Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 250502
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1202 AM EST Sat Feb 25 2017

.SYNOPSIS...
Isolated to scattered rain showers associated with a warm front
lifting northward into the region this afternoon will dissipate
during the evening hours with a mainly dry night expected ahead of a
strong cold front which will move through the North Country on
Saturday. Very warm temperatures ahead of this front combined with
widespread rainfall will lead to increased snowmelt and runoff which
in turn will increase the potential for ice jams and river flooding
through the weekend. 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 1 PM THIS AFTERNOON/...
As of 942 PM EST Friday...Have again adjusted forecast
temperatures to reflect current observations. Still wide range
of temperatures across the area, mainly elevational. Warm front
is pushing into our forecast area from the south. An area of
showers with embedded thunderstorms passed Northwest of the St
Lawrence Valley. Lots of fog across the area and have continued
with mention of fog areawide, some areas dense. Minimum
temperatures for the overnight have likely already been achieved
and temps will rise through morning. Across the Saint Lawrence
valley and along the international border where northeast winds
will remain locked in will have the coldest temps, just a bit
warmer than freezing. Previous discussion follows.

Forecast is playing out well this afternoon with this morning`s
stalled front near the VT/NH/MA border is providing isolated to
scattered showers across central and northern portions of the
North Country as the boundary slowly moves back northward as a
warm front. Widely variable temperatures have been observed
across the forecast area today with mid/upper 30s in the
northern St. Lawrence Valley, to 50s in the Northeast Kingdom of
Vermont, to lower 70s in far southern Vermont in the north
Bennington area which broke out into the warm sector and some
sun. As the boundary continues to move north tonight, in general
precipitation will wane and surface temps will gradually rise
to the upper 40s and low 50s by sunrise tomorrow. Exception will
be the northern St. Lawrence and northern Champlain Valley`s
very shallow north-northeasterly flow will keep temps locked in
the upper 30s to lower 40s through much of the night.

Saturday and Saturday night continue to look like a very active and
potentially hazardous period of weather with continued record
warmth, gusty winds, and widespread moderate to heavy rain. The most
critical aspect of the next 36 hours continues to be the flood
potential due snow melt and ice breakup/jams with a FLOOD WATCH
remaining in effect from tonight through Sunday. Please refer to
hydrology section below for full details relating to flooding
potential.

Synoptic setup hasn`t changed a bit from previous thinking with a
deepening low pressure system near Georgian Bay at 12Z Saturday
lifting northeastward to the eastern tip of James Bay by 00z Sunday.
Strengthening low-level southerly flow east of the low and ahead of
a strong cold front will allow the aforementioned surface warm front
to lift north of the international border early Saturday morning,
with very warm 925mb temperatures of +10-12C moving into the region.
Already in the warm sector this afternoon we`re seeing sunshine
and temperatures well into the 60s and even low 70s so there`s
no doubt in my mind that the North Country will see some
filtered sunshine tomorrow and sufficient heating to produce
another round of record high temps area-wide in the low-mid 60s
(see our climate section below for the current records). Through
16-17Z or so expect not much in the way of precipitation in the
warm sector ahead of the approaching cold front, but it will be
rather windy with southerly winds of 15-25 mph with gusts 35-45
mph, especially in the northern Champlain islands due to valley
channeled flow.

Main action comes after 16-17Z where strong low-level convergence is
expected along cold front moving into the St. Lawrence Valley around
18Z, across the Champlain Valley from 21-00Z, and generally east of
VT by 03Z Sunday. Just about all of the available high resolution
models, including our locally run 2km and 4km WRF`s continue to
suggest a very narrow reflectivity fine line on the leading edge of
the front, so we`re looking for a brief period of moderate to heavy
precip, with possible embedded convective elements which will
enhance runoff and bring some main stem rivers to minor and possible
moderate flood stages late Saturday into Saturday night. Overall
rainfall amounts are expected in the 0.50 to 0.75" range, with
locally up to 1" with orographic enhancement across the northern
Adirondacks.

Behind the front for Saturday night, temperatures fall rapidly with
strong low-level cold air advection and wind shift developing where
precipitation lingering across portions of central/northern Vermont
will end as a period of rain/snow in the valleys and wet snow in the
higher elevations where several inches of accumulation are likely. A
dusting to perhaps and inch is possible at the valley floor. Lows
return to more seasonal values ranging through the 20s by early
Sunday morning.

&&

.SHORT TERM /1 PM THIS AFTERNOON THROUGH MONDAY NIGHT/...
As of 350 PM EST Friday...As the low pressure system exits
east, moisture will decrease across the North Country in west to
northwest flow. The pressure gradient force will continue to
produce gusty winds as lingering showers focus around the higher
terrain. Temperatures will be closer to normal Sunday, but
remain about 5 degrees above normal for this time of year. With
clouds increasing from the north, due to approaching shortwave
that may bring some light showers on Monday. Temperatures on
Monday will range from the mid 30s to mid 40s.

Monday night will be relatively quiet with temperatures falling
below freezing.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 350 PM EST Friday...Complex system or two follow each
other Tuesday and Wednesday. Differences continue to exist in
the model guidance. More significant trough possible Thursday,
but again models continue to differ, with GFS having the system
develop north and east of the forecast area and ECMWF having it
along the New England coast.

Overall expect above normal temperatures throughout the period
with several chances for precipitation.

&&

.AVIATION /05Z Saturday THROUGH Wednesday/...
Through 06Z Sunday...A warm front will lift slowly north
overnight. Surface observations showing quite a bit of fog and
low ceilings developing north of the warm front at this time, so
expecting widespread IFR/LIFR in fog and low ceilings
overnight. expecting ceilings and visibility to improve to
MVFR/VFR between 12Z-15Z Saturday, as warm front lifts north
into Canada. Surface pressure gradient to tighten across the
region during the day on Saturday, with southerly surface wind
gusts at or above 20 knots developing between 12Z- 15Z Saturday
and continuing at least through 22Z Saturday. Showers ahead of
an approaching strong cold front Will move into northern New
york between 18Z to 21Z Saturday, and then into Vermont between
21Z- 24Z Saturday. Guidance also indicating some isolated
thunderstorms are possible ahead of the cold front Saturday
afternoon.

Outlook 06Z Sunday through Wednesday...

06Z Sunday - 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 - 12Z Tuesday...Mainly VFR under high pressure.

12Z Tuesday - 00Z Thursday...VFR with chance MVFR/IFR snow
showers.

&&

.HYDROLOGY...
As of 600 pM EST Friday...The flood watch remains in place and
is effective through 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 afternoon, 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.

&&

.CLIMATE...
Several more record high temperatures are expected to be broken
tomorrow on February 25, 2017. Here are the current records for
our long standing sites:

Burlington, VT       55|1985
Montpelier, VT       59|1985
St. Johnsbury, VT    60|2016
Massena, NY          50|1956
Mount Mansfield, VT  49|1961

&&

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

&&

$$
SYNOPSIS...Lahiff
NEAR TERM...Lahiff/Neiles
SHORT TERM...KGM
LONG TERM...KGM
AVIATION...WGH/KGM
HYDROLOGY...Banacos
CLIMATE...Lahiff



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