Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 192058

Area Forecast Discussion
National Weather Service Burlington VT
358 PM EST Thu Jan 19 2017

A building ridge of high pressure both aloft and at the surface will
occur across our region...while plenty of low level moisture will
produce lots of clouds and patchy fog overnight. Given the depth of
moisture...expecting the higher summits to be above the low cloud
deck on Friday...with other areas of clearing also possible.
Temperatures will continue to be very mild with highs mainly in the
30s to near 40 and lows in the mid 20s to mid 30s. The next system
will impact our region late Monday into Tuesday with a wintry mix


As of 254 PM EST Thursday...Forecast challenge will be areal
coverage of low clouds/patchy fog...followed by impacts on
temps. Water vapor shows building ridge of high pres with
subsidence/dry air aloft...which has suppressed moisture enough
to support a few breaks across the dacks. Very difficult to
determine if dry air aloft is strong enough to erode low level
moisture/clouds and produce some clearing. If this was to
occur...fog/br would quickly develop as temps would drop below
cross over values...given current dewpoints in the upper 20s to
lower 30s. Have continued to show some clearing across the
summits tonight...with temps holding mainly in the mid 20s to
lower 30s most locations. Given boundary layer moisture and
light winds...combined with additional snow melt...some areas of
patchy fog/br are possible overnight...especially Saint
Lawrence Valley and protected valleys.

Friday...little change in overall pattern expected...except
soundings show shallower moisture. This will support more
clearing especially southern sections and summits. Expect clouds
to linger the longest over the Saint Lawrence and Champlain
Valleys on light winds will limit mixing of boundary
layer moisture. Temps will be tricky with progged 925mb to
850mb values around 0C...but mixing will be limited with high
pres directly overhead. Thinking mainly near freezing summits to
lower 40s warmer valleys which experience some sun on Friday
afternoon...if more sun develops than expected highs will be 3
to 5 degrees warmer across the board.


As of 330 PM EST Thursday...Relatively quiet weather continues
into the Friday night/Saturday time frame. An upper level ridge
will be building into the North Country which should block some
of the energy that will be heading into the Great Lakes. The
strong inversion that will be trapping low level moisture will
become eroded on on Friday evening and so low level clouds begin
to move out of the area but a persistent mid deck will keep the
area mostly cloudy through the day. There will be some light
warm air advection that pushes into the region and so it
wont take much to generate some light stratiform rain/snow
especially in the higher terrain. Temperatures will remain above
normal in the  continue very mild with lows Friday night from
the lower 30s and Saturday highs in the upper 30s to lower 40s.


As of 330 PM EST Thursday...The end of the weekend should remain
quiet before the weak upper level ridge breaks down. Then we
move into a more uncertain time period from Monday through
Wednesday. The one certainty is that we should be well above
normal for temperatures to start the week.

The most significant portion of the forecast is the potential
for a phased low pressure system that will impact us early in
the week. A southern stream upper level trough will dive south
of the mid-Atlantic region over the Carolinas while a surface
low develops over the DelMarVa peninsula. By Monday evening the
GFS and EC show the system phasing and increasing in intensity.
The trend in the guidance has been to bring the low close to the
coastline Monday Night into Tuesday sliding the surface low
inside of the benchmark. If that scenario plays out then we
would trend colder than previous runs and we will see a wintry
mix. I`m hesitant to go straight snow, however, because
1000-500mb thicknesses remain closer to 542-545 dam. So I chose
to use a thickness based approach for precip type for the event
leading to a mix of snow and sleet. The big take away is that
the trend was more from rain and freezing rain to more of a
wintry mix. This will be something continue to keep an eye on
moving forward.

I agree the previous forecaster in that there is some concern
based on the potential for gusty southeasterly downsloping
winds, especially along the western slopes of the Green
Mountains later Monday afternoon into Monday night as mean
925-850 mb flow may exceed 40 knots. This will eventually
predicate some adjustment of model qpf fields as valley/terrain
shadowing/enhancement effects will be magnified. Higher-res
guidance will certainly provide more detail as we draw closer in

Variable conditions are expected behind that system as with
synoptic flow stays southwesterly. This results in no
significant change in our airmass through the mid week. Expect
several shortwaves to pass through the area but at this point
there isn`t a significant amount of confidence in timing. The
idea of a daily threat of scattered rain/snow shower activity
seems the way to go so I continued that from the previous
forecast. Temps will continue in the mid 30s to lower 40s during
the day time with overnight lows averaging in the upper 20s to
lower 30s.


.AVIATION /21Z Thursday THROUGH Tuesday/...
Through 18Z Friday...Challenging aviation forecast with regards
to ifr potential this evening into the overnight hours...with
models struggling with boundary layer moisture profiles. Current
obs show a mix of ifr at slk to mvfr at rut/btv/mpv/mss and vfr
at pbg. Expecting mvfr cigs to prevail at most sites thru 00z
this evening...with occasional ifr vis/cigs continuing at slk.
Tonight...expecting additional ifr cigs with areas of
fog/freezing fog to develop associated with light winds and
plenty of boundary layer moisture. Difficult to determine areal
coverage of fog...but thinking most likely ifr/lifr conditions
will be at slk/mss with ifr possible at pbg/btv/rut/mpv.
Conditions will slowly improve to mvfr cigs by 16z Friday with
light winds.

Outlook 18Z Friday through Tuesday... Areal coverage and cigs
heights associated with plenty of low level moisture continues
to be the aviation focus for Friday into Saturday. Expecting
mainly mvfr from 18z Friday to 00z Saturday...with trends toward
ifr/lifr in low cigs and areas of fog/br. A few sprinkles or
flurries possible Saturday with general mvfr mountains to vfr
valleys. Next system spreads a wintry mix across our taf sites
late Monday into Tuesday with widespread mvfr to ifr conditions
likely...along with breezy southeast downslope winds possible at




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