FXUS61 KBTV 222354
Area Forecast Discussion
National Weather Service Burlington VT
754 PM EDT Wed Mar 22 2023
Widespread light to moderate rain will push across the region
late tonight through Thursday in advance of a cold front. This
front will supply cool and dry weather for Friday with some
sunshine returning. Then as a stronger low pressure system
passes to our northwest, an elevationally dependent snowstorm
will unfold over the region.
.NEAR TERM /THROUGH THURSDAY NIGHT/...
As of 739 PM EDT Wednesday...No major changes were made with
primary adjustment to linger warmer temperatures longer until
wet-bulbing begins contributing more to cooling tonight. Current
radar returns are outlining areas of virga over the North
Country with 12+ dew point depressions drying rain before it can
reach the surface. Some light rain/snow may be falling on top of
White Face Mtn, but lower elevations remain above freezing.
Primary concern through the first half of the night continues to
be the potential for light freezing rain towards the Canadian
border where northerly flow has kept temperatures in the low/mid
30s. It won`t take much dynamic cooling related to wet-bulbing
to drop sfc temperatures below freezing with dew points around
20 degrees. A slug of rain is falling just north of Lake Ontario
and tracking eastward towards northern New York. In the next 3-6
hrs, this precipitation will be moving through with some slick
areas possible mainly for bridges and overpasses.
Previous Discussion...Stubborn northerly flow roughly below the
925 millibar level has kept modest cold air advection going in
our northern valleys while strong warming aloft has taken place
with synoptic scale southerly flow in advance of a series of
shortwave troughs to our west ahead of a cold frontal passage
tomorrow evening. The first one of note will pass to our north
this evening but skim northern areas with showers. Given the
marginally cold surface temperatures, have at least a moderate
confidence in spotty freezing rain in the typical coolest spots
near the international border from near Highgate to Massena.
Greatest likelihood of precipitation appears to be from around
10 PM to 3 AM, and have indicated up to 0.05" of ice accretion
supporting only a glaze of ice on colder surfaces. This
potential hazard is currently highlighted in the HWO but an SPS
may be needed if confidence increases tonight. Luckily, the
threat is relatively localized and timed fairly late at night,
mitigating potential impacts.
The surface high to our north will be retreating tonight which
will allow the colder spots to gradually warm by daybreak as
southerly flow deepens, as suggested by increasingly southerly
winds down to 925 millibars. A stronger frontal wave with more
abundant moisture will cross our area through the daytime hours
with widespread light to moderate rain, although briefly heavier
showers are possible with any elevated instability favored in
our southern areas. This rain along with dew points rising
above freezing should support increasing snow melt in the
valleys; see hydro section of AFD for details. Precipitation
should be fairly uniform during the morning before becoming more
terrain influenced in the afternoon as 850 millibar flow turns
more westerly. Tomorrow night, cold air advection and low level
northwesterly flow will support cooling aloft that should
change rain showers to light snow showers in the mountains.
.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
As of 327 PM EDT Wednesday...We`ll see a brief dry spell this period
as ridging quickly builds in between low pressure departing to our
east and our next system approaching from the west. Ample low level
moisture will be hanging around Friday, especially over the northern
Greens/Northeast Kingdom, where a few snow showers will be possible
until about mid day. Sunshine will increase thereafter, and expect
just about everyone will see at least a few peeks of sun by late
afternoon. Partly cloudy skies persist overnight, with some
increasing clouds late as low pressure starts to lift up through the
Great Lakes. Friday`s highs will top out in the mid 30s to around
40, but cooling off Friday night, especially if we can get more
clearing than currently anticipated. Lows could get into the lower
teens in the usual cold spots in the higher terrain, with mid 20s in
the wider valleys.
.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 327 PM EDT Wednesday...Active weather pattern setting up for
the weekend and potentially into the middle of next week. First
comes what looks like a messy mix of wintry precipitation Saturday
and Sunday. Overall thinking hasn`t changed overly much from
previous forecast package; a potent primary low will lift up through
the Midwest into the Great Lakes Saturday, and eventually to our
north by Sunday morning. Meanwhile, a weaker secondary low will
develop somewhere near the southern New England coast late Saturday,
trekking east to near Nova Scotia by Sunday. The question remains
temperature profiles and the resultant precipitation type.
Precipitation will start by midday Saturday, and given the amount of
dry air aloft, would expect things to start out as snow, perhaps a
rain/snow mix, but cooling of the column should allow things to turn
over to mainly snow, especially in the higher terrain and as
intensity increases due to strong frontogenetical forcing during the
afternooon/evening. However, strong south/southwest flow of 50+ kt
at 850mb will usher in warmer air, with a deep warming layer
developing late Saturday/Saturday night. Hence a period of mixed
precip is possible; exact timing/amounts/ptypes are difficult to
pinpoint at this time as temperature profiles will be very close
with just a degree or two one way or the other driving a wintry mix
or all snow, especially east of the Greens where warming will be
slowest. Have updated the weather grids to show this potential for
mix (rather than just rain/snow), but fully expect subsequent shifts
will make changes as exact details become more clear. Long story
short...stay tuned, as even the wider valleys could see a little
snow and/or wintry mix. Colder air starts to spread in from west to
east on Sunday, mainly aloft, as the secondary low takes over.
Therefore it should be more of a rain/snow scenario, with boundary-
layer temps/elevation driving ptype. Still, it won`t be all that
cold on Sunday, with highs reaching into the mid 30s to lower 40s.
Considerable uncertainty from Sunday night onward as model guidance
continues to differ from model to model and run to run. Latest GFS
and ECMWF continue to flip flop with the evolution of a digging upper
trough swinging in from the west and the potential for coastal low
development by midweek. Meanwhile, the CMC does not phase the system
and keeps any surface low well to our south, with just a weak upper
trough pushing through. This, too, will bear watching as we draw
closer to next week.
.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Through 00z Friday...All terminals are currently VFR, which will
persist through the early overnight hours. There is the
potential for some FZRA at MSS between 02-06z when the best
chance for light precipitation occurs with surface temperatures
below freezing. Rain will become widespread around 12z, with
MVFR conditions becoming widespread with some localized IFR
possible. Some LLWS is possible at RUT around 15z as directional
shear increases with southwest flow aloft.
Friday: Mainly VFR, with local MVFR possible. NO SIG WX.
Friday Night: VFR. Slight chance SN.
Saturday: Mainly VFR, with local MVFR possible. Definite RA,
Saturday Night: Mainly MVFR and IFR, with areas VFR possible.
Definite RA, Definite SN, Definite PL.
Sunday: Mainly MVFR, with local IFR possible. Chance SHRA, Likely
Sunday Night: Mainly MVFR, with areas VFR possible. Slight chance
SHRA, Slight chance SHSN.
Monday: Mainly MVFR, with areas VFR possible. Slight chance SHRA.
A low chance of river flooding will be monitored for the late
Thursday into Friday morning period in response to light to
moderate rainfall with basin averages of 0.25" to 0.5" and snow
melt up to 1". Rainfall will mainly be absorbed into the
existing snowpack at higher elevations where snowpack
temperatures will remain cold. As dew points rise through the
30s tomorrow, however, snow will ripen with continued melting
at lower elevations. Therefore, modest to sharp river rises may
occur by late Thursday into Friday in the watersheds with runoff
primarily due to snow melt.