Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
533 am CST Sat Dec 27 2014
Short term...(today and tonight)
issued at 325 am CST Sat Dec 27 2014
Challenges include timing the exit of the snow today...sky cover/
temperatures through the seven-day period and low precipitation chances
late Sunday night through Monday and next Friday. 27.00z GFS/NAM/
European model (ecmwf)/Gem in good overall agreement with few initialization
errors...so will use a blended solution.
27.09z surface low centered across the Iowa/WI/IL borders will
rapidly move northeastward and into southeast Ontario by 28.00z.
925 hpa zero celsius isotherm bisects forecast area from
southwest to northeast early this morning and this matches well
with the radar 0.5 degree dual-pol products showing snow to the
northwest of a line from Charles City to La Crosse to Wisconsin
Rapids and a rain/snow mix to the southeast. All precipitation expected
to change over to snow later this morning as colder air oozes
south behind departing cold front. Widespread deformation
snow/rain will slide through the region the remainder of the
morning with the latest rap/hrrr drying out the forecast area
between 27.18 and 27.21z. When all is said and done...snow totals
will range from 2 to 4 inches along and northwest of a line from
Austin to Winona to Wausau. To the south and east...expect snow
amounts around 1 to 2 inches. Roads will continue to be slippery
through the morning hours.
Northwest winds will increase within cold air advection regime
given 20 kts to mix in the boundary layer. Broad subsidence
behind the departing wave should allow for some breaks in the
cloud cover late this afternoon and evening with mostly clear
skies by daybreak Sunday. Highs for the day will be met by middle to
late morning before slowing falling through the afternoon.
Nighttime lows will be dependent on cloud cover...but generally
expected to drop into the teens to lower 20s.
Long term...(sunday through friday)
issued at 325 am CST Sat Dec 27 2014
Early Sunday should see some sun before clouds increase again from
north to south ahead of the next cold front. Temperatures will top
out in the middle 20s to lower 30s with light and variable wind.
Light snow chances increase Sunday night through Monday along
middle-level cold front and the next shot of reinforcing Arctic air.
GFS/NAM/Gem more bullish with quantitative precipitation forecast compared to the dry European model (ecmwf)...so
will keep probability of precipitation in the 20 to 30 percent range for now. Even still...
only a few hundredths are possible...so any snow accumulation will
Temperatures bottom-out Tuesday morning through Wednesday morning
as 925 hpa values behind the secondary cold front plummet to
between 18 and 20 below celsius by 31.06z. Lows Tuesday/Wednesday
mornings likely to be in the single digits above/below zero. With
winds around 10 miles per hour...wind chill values as cold as 20 below not
out of the question. Highs Tuesday afternoon will struggle to
reach 10 above. However...partly to mostly sunny skies should
provide some welcome relief to the persistent cloud cover/fog of
the past few weeks.
Surface high pressure will dominate the regional weather pattern
for Wednesday/Thursday. Some moderation in temperatures is
expected with a warming thermal profile so that highs by New Years
Day will be in the middle teens to lower 20s. The next threat for
snowfall will come Friday with dueling northern/southern stream
troughs. Both GFS/European model (ecmwf) suggest two separate areas of precipitation that
may combine/interact somewhere across the upper Midwest...but too
much uncertainty at this time precludes mention of probability of precipitation higher
than 20 percent. Temperatures by weeks-end may rise back into the
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 533 am CST Sat Dec 27 2014
Snow and IFR conditions were already lifting northeast of the taf
sites early this morning. Some mainly MVFR visibilities and MVFR/IFR ceilings
will linger into middle morning...with the precipitation coming to an end by
15z. MVFR ceilings with VFR visibilities then expected for the late morning and
much of the afternoon as the low moves away. Tighter pressure
gradient on the back side of the low will produce brisk/gusty
northwest for the late morning/afternoon hours.
Colder/drier Canadian high pressure builds in tonight...with MVFR
clouds becoming scattered around 00z...then clearing out by middle evening.
Winds diminish quickly around 00z as well. Clear skies/sct200 then
expected overnight into Sunday morning. Temperatures Sat night/sun
will return to late December normals as the Canadian high builds
into the region.
WI...Winter Weather Advisory until noon CST today for wiz017-029-
Minnesota...Winter Weather Advisory until 9 am CST this morning for mnz079-
Iowa...Winter Weather Advisory until 9 am CST this morning for iaz008.