Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1139 PM CST Wednesday Feb 10 2016
Short term...(tonight through Thursday night)
issued at 243 PM CST Wednesday Feb 10 2016
Main forecast concerns this period...cold temperatures...small -sn
chances southwest late Thursday night.
18z data analysis had a weak ridge of high pressure from northern Minnesota
to southern Illinois...with low pressure centered over neb/SD. Band of fn
convergence/isentropic lift in a tight baroclinic zone snow was
spreading southeast across southwest Minnesota through central into southeast
Iowa. Heaviest of this passing south of I-80 as of mid-day. This
forcing/lift and a shortwave dropping southeast across Minnesota producing
clouds over the southwest half of the region...with partly cloudy/
mostly sunny skies over much of the northeast half of the region.
Even with some sunshine...temperatures across the upper Midwest on the cold
side today with middle-day temperatures mostly in the 5 to 20 above range.
Model runs of 10.12z initialized well. Solutions very similar as
rather channelized shortwave energy slides southeast across the
region this afternoon/this evening...with the middle level jet axis
then shifting east of the area for later tonight/Thu. Tight
consensus for another channeled shortwave to slide southeast across
the Dakotas Thursday night while another wave drops toward northern Minnesota
and Lake Superior. Trend favors a compromise of the earlier runs on
these feature with the short-term forecast confidence good this cycle.
In the short term...with passage of the shortwave this afternoon/
evening renewed low level cold advection spreads across the area.
This as the next Canadian/Arctic high builds south into the upper
Midwest. Light/decoupled winds tonight near the surface ridge axis...
under what should be clear/mostly clear skies. 925mb temperatures in the
-15c to -18c range at 12z Thursday...for what promises to be a night of
widespread sub-zero lows. Naefs shows 850mb temperatures around 1 Standard
deviation below normal Thursday morning. Favored colder of guidance lows
tonight. Wind chills later tonight/Thursday morning only looking to be in
the -10 to -20 range...mainly due to the light/variable winds.
Little low level advections over the area Thursday with the surface-850mb
ridge axis near/over the area. With what should be plenty of
sunshine Thursday...model soundings showing mixing to about 925mb.
However 925mb temperatures Thursday afternoon only look to support highs teens.
Wave dropping southeast across the Dakotas Thursday night spreads a round
of lower level warm advection/isentropic lift across the area.
Stronger of the lift/greater of the moisture is west/south of the
forecast area Thursday night...in the tighter baroclinic zone near the track
of the shortwave. Appears a band of -sn will pass west/south of the
forecast area Thursday night...with the question of where will the northeast
edge of -sn band be. Much of the lift over the forecast area looks to
work to saturate the column and by the time this occurs the lift is
on its way out. Few of the models indicating column would saturate
enough for some -sn over the southwest end of the forecast area during
the late evening into overnight hours. Continued some 20-30 percent
-sn chances over about the southwest 1/4 of the forecast area for this.
Any -sn amounts Thursday night looking to be little more than a dusting.
Used a blend of the guidance highs/lows for Thursday/Thursday night.
Long term...(friday through wednesday)
issued at 243 PM CST Wednesday Feb 10 2016
For Friday through Saturday night...
Main forecast concerns this period...wind chills late Friday night/Sat
morning...returning -sn chances Sat night...below normal temperatures
through the period.
10.12z models in good agreement for the Dakotas shortwave energy
from Thursday night to quickly pass well South/West of the forecast area Friday
morning. This while the northern Minnesota/Lake Superior energy rotates
southeast across the western Great Lakes. Tightening consensus for
heights to start rising Friday night...behind the departing trough and a
ahead of a stronger shortwave headed into the northern plains by
late Sat. By Sat night the wave is prognosticated to be moving into the
upper Midwest...but timing differences develop among the models.
Forecast confidence remains good Friday through Sat then is average for Sat
Wave passing across the western Great Lakes Friday looks to do little
more than enhance the lake effect snow in the Lee of the lakes as it
passes. Drier air above 850mb and deep layered subsidence spreads
over the forecast area Friday west of/behind this wave...for a dry period
Friday/Friday night. This wave does open the door for one last large...
cold can/Arctic high to build south into/across the upper Midwest
Friday into Sat. Center of this high is over Minnesota Friday night...for what
would be northwest gradient winds 5-10 miles per hour across the forecast area Friday
night into Sat morning. This with 925mb temperatures in the -19c to -23c
range and naefs indicating 850mb temperatures 1 to 2 Standard deviations below
normal at 12z Sat. Friday night lows trending to be the coldest of the
past several nights...but the gradient winds look to keep the
boundary layer stirred a bit and lows from fully bottoming out. Did
favor colder of guidance lows Friday night and trended some lows toward
-20f in the lower laying areas along/northeast of I-94 where winds
are more prone to decoupling. The winds and lows in the -5f to -20f
range will send wind chills later Friday night into Sat morning into
the -20 to -30 range. A Wind Chill Advisory is looking necessary for
later Friday night through much of Sat morning...but plenty of time to
detail and issue that before Friday night. Sat to be a cold day with
the cold start and the surface-850mb ridge axis near/over the area for
little in the way of advections to change the airmass. The high is
quickly pushed east of the area Sat night...with increasing
southeast winds on its backside and increasing clouds ahead of the
next low/trough/shortwave into the northern plains. Lows Sat night
to be in the evening...with steady/slowly rising temperatures after
midnight...especially west of the MS river. By later Sat night...
increasing moisture and lower level Thermo-dynamic forcing already
prognosticated into the west side of the forecast area by the faster of the
models. Continued with some 20-40 percent -sn chances mainly along/
west of the MS river and mainly after midnight Sat night. Used a
blend of the guidance highs/lows Friday/Sat/Sat night.
For Sunday through Wednesday...days 4 to 7...
Main forecast concerns this period...snow chances sun into Sun night and
mainly snow chances again Tuesday/Tuesday night...warming temperatures.
Medium range model runs of 02.10z/02.12z offer improving agreement
on a stronger shortwave/trough to be moving into the upper Midwest
sun then across the region Sun night. Looser agreement for heights to
rise and weak shortwave ridging to build across the region Monday/Monday
night. Even lesser agreement on the details by Tuesday...but some signal
among GFS/European model (ecmwf) for another northwest flow shortwave to drop into
the northern plains Tuesday then across the upper Midwest Tuesday night...
with rising heights/ridging build in Wednesday. Forecast confidence is average
sun...then with the increasing detail differences below average by
Cold surface high to retreat east Sunday as the next stronger shortwave
trough brings low pressure and increasing surface-700mb moisture/warm
advection/isentropic lift eastward into the region. Depth of the
moisture column/precipitable water values looking suitable for the shortwave and
lower level Thermo-dynamic forcing to squeeze out some snow across
the area sun into sun evening. Consensus snow chances Sunday in the
50 percent range are likely a bit low but will leave them as is for
now as trend of recent model runs is toward less precipitation with this
wave/trough. Monday would be a between-systems day with passage of
the middle level trough axis. However models wanting to drop a weak
secondary shortwave into the trough...keeping cyclonic flow aloft
over the area on Monday. Lingering small -sn or flurry chances okay at
this point. Bigger thing by Monday is westerly lower level flow/warm
advection returning lower level temperatures toward normal. After a week in
the deep freeze...highs Monday trending to be near 30f. Next wave
through the flow /stronger in European model (ecmwf)/ would spread another round of
stronger moisture advection/Thermo-dynamic forcing/lift across the
area Tuesday/Tuesday night into Wednesday. Bigger time spread Tuesday-Wednesday with the
model detail differences. Depending on model...perhaps enough warm
air drawn northward for precipitation type issues Tuesday/Tuesday night...
especially across the south 1/2 of the forecast area. Precipitation chances in
the 20-30 percent range Tuesday-Wednesday reasonable for now. Lower level
warming would continue Tue/Wed...with temperatures trending near/above
normal. Model/ensemble consensus of highs/lows for sun-Wednesday appear
well trended at this time.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1139 PM CST Wednesday Feb 10 2016
VFR conditions are expected through Thursday afternoon...then MVFR
conditions are possible Thursday evening into early Friday
morning. Scattered to broken at times altostratus will persist across the
taf sites through at least 10z then we should start to see some
clearing working in...especially at klse. Plan on cloud bases
overnight in the 10-12kft range. Clouds will then again be on the
increase Thursday afternoon into Thursday evening as a weak upper
level disturbance grazes the area to the southwest...and a cold
front pushes in. We could see a brief period of flurries at krst
Thursday evening. Also...we may see some mfvr to possibly IFR
stratus along and just behind an Arctic cold front sliding in late
Thursday night into Friday.