Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
545 PM CST Thursday Dec 12 2013
Short term...(tonight through Friday night)
issued at 230 PM CST Thursday Dec 12 2013
Water vapor imagery continues to feature prominent/longwave trough/
northwest flow across the upper Mississippi River valley region
through the eastern Continental U.S.. meanwhile...a fairly vigorous middle-level
trough was passing over the Cascades/northern Montana rockies. This was
spreading middle-clouds as far east as the Dakotas into Minnesota.
For tonight...look for an increase in cloud cover through the night
with a chance of snow along and mainly northeast of I-94 after
midnight but mainly toward morning. This area will be closer to the
right entrance region of 300mb jet with the NAM showing pretty
decent 850-800mb frontogenesis focused around the 09-12z time
frame. Otherwise...look for overnight lows in the single digits
above zero with light winds generally 5 miles per hour or less.
On Friday...areas north of I-94 will still be in the vicinity of the
300mb right entrance region and weakening 800-850mb frontogenesis
for a continued chance of light snow. Meanwhile...the rest of the
area will see weak isentropic upglide during the afternoon in advance
of an approaching middle-level trough out of the northern plains. This
will generate clouds as well as a slight chance of light snow in the
afternoon. Plan on highs in the teens to lower 20s.
As the middle-level trough swings from the northern plains into the
upper Mississippi River region Friday night...look for scattered
snow showers to spread across the area with perhaps up to 1/2 inch
of accumulation in places. Overnight lows are expected to dip into
the upper single digits above zero to the middle teens.
Long term...(saturday through thursday)
issued at 230 PM CST Thursday Dec 12 2013
Plan on scattered snow shower activity to continue Saturday as that
mid-level/surface trough rotates through the area with steepening 0-
1km lapse rates increasing into the 8-10c/km range by afternoon.
With higher snow-to-liquid ratios in the 16-20:1 range...
accumulations through the day could be in the order of 1/2 to 1
inch. Lingering snow showers look to hang on through Saturday night
as cold cyclonic flow overtakes the area. Plan on lows to bottom out
in the single digits above and below zero with wind chills in the
minus 15 to minus 20 degree range.
Sunday looks dry and cold as a weak ridge of high pressure drifts
overhead. Highs Sunday are only expected to top off in the
Northwest flow aloft will bring another weak middle-level trough across
the region Sunday night through Monday night for a chance of snow.
It appears the best chances will focused north of I-90.
GFS/European model (ecmwf) show a ridge of high pressure building into the area
Tuesday through Wednesday night for a quiet/cold period. Look for
some moderation in highs Tuesday and Wednesday...topping off in the
upper teens to middle 20s.
Both the GFS/European model (ecmwf) continue to show some sort of cyclogenesis
affecting the area on Thursday. The latest 12z European model (ecmwf) now shows low
pressure tracking farther north across Minnesota into northern WI which
could bring a wintry mix to the region whereas the 12z GFS still
showing a slower/farther south solution which would keep the
precipitation in the form of snow.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 545 PM CST Thursday Dec 12 2013
A weak disturbance will move from northern Minnesota into
northern Wisconsin tonight...with the only noticeable impact to
krst/klse being a broken middle level clouds moving in. Winds then
become easterly after 12z Friday as a surface low pressure
develops across the Central Plains. This easterly fetch will help
to saturate the low levels /900 to 950 mb/...and a low-end MVFR
deck is expected to develop by middle day. For now...guidance agrees
with forecast thinking that bases will be between 1-2 kft.
However...there are some signals that krst could lower to
IFR...especially by middle afternoon. Did not include that in the
forecast at this time since confidence is below normal. Not
expecting any precipitation during this period...as the better
forcing /albeit still weak/ arrives after 00z Saturday.