Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
634 PM EST Sat Dec 20 2014
a large area of high pressure will bring dry weather to the north
country tonight through Monday. Below normal temperatures are
expected on Sunday with a return to normal temperatures on Monday.
The pattern will change starting Tuesday as moisture increases
from the south and a storm system moving into the Ohio Valley will
ultimately bring widespread rain to the area on Wednesday along
with windy conditions...especially Christmas evening day and night.
Near term /until 7 am Sunday morning/...
as of 634 PM EST Saturday...rather quiet evening across our area
under weak high pressure...light winds and temperatures falling
through the 20s and teens. Forecast challenge tonight will once
again be degree of stratus cloud cover...and the extent to which
it advects northward around western periphery of departing high.
Return flow aloft is initially very weak so expecting only a slow
northward advance overnight. Also dealing with some stratus up
across the northern border counties where lingering pressure-
driven channeling of light northeasterly flow has allowed clouds
to back southwest into Highgate...Montreal...Massena et cetera. Expect
these clouds to essentially stay put or begin to slowly lift
northeast as the night GOES on...with lingering clear skies
central/north becoming caught in a squeeze play. 18z nam12 1 km
above ground level relative humidity appears to be capturing current trends the best so leaned
in this direction accordingly. Otherwise forecast remains in good
shape and no other changes needed as of early evening. Prior
discussion follows. Have a great evening.
Prior discussion from 342 PM EST Saturday...
high pressure remains over the area this afternoon with generally
weak low level flow. This should keep most of the clouds across
southern New England from returning into our area...but a few
clouds should make it into south central Vermont. Below normal
temperatures are expected tonight with generally clear
skies...light winds...and colder air mass over the region.
Short term /7 am Sunday morning through Monday night/...
as of 342 PM EST Saturday...quiet pattern continues right through
Monday night with high pressure generally remaining in control.
The flow gradually turns more to the south and
southeast...especially Monday and Monday night. We should still
see some sun on Sunday...but clouds will be on the increase Monday
and Monday night. High temperatures will generally be in the 20s
on Sunday...which will be a few degrees below normal. A slow
warming trend on Monday results in high temperatures right around
seasonal normals. Dry weather should generally prevail Sunday
through Monday night with more active weather expected toward
Long term /Tuesday through Saturday/...
as of 342 PM EST Saturday...guidance still points to a wet and
rainy Christmas evening day and night. As of the 12z guidance the
threat of high winds is becoming slightly weaker over the lower
elevations while the threat of flooding continues to remain.
Beginning Tuesday a ridge of high pressure will be continuing to
keep the area under cloudy skies as the models are hinting at a weak
temperature inversion with a shallow saturated layer of stratus
under the inversion.
As the low pressure system closes at 500mb on Wednesday morning
ridge will begin pushing offshore and out of the area. Rainfall
chances increase quickly from Wednesday morning through Wednesday
evening at the first significant bursts of energy from the system
move into the region. That will be driven by a strong low level
jet but the GFS and European model (ecmwf) continue to show slightly different
solutions as to how strong the jet is. The European model (ecmwf) brings at 50-60kt
jet in at 850mb whereas the GFS is about 10-15 kts slower. The
problem with that jet maximum is that its occurring near the same
time as the heaviest precipitation so the threat of some of those winds
mixing down does exist. Right now the biggest area of concern for
localized high winds will be over the higher terrain and over the
western slopes of The Greens and the Adirondacks as some
downsloping will be possible.
As far as quantitative precipitation forecast amounts for this system, we still expect total quantitative precipitation forecast to
be in the range of 1-2 inches. With temperatures on Wednesday expected
to reach the middle to upper 40s there will be a significant impact
on snowmelt. That snowmelt combined with the 1-2 inches could
cause some localized flooding in areas of poor drainage and in
small streams. Right now river guages are showing the rivers are
quite low so while some minor flooding will be possible we dont
expect at the moment significant river flooding issues. Christmas
day the dry air from the low will move in and the region should
get dry slotted allowing for the colder temperatures that will be coming
from the South West to move in. With the colder air moving in
there will be a slight chance for some snow showers late in the
day on Christmas and into the early evening.
Temperatures during the period are expected to remain above normal
through the period with no significant Arctic air expected through
Aviation /00z Sunday through Thursday/...
through 00z Monday...mix of flight categories (vfr/MVFR/local
IFR due to ceilings) through 03z and then becoming widespread MVFR
after that point.
Satellite fog product reveals an MVFR stratus deck working
northward from southern New England, while at the same time MVFR
to IFR stratus snaking its way southwest/south along the St.
Lawrence River. Weak surface high over northeast Quebec will help
advect IFR ceilings at mss and perhaps temporarily at btv (per 555
PM fso observation), with other sites mainly VFR through about
After 03z, expect widespread MVFR ceilings to develop at all
terminals as stratus layer over the middle- Atlantic/southern New
England lifts northward, occurring the latest at btv. Light bl
winds also suggests potential for some mist at times in the
stratus. Some question as to when these MVFR ceilings lift...but
thinking not until later in the day Sunday (around 17z mss and
between 20-00z rest of the tafs). Winds to stay light/variable
through the period.
Outlook 00z Monday through Wednesday...
00z Monday-00z Tuesday...some periods of MVFR ceilings with areas of low
clouds...otherwise mainly VFR conditions under high pressure.
00z Tuesday-12z Wednesday...periods of MVFR in light mixed precipitation.
12z Wednesday-00z Thursday...periods of MVFR/IFR in rain. Gusty winds possible.