Scientific Forecaster Discussion
fxus61 kbtv 240502
Area forecast discussion
National Weather Service Burlington Vermont
1202 am EST Fri Feb 24 2017
a weak cold front moving through the region this afternoon with
bring cooler temperatures back to the area to end the week
on Friday before abnormally warm temperatures ranging 15 to 25
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. Cooler
and drier weather returns for Sunday.
Near term /until 1 PM this afternoon/...
as of 937 PM EST Thursday...temperatures remain in the 50s as
of 10 PM in areas that still have cloud cover in place. The CT
River Valley has had some clearing and temperatures have dropped
into the 30s there. Still forecasting temperatures to drop into
the 30s areawide, possibly right around 40 here in the
Champlain Valley. Isolated light rain showers have ended. Still
thinking that winds will remain mixed enough to keep fog or br
from forming. Previous discussion follows.
What an unbelievable February afternoon with several high temp
records broken with many locations in the mid/upper 50s and
several in the lower 60s! As i'm writing this kbtv just broke
the all time February high temp record sitting at 63f!!! window
for warming is closing rapidly though as a weak cold front is
shifting through northern New York with some isolated rain
showers and increased cloud cover associated with it. Front
works its way through the area later this afternoon and early
evening, stalling out over southern Vermont tonight with gusty
southerly winds ahead of the boundary abating to nearly calm for
most areas overnight. Low levels remain fairly saturated
overnight and the combination of abating winds and warm temps
over snowpack normally would lead to fog developing, but my
confidence is low at this point based on mean mixed boundary
layer winds in the 5-15kt range. Will allow the evening shift to
assess that potential further.
On Friday, the aforementioned stalled front lifts back
northward as a warm front on the nose of a modest 30-40kt 850mb
jet. Overrunning precipitation develops from southwest to
northeast through the morning, and clearing the international
border by the late afternoon. Latest NAM based guidance shows
some minor instability along the boundary with slightly negative
Li/sw's so I can't rule out a few rumbles of thunder. A dry
Friday night is then expected in the warm sector ahead of a more
impactful system on Saturday.
Temps through the period remain mild for February standards
with lows tonight ranging through the 30s and Friday night in
the 40s. Fridays highs won't be quite as warm as today, but
still mid/upper 40s central/north and upper 40s to lower 50s
Short term /1 PM this afternoon through Saturday night/...
as of 345 PM EST Thursday...very complex and dynamic situation
on the way. 12z guidance not offering any significant variations
on the prior forecasts, so confidence is fairly high in the
overall scenario. Thought i'd break things down by components.
see separate section in the afd
looks like we will be solidly in the warm
sector, with 925mb temperatures perhaps pushing 10-12c. Crazy
for this time of year. Uncertainty in how much sun we'll see,
but even with more clouds than sun, all signs point to highs
well into the upper 50s to lower 60s. With snow gone in the
valleys, mid-60s is not out of the question. We may end up
breaking some all-time February temperature records that we
just set today. Again crazy. Did shade toward the warmer side of
guidance, but not the warmest.
Potent front will roll across the region during the day. Not out
of the question for a 20f degree drop in an hour. Tried my best
to show this huge change by folding in some of the hi-res data
from btv 4km WRF, but uncertainty in specific frontal timing
makes it difficult.
models all show instability that is surface based and aloft, at
least enough to suggest isolated thunder is possible.
However, strong jet, low level convergence and pressure rises
associated with the front strongly suggest a line of convection
will develop and accompany the front. Hi-res models show this
orientation very well. Expect we will need to focus on radar and
dust off some of our Summer related procedures. Thinking as
this line of primarily heavy rain showers moves through, we'll
have the potential for wind gusts 40-50 mph.
strong low/mid level southerly jet moves across the region. With
enhanced channeling up the Champlain Valley and to an extent the
St Lawrence Valley, frequent gusts in the 30-35 mph range are
expected. As noted above, with the convective line there is the
potential for localized stronger gusts. Behind the front,
westerly winds will remain on the gusty side into Saturday
believe it or not, in a span of a few hours we'll go from record
heat to potential for some snow, especially the higher terrain.
Tells you how strong this front is. At this point looks like
any snow late Saturday/Saturday night will be at the higher
elevations and be nothing more than 1-2".
Long term /Sunday through Thursday/...
as of 345 PM EST Thursday...honestly, much of my focus was on
Saturday into Sunday. Went with the model blend for the extended
portion of the forecast as it looked like the 12z suite was in
decent agreement with one another. Did note that mid-week it
looks like another fairly potent system will push across the
region. Could be a light wintry mix to rain situation per GFS,
unless the 12z Euro is correct. It's got 925mb temperatures
pushing 12-14c again, so if that pans out it's another warm/wet
system. The model blend I used, which also incorporates prior
00z runs indicated the cooler scenario to be more likely at this
point, so i've got the snow/rain mix type of forecast. However
don't put too much stock in that right now.
Aviation /05z Friday through Tuesday/...
through 06z Saturday...mainly VFR conditions are expected
overnight as a weak ridge of high pressure will be over the
region. A warm front will approach Vermont and northern New York
on Friday. Expecting light rain showers to develop ahead of
this warm front across the region between 15z-18z Friday, with
areas of MVFR ceilings and visibilities developing across
Vermont and northern New York by 18z Friday and continuing
through 06z Saturday.
06z Saturday - 15z Saturday...mainly VFR/MVFR. Light winds
becoming gusty from the south 12-15z Saturday.
15z 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
as of 330 PM EST Thursday...the Flood Watch remains in place and
effective from Friday 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 this morning, 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, this seems quite reasonable. Add to that a period of
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 Vermont 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 New York 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 expect forecast changes as we move closer
to the event.
Vermont...Flood Watch from this evening through Sunday evening for
New York...Flood Watch from this evening through Sunday evening for