Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
921 PM EST Friday Nov 28 2014
surface high pressure across southern Canada will build over the
north country tonight...resulting in cold temperatures and
clearing skies. Temperatures will drop to near zero in the colder
mountains valleys to upper teens in the Champlain Valley. A warm
front will increase the clouds on Saturday with light snow
arriving by afternoon. A dusting to 2 inches of snow accumulation
is possible with temperatures rebounding to above normal levels by
Near term /until 7 am Saturday morning/...
as of 921 PM EST Friday...forecast still in good shape for
remainder of the night with lingering northern mountain shsn/flurries slowly
coming to and end leaving variably cloudy/partly cloudy skies across
the area. Some natural variability starting to crop up in hourly
T/dew point observations...but a quick look at forecast overnight mins
still appear most reasonable out of available guidance so left
those values unchanged. Primary adjustment was to delay onset of
precipitation across the area tomorrow...keeping bulk of light snows/mixed
precipitation out of the area until after sunset. This is in better
agreement with nearly all model relative humidity/Omega forecasts and MOS output.
Still looking at a period of light mixed precipitation tomorrow
evening/overnight however so please refer to primary short term
discussion below describing dynamical forcing/reasoning et cetera. Have
a great night.
Prior near term discussion from 628 PM EST Friday...
forecast remains more or less on track as of early evening.
Adjusted sky cover and T/dew point data to match current trends. Snow
showers/flurries continue over the next few hours then wane/end.
Potl bust on temperatures overnight given fresh snow cover and
clearing skies...but warm thermal advection- driven middle clouds
arrive late so tough call. Values ranging from 5-15 above with a
few unlucky spots around zero still look reasonable at this time.
Short term /7 am Saturday morning through Sunday night/...
as of 340 PM EST Friday..a large swing in temperatures are expected from
a cold morning on Saturday to much above normal values by
Sunday...along with a period of light accumulating snowfall. The
combination of strong low level warm air advection/fgen forcing from 850 to
700mb...good ribbon of relative humidity >80%...and some 500 mb energy will produce a
band of snow across our County Warning Area on Sat afternoon into Sat evening. Snow
accumulations will range from a dusting to 2 inches with quantitative precipitation forecast <
0.10". Using the relative humidity fields and composite reflectivity proggs
places the snow band across the southern slv/western dacks between 18z-
21z Sat...into the northern dacks and cpv around 00z Sunday...and
impacting central/northern Vermont between 00z-06z Sunday. Soundings show
good relative humidity/Omega and favorable snow growth all intersecting each
other...supporting big flake size. Expecting only a 1 to 3 hour
window of precipitation late Sat after into Sat night associated with this
warm front feature. Prognosticated 850 mb temperatures remain between -8c and -10c on
Sat...but warm to near 0c by 12z Sunday. This low level warm air advection combined
with increased southwest winds will result in temperatures warming
overnight Sat into Sunday...with coldest readings in the deeper
valleys of central/eastern Vermont...where the cold air remains
trapped. Will mention highs mainly in the 20s to l30s on
Saturday...with lows ranging from the upper teens nek/eastern Vermont
to near 30f cpv/slv...with temperatures warming toward 12z Sunday. Winds will
increase sharply after 00z Sunday...especially mountain summits
associated with 925mb to 850mb jet of 40 to 50 knots...helping to
advect the warmer air into our County Warning Area.
For Sunday...thickness and low level temperatures increase quickly under board
SW follow...while best 850 to 500mb relative humidity is northeast of our County Warning Area by 12z.
However...soundings show plenty of moisture between surface and 850mb on
Sunday...along with weak upward vertical velocities fields...so thinking areas of drizzle
will be likely...with some freezing drizzle possible between 1800 feet
and 2500 feet through 18z Sunday...before entire thermal profile warms
above 0c. Also...high resolution model data shows potential for cold
air to get trapped in the deeper valleys of Vermont on Sunday
morning...resulting in areas of light freezing drizzle. Ice
accumulation will be minor...with no significant impacts anticipated
at this time. Temperatures by Sunday afternoon will warm into the 40s slv/cpv with
middle/upper level nek/mtns. A very warm night expected on Sunday
night...with temperatures holding steady in most locations with breezy
southwest winds....ahead of sharp surface boundary. This boundary will
increase the chances for mainly rain showers after 06z...along with
falling temperatures across northern New York toward 12z Monday. Precipitation amounts will be
light associated with surface cold front through 12z Monday.
Long term /Monday through Friday/...
as of 340 PM EST Friday...looks like a week of fast-changing
weather. 12z guidance suite in decent agreement up to Wednesday,
then the GFS and European model (ecmwf) go their own way. Have stuck with a blend
for Monday-Wednesday, and leaned a bit more toward the European model (ecmwf) after
that. Rambling thoughts for each day are below:
Monday: sharp cold front will quickly push east of the forecast
area by late morning (if not a bit earlier). Assuming we've got
the timing of the frontal passage correct, high temperatures will
occur between 6am-10am, and then fall. So even though the forecast
words may say "high of 40", the reality is that most of the day
will be in the 20s! Could be a brief period of gusty winds with
the frontal passage and just behind it as strong cold air
advection commences. Not a lot of moisture with the front, but
should be a few showers around. Initially rain showers, but will
turn quickly to snow showers -- and by early afternoon be focussed
more across the higher terrain. Perhaps a dusting in lower
elevations and an inch or so in the higher terrain. Very
progressive/fast flow pattern. High pressure quickly builds in
overnight and should result in clear skies. The high pressure does
have it's source in far northwest Canada and will be associated
with 925mb temperatures of -12 to -18c. This means temperatures
generally in the 0f to +15f range here at ground level. By the
way, the pressure in the center of that high will be about 1044mb
Tuesday: strong high pressure begins to push to our east, but
keeps its influence over US for most of the day. With 925mb
temperatures still in the -12 to -15c range, highs everywhere will
be sub-freezing. Perhaps just middle 20s for the "warmer" valleys.
This is about 10-12 degrees below normal. Southwest return flow
begins Tuesday night as the high moves east. In general, we'll
probably see a quick fall of temperatures early in the evening and
then steady/slow rise later at night. Timing of the return flow
this far out is quite difficult, so just went with model blend for
lows. So generally 10-20f, coldest in the Northeast Kingdom.
Rather high potential for temperature busts in this forecast.
Wednesday: low pressure moves quickly across the Great Lakes with
a strong low level southerly jet of 40-50kt developing. This
strong jet will bring much warmer air aloft -- and thus make for
issues with precipitation type. Liked the European model (ecmwf) depiction of the
temperatures aloft (925mb temperatures +1 to +3c by 18z), and
thus expect intial bout of light snow to turn into a brief period
of light sleet before turning into a bout of light rain. Can't
rule out some isolated light freezing rain in parts of eastern
Vermont where cold air remains stuck in various nooks and
crevices. Surface pressure pattern shown by both the European model (ecmwf) and
GFS indicate the liklihood of cold air damming down along the New
England coast. All of this transition should happen in 6-8 hours
time. Highs should range from the middle 30s to lower 40s in the
valleys. Could be a little breezy in the Champlain & St Lawrence
valleys with the strong low level jet, but precipitation should
stabilize the boundary layer and keep the strongest of the winds
from fully mixing to the surface. Though this will be a mix of
wintry precipitation event, it does look like overall
precipitation amounts will be less than 1/3", so not anticipating
anything major. The "warm up" will be short lived. The fast moving
low will pass by to our north Wednesday evening, dragging a cold
front through. Thus temperatures fall back below freezing. Any
remaining precipitation will end as snow.
Thursday/friday: as mentioned above, liked the European model (ecmwf) solution over
the GFS. In this scenario, another fairly strong (but only 1035mb
this time) high quickly builds in and pushes the cold front well
to the south. Leads to a dry Thursday, and most of Friday. However
with the high pushing east during the day Friday, our flow again
turns southerly and starts bringing up the moisture from the front
to the south by the afternoon. For that reason, I did put in some
low probability of precipitation (20-30%) and painted in mostly cloudy skies. To contrast,
the GFS never pushes that Wednesday cold front too far away. The
high it brings in for Thursday is to our north, with southwest
flow aloft overrunning the front resulting in clouds and light
precipitation (in the form of a winter mix) Thursday and Friday.
Looking at the various 12z GFS ensemble members, there was not any
consistency with the operational run, so what that all means is
low confidence in the forecast to end the week.
Aviation /02z Saturday through Wednesday/...
through 00z Sunday...VFR conditions expected through the period as
a ridge of high pressure will be over the region through the
period. Expecting warm advection to develop across the region
after 18z Saturday as surface winds turn southerly. Expecting
cloud cover to be on the increase after 18z Saturday in this
developing warm air advection pattern...but will still remain VFR
through the period.
Outlook 00z Sunday through Wednesday...
00z Sunday-00z Monday...light snow resulting in MVFR/IFR
condition Saturday night. MVFR continues Sunday with spotty light
mixed preciptiation or drizzle.
00z Monday - 00z Tuesday...scattered rain showers turning to
scattered snow showers, producing periods of MVFR conditions.
00z Tuesday - 12z Wednesday...primarily VFR.
12z Wednesday Onward...possible light mixed precipitation with