Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
820 PM CST Thursday Dec 5 2013
Short term...(tonight through Friday tonight)
issued at 200 PM CST Thursday Dec 5 2013
Invasion of Arctic air and biter cold wind chills are the main
concern in the short term. Likelihood of accumulating snow Sunday
into Sunday night are the main topics of concern through the
Arctic airmass is currently spreading into the region behind
departing deep low pressure over southern Ontario Canada. Under
mostly sunny skies...temperatures this afternoon were only in the 10
to 20 degree range. Brisk west in the fairly tight pressure gradient
around the low was producing wind chills in the single digits above
and below zero.
For tonight...Arctic high pressure/airmass will continue to build
into the region with west to northwest winds remaining around 8-12
miles per hour through the night. Under mostly clear skies...temperatures
bottoming out in the single digits above and below zero will combine
with these winds to produce bitter cold wind chills in the minus 10
to minus 25 degree range. A Wind Chill Advisory will be in effect
from midnight tonight through 9 am Friday morning across portions of
north central WI into southeast Minnesota.
Quiet but very cold conditions will be experienced Friday through
Friday night as the modified Arctic airmass moves overhead. Look for
highs Friday in the single digits to teens above zero with overnight
lows Friday night dropping back into the single digits above and
below zero. Will have to watch for the likelihood of another wind
chill headline late Friday night into Saturday morning.
Long term...(saturday through thursday)
issued at 200 PM CST Thursday Dec 5 2013
Likelihood of accumulating snow Sunday into Sunday night and
continuing unseasonably cold temperatures are the main topics of
concern through the long term.
Look for the Arctic airmass to push off to the east Saturday night
as a broad/deepening middle-level trough pushes into the plains from
the intermountain west. Broadscale isentropic lift depicted on the
GFS 270-275k surface headed into areas west of the Mississippi for
thickening cloud cover and a chance of snow after midnight Saturday
Things get a bit interesting for Sunday into Sunday night as the
trough continues to push east out of the plains and into the upper
Mississippi River valley region. Isentropic lift will increase into
the area during the day Sunday aided by the left ext region of the
middle-upper level jet. These dynamics combined with dendritic growth
zone increasing to nearly 200mb should produce fluffy 20:1 snow-to-
liquid ratio snowfall. Right now there is a strong indication of a
widespread 3-5 inch snowfall for the area. Luckily the surface winds
will not be that strong for widespread blowing of this fluffy snow.
This snow signal has been getting increasingly consistent from run
to run...so went ahead with categorical snow probabilities with the
need for an eventual headline looking probable at this time. A few
lingering flurries in the cold cyclonic flow expected on Monday with
the main trough axis shifting into eastern WI. Otherwise...look for
highs Sunday in the 12-22 degree range with lows Sunday night in the
4-12 range...with a 2-5 degree diurnal rise during the day Monday.
The rest of the period //Tuesday through Thursday// is marked by
cold northwest flow aloft. A weak embedded wave within this flow
comes through Monday night/Tuesday for a shot at some light snow.
Wednesday and Thursday look dry at this time. Otherwise...plan on
the colder than normal temperatures to continue with highs Tuesday
and Thursday in the single digits/lower teens above
zero...moderating a few degrees on Thursday into the teens to middle
20s. Overnight lows will remain in the single digits above/below zero.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 820 PM CST Thursday Dec 5 2013
Update...2 kft scattered-broken clouds have manifested across the taf sites.
Fog/stratus imagery shows some cloud streets...with some hints in
the 88-d radar imagery. Relative humidity fields and BUFKIT soundings point to some
low level moisture hanging around...under an inversion near 925 mb.
Have added in scattered-broken clouds for a few hour period this evening.
Relatively tight pressure gradient will persist tonight into
Friday...keeping the winds near or above 10 kts. Decent near surface
lapse rates should keep some gustiness at krst.
Otherwise...VFR conditions are anticipated into Friday night with
only a few high clouds here and there. High pressure builds across
the region on Saturday...continuing VFR conditions...but a low
pressure system will bring some -sn to the region Sunday/Sunday
night. A few inches of accumulation looks likely at this time.
WI...Wind Chill Advisory from midnight tonight to 9 am CST Friday for
Minnesota...Wind Chill Advisory from midnight tonight to 9 am CST Friday for