Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 150920

Area Forecast Discussion
National Weather Service La Crosse WI
320 AM CST Fri Dec 15 2017

.SHORT TERM...(Today and tonight)
Issued at 320 AM CST Fri Dec 15 2017

12.09Z water vapor loop shows a short-wave moving through western
WI, which has resulted in scattered flurries across much of the
region early this morning. Northwest cyclonic flow and sufficient
low-level moisture behind the wave will favor additional flurries
for much of the day, especially this morning for areas along and
west of the MS River. A strong northwest to southeast thermal
gradient will bisect the forecast area today, resulting in highs
ranging from the upper teens across north-central WI to the lower
30s across portions of northeast IA. Winds will generally be light
from the west.

For tonight into Saturday morning, increasing 850-700 hPa warm
air advection on the leading edge of ridging aloft, combined with
modest lift on the 285 to 295 K isentropic surfaces will result in
a band of light snow developing for areas along and east of the
MS River. Best chance for accumulations in the 1-2 inch range will
be north of I-94 with less than a 1/2 inch southwestward to the
MS River. Temperatures tonight will range from the single digits
across north-central WI to the lower 20s across MN/IA into
southwest WI.

.LONG TERM...(Saturday through Thursday)
Issued at 320 AM CST Fri Dec 15 2017

Some light snow will continue to accumulate Saturday morning
north of I-94 before gradually tapering off into the afternoon.
Forecast area remains within the low-mid level thermal gradient,
but with ridging aloft, some warmer air will start to make its way
into the area, especially south of I-90. Highs will climb into
the lower 20s across the north to the upper 30s across the south.

Flow aloft becomes quasi-zonal Sunday through Wednesday with the
thermal gradient remaining near the forecast area. Primary impact
to sensible weather will be near or above normal temperatures with
daily highs generally in the 30s, but cannot rule out some spots
across the south hitting 40 degrees. Dry conditions will be the
general rule for most areas through mid-week, although some light
precipitation is possible Sunday afternoon across northeast IA
into southern WI as a weakening short-wave ejects northeastward
from the Central Plains. Precipitation type will be dependent on
near surface temperatures, but right now, rain looks more probable
than snow. Could also see some light snow across the north Monday
night with another passing short-wave.

All eyes are on Thursday as both the GFS/ECMWF move a potent
Eastern Pacific trough into the western CONUS, allowing flow
aloft across the Upper Midwest to shift to the southwest. The GFS
is 6-12 hours faster than the ECMWF, but both models bring a
strong lee cyclone northeastward across IA and into southern WI.
With ample moisture, a deformation precipitation band moves
across the forecast area and at this time appears cold enough for
snow. A LOT can change over the course of the next 7 days, so will
need to monitor model trends to see if this idea holds or things


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1115 PM CST Thu Dec 14 2017

Cigs: probably the most challenging part of the forecast as
observations, upstream cigs, and models all suggesting periods of
mvfr and vfr (within around 1kft of each other) through Fri night.
Latest GOES16 Fog/Stratus satellite imagery has a pretty good handle
on where the back edge of the current mvfr cigs lie - and if
trends hold - should be through KRST before 06z and KLSE by 07z.
Then, it looks mostly VFR into Fri afternoon, with a drop back
into MVFR looking more likely for KLSE than KRST. That said,
confidence remains shaky on when/if the back and forth between
flight categories will occur at KRST/KLSE. Will lean on current
trends with model consensus to try and narrow down most likely
time periods.

WX/vsby: upper level shortwave sliding across the region tonight,
bringing areas of light snow/flurries but little if any vsby
reductions. Don`t expect any accumulations as a result. Will
continue a mention for tonight at both TAF sites. Another shortwave
trough and a good surge of low level warm air advection push in for
later friday afternoon/night. Models still in good agreement with
developing a narrow band of snow from central MN through central
WI, northeast of the TAF sites. That said, might be enough lift
for at least some flurries of light vfr snow farther south - and
will continue mention.

Winds: should hold westerly through Friday, a swing to more
southerly Friday night.




LONG TERM...Rogers
AVIATION.....Rieck is the U.S. government's official web portal to all federal, state and local government web resources and services.