftn with valley
channeled flow.

Strong low-level convergence expected along cold front approaching
from the west...reaching St. Lawrence county around 18Z Saturday,
and across the Champlain Valley by 22-00Z, and generally east of VT
by 03Z Sunday. High resolution models, including the BTV4kmWRF, and
NCEP 4km NAM suggest a reflectivity fine line associated with low
CAPE and strong dynamical/frontogenetic forcing right along the
bndry. With PW values near 1" along the front, looking for a brief
period of moderate to heavy precip, with possible embedded
convective elements. This rainfall will enhance runoff and bring
main threat of minor to locally moderate flooding late Saturday into
Saturday night. Overall rainfall amts generally 0.50 to 0.75", with
locally up to 1" with orographic enhancement across the nrn
Adirondacks.

Negative tilt mid-level trough and sfc bndry shift east of our
region after 03Z Sunday, with strong low-level CAA and a wly wind
shift following FROPA. Should see rain end as a period of wet snow
or snow showers, especially across the higher terrain during
Saturday night, with temperatures falling into the upr 20s to lower
30s by daybreak Sunday. Some anafrontal character to the frontal
zone per NWP time-height cross sections, and as such, should see 1-
3" snowfall across the higher summits late Saturday night as
vertical temperature profiles cool sufficiently to support snow as
the p-type. Even in valley locations, may see precipitation end as
isold/sct snow showers, but with little or no accumulation expected.

Will see sfc high pressure cresting into the nern CONUS by
Sunday afternoon. Prevailing W-NW low-level flow on Sunday will
result in much cooler conditions with afternoon highs generally
in the low-mid 30s, except close to 40F in the CT River Valley
with local downslope effects. Should see gradual clearing as the
day progresses on Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 428 AM EST Friday...A relatively fast and active WSWLY
mid-level flow regime will persist through the extended
forecast period, though the 00Z ECMWF and 00Z GFS differ on
timing and track of embedded shortwave troughs potentially
affecting the North Country. It appears that surface high
pressure will remain in control Sunday night through 18Z
Tuesday, providing a brief period of mainly dry weather
conditions across the North Country. Should see lows in the 20s
Sunday night, followed by highs in the low-mid 40s for Monday,
and lows again in the 20s Monday night. Developing low pressure
in SWLY 500mb flow takes shape across the lower OH river valley
and srn Great Lakes by 00Z Wednesday. Will see ewd extending
warm front bringing initial surge of isentropic ascent and
precipitation later Tuesday afternoon through Tuesday night
based on 00Z GFS. It appears primary low track is to our west,
with late development of a secondary low off the Maine coast
later Wednesday. Overall synoptic evolution favors initial mixed
wintry precipitation, changing to rain, and perhaps ending as
snow showers toward Wednesday evening into Thursday morning with
deep layer NW flow as secondary low departs toward the Gulf of
St. Lawrence. Given potential track/timing uncertainty, current
forecast calls for a rain/snow mix, though a period of
sleet/freezing rain is possible in advance of the warm front as
precipitation commences. Maintained idea of likely PoPs (60-70%)
Wednesday into the first half of Wednesday night. Overall,
temperatures during the long-term period will remain above
seasonal averages as we head into the beginning of March.

&&

.AVIATION /15Z Friday THROUGH Tuesday/...
Through 06Z Saturday...VFR conditions at most locations, with
KMSS/KSLK currently MVFR/IFR respectively. Conditions will
deteriorate to MVFR for all locations ahead of approaching warm
front after 15Z. Showers and thunderstorms are embedded in the
the front which could lead to a potential thunderstorm on
station through 00Z. Ceilings will continue to degrade down to
borderline IFR late in the period after 07Z.

Winds will start light and variable this morning before turning
southerly at 05-10 knots after 00Z. KMSS Will be the exception
with northeasterly flow at 05-10 knots locked in the entire
period. KPBG/KSLK/KMPV will also see wind shear conditions this
evening from 210 at 35-40 knots at 2000 feet.


12Z Saturday - 12Z Sunday...MVFR/IFR in rain with embedded
heavy rain and possible thunder. Very strong southerly winds
with localized areas of shear and turbulence likely.

12Z Sunday - 00Z Tuesday...Mainly VFR under high pressure.

00Z Tuesday - 00Z Wednesday...VFR with chance MVFR/IFR snow
showers.

&&

.HYDROLOGY...
As of 330 AM EST Friday...The flood watch remains in place and
is effective from this evening all the way to Sunday evening.
Still looking like a very warm stretch producing quite a bit of
snow melt combined with some rain on top for good measure will
be enough to produce significant rises on nearly all rivers and
streams across the region. As of Thursday, there was still a
decent snowpack across the Adirondacks and most of Eastern VT,
with depths averaging 12-30 inches. Latest guidance suggests
we`ll melt enough snow to the equivalent of 3-4" of rainfall by
Saturday. Given the all-time February temperature records set
Thursday and perhaps again Saturday along with fairly high
dewpoints (approaching 50F), this seems quite reasonable. Add to
that a period of warm frontal precipitation Friday aftn, and
the moderate/heavy rain showers on Saturday that could produce
another 1/2 to 1" of liquid, we are basically dealing with the
equivalent of a 3-5" rainstorm. That`s a lot of liquid for our
rivers to handle. And the ground is also frozen, so not much
will soak in. Complicating matters is that some rivers,
especially across northern/northeast VT and in the Adirondacks
are still ice covered. Thus as the water levels rise and the ice
weakens, we`ll start to see ice movement. Which could very well
then get stuck in bends in the river or along bridges or where
the rivers go from a steeper to more gentle slope. By their
nature, ice jams are impossible to predict but can result in
localized flooding occurring quickly. In general, it takes the
river to rise 1.5 to 3 times the thickness of the ice to start
the breakup process. Based upon NERFC forecasts as well as
ensemble river simulations we continue to look at the Ausable @
Ausable Forks, Winooski @ Essex Jct., and the Mad River @
Moretown for the primary focus of more significant river
flooding. Best river response will be late Saturday through
Saturday night due to runoff from snowmelt and expected
rainfall. Thickest river ice is in place across nern NY basins
and nrn VT, including the Chazy, Ausable, Winooski, Lamoille,
Missisquoi, and Passumpsic. Uncertainty in the river forecasts
are due to unknown exactly how much snow melt we are getting and
how much rainfall we will see on Saturday. Those uncertainties
are pretty large, so forecast changes are possible as we move
closer to the main event on Saturday.

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...Flood Watch from 7 PM EST this evening through Sunday evening
     for VTZ001>012-016>019.
NY...Flood Watch from 7 PM EST this evening through Sunday evening
     for NYZ026>031-034-035-087.

&&

$$
SYNOPSIS...MV
NEAR TERM...Lahiff/MV
SHORT TERM...Banacos
LONG TERM...Banacos
AVIATION...MV
HYDROLOGY...Banacos



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