Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
733 PM EDT Wednesday Oct 1 2014
an upper low will pass south of the region overnight. A ridge of
high pressure at the surface and aloft will build into the north country
on Thursday and remain through Friday. A strong cold front will
approach the region on Saturday with rain likely Saturday and
Saturday night. Much cooler air will move into the north country
Near term /until 6 am Thursday morning/...
as of 733 PM EDT Wednesday...nearly stationary upper level low
pressure over southern New England will continue to dominate the
north country's weather tonight. We saw a few peaks of sun here in
the Champlain Valley right around sunset...but that was short
lived as cyclonic flow around the upper low is shifting additional
low level moisture westward through the btv County Warning Area at this time. Most
if not all of Vermont is cloudy now...and feeling is that these low
clouds will continue to progress westward overnight. In
addition...surface dewpoints remain fairly mild in the low/middle 50s
and with very little wind in the boundary layer we should see some
fog develop despite the cloud cover. Expect we'll see similar
conditions to last night...fog in some places...low stratus in
others. Lows should remain on the mild side of normal where more
cloud cover persists from the Champlain Valley eastward...
lower westward with more breaks in sky cover...40s. A few spits of
rain will also be possible across southern Vermont in closer
proximity to the low center.
Short term /6 am Thursday morning through Friday night/...
as of 411 PM EDT Wednesday...expecting morning clouds and fog
across the region early Thursday morning and then becoming partly
sunny by Thursday afternoon. A ridge of high pressure at the
surface and aloft will build into the region on Thursday and
remain through Friday. Expecting more patchy fog to develop over
north central and eastern Vermont after midnight Thursday night.
Southerly winds to develop across the region on Friday under sunny
skies...so expecting temperatures to climb into the lower 70s on
Friday. Surface pressure gradient to tighten Friday and Friday
night. Have leaned toward the warmer NAM MOS guidance for min
temperatures Friday night...as GFS MOS guidance looks too
cool...especially given southerly wind flow. Models have been
trending slower in bringing a strong cold front eastward from the
Great Lakes. Expecting most of Friday night to remain dry across
the region...with the best chance for any rain late Friday night
across the Saint Lawrence valley and Adirondacks in northern New
Long term /Saturday through Wednesday/...
as of 330 PM EDT Wednesday...finally a pattern change expected for
the weekend into early next week...as middle/upper level ridge breaks
down and trough develops. This will result in cooler temperatures...along
with chances for precipitation...especially on Saturday and again early next
week. A much more active weather pattern is anticipated with chances
for rain showers every couple of days...along with a slow trend
toward cooler temperatures.
Latest 12z GFS and 00z ensemble data in excellent agreement with
closed 500 mb/7h cyclonic circulation lifting across the central Great
Lakes on Saturday and into central Canada on Sunday.
Meanwhile...potent 500 mb vorticity will round developing trough axis and
create a negatively tilted middle/upper level trough across our
region...which will help advect deep Atlantic moisture into our
forecast area. Have noted GFS ensemble precipitable water values between 1.50 and 1.75" on
Saturday afternoon...which is 1 to 3 Standard above normal. This combined
with favorable 500 mb positive vorticity advection from vorticity and strong ribbon of uvv's will
result in a 6 to 10 hour window of rain across our County Warning Area on Saturday.
Thinking arrival time will be around 12z slv...and 18z dacks/western
cpv...and into eastern Vermont by 00z Sunday. Given the available
moisture/dynamics and slow progression of surface boundary...thinking
quantitative precipitation forecast will range between 0.25 and 0.75" with localized amounts near 1.0
likely. No Hydro issues are expected given recent dry spell.
Also...watching the potential for breezy/gusty south to southeast
winds on Sat afternoon. Latest 12z GFS continues to show 925mb winds of 45
to 55 knots...along with 850 mb jet of 60 to 65 knts. However...these
strong winds are closely associated with the heaviest rain
axis...which may limit mixing to the surface. Thinking gusts between 35
and 40 miles per hour will be possible the slv/dacks/western slopes...and parts
of the cpv on Sat. This will need to be watched closely as event
becomes better sampled by our local hi resolution models. Temperatures will
be tricky Sat with developing sharp west to east thermal gradient
across our forecast area...but thinking near 70f east to u50s/lower 60s dacks/slv.
Sunday-Monday...deep closed 500 mb circulation continues near Hudson
Bay...with a progressive west to southwest follow aloft across our County Warning Area.
This follow aloft...combined with several disturbances and ribbons of
middle level moisture...will create an unsettled weather pattern. In
addition...expect enhanced low level moisture from the Great
Lakes...which will impact the dacks with plenty of clouds and off
and on showers. Prognosticated 850 mb temperatures fall between 2-4c by
Sunday...supporting highs only in the u40s mountains to u50s/lower 60s
warmer valleys. Will mention highest chance probability of precipitation across the dacks into
parts of the central/northern Green Mountains during this time period.
Lows with the clouds and expected pressure gradient will range from the
u30s mountains to upper 40s/lower 50s near Lake Champlain.
Better chances for rain showers arrive on Tuesday associated with
another surface cold front and potent 500 mb energy...along with a ribbon
of deep layer moisture. Will increase probability of precipitation to high chance/low likely
for this system. Still some questions on timing...especially with
fast follow aloft. Temperatures with more clouds/precipitation will range from the
u40s mountains to u50s/lower 60s warmer valleys.
Aviation /00z Thursday through Monday/...
through 00z Friday...quasi-stationary upper low over southern New
England continues to be associated with relatively moist low-level
air mass across the north country with variably cloudy conditions.
General consensus of model solutions is for ceilings/visibilities to
gradually deteriorate once again during the nighttime hours...as
low-levels become saturated. Challenge is timing and areal extent
of MVFR/IFR conditions. Taf sites are VFR at 2330z...with the
exception of MVFR ceilings at rut. Anticipate ceilings reforming
with MVFR stratus overnight. Also...anticipate localized IFR
after 04z at slk/mpv with lower ceilings and potential for dense
fog. Winds will be light and variable overnight...except southeast around
10 kts at krut. During the day tomorrow...conditions gradually
improve back to VFR ceilings by late morning or early afternoon...with
skies becoming partly sunny during the middle-late afternoon. Winds will
Outlook 00z Friday through Sunday... fog/br possible Thursday night
into Friday...especially at slk/mpv. A strong cold front will produce
widespread showers on Saturday...with a rumble of thunder
possible. In addition...gusty south to southeast winds are likely
with areas of low level wind shear and turbulence expected on Saturday.
Scattered showers with MVFR conditions linger at slk/mpv on Sunday
with breezy southwest winds.