Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 240532
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1132 PM CST Mon Jan 23 2017

.SHORT TERM...(Tonight through Thursday)
Issued at 310 PM CST Mon Jan 23 2017

High Impact Weather Potential: Accumulating snowfall expected late
Tuesday into Wednesday, with amounts of 6 inches or better
increasingly likely for many areas. This will be a heavy, wet,
greasy snow with significant travel issues expected. Winter Storm
Watches have been upgraded for many areas west of the Mississippi
River, with watches still in effect to the east given some
remaining uncertainty and later start time for precipitation.

January thaw is over - back to our regularly scheduled winter
weather. Main focus obviously is on the later Tuesday-Wednesday
window with quite the strong shortwave energy coming ashore into
southern California right now emerging on the Central Plains and
redeveloping a closed mid level wave. That feature is progged to
track just south of the region per a multi-model consensus, with
decreasing model spread as of the 12Z runs finally helping to
greatly increase confidence. Expected track of the system places the
axis of strongest frontogenetic forcing directly across the region,
with quite a nice tap of moisture also being worked north into the
area as PWAT values climb toward 0.6 inches. All in all, this one
looks like a messy system for the local area, with warning criteria
snow increasingly likely.

Looking at the specifics, not much going on tonight, save for more
residual lower stratus (what`s new?). Don`t envision any more
drizzle overnight given transient shortwave ridging working overhead
and lack of any shallow lift (unlike this past morning), but will
monitor. Increasing mid level difluence quickly arrives Tuesday
afternoon and evening ahead of the upper wave, with a narrow but
rather robust axis of 850-500mb frontogenetical forcing overtaking
the area. Have no doubt we are in for a period of moderate to
briefly heavy precip as that unfolds, with just enough cold air
aloft in the presence of strong dynamical forcing to deliver mainly
snow for southeast MN and western WI. The biggest question is areas
farther south with some hints of a warm nose and/or warmer boundary
layer for a time, especially through Tuesday evening. In addition,
will really need to watch the potential for thunder perhaps as far
north of I-90 with a pronounced axis of negative EPV/reduced
stability overtopping a nicely sloped FGEN axis, and even hints of
possibly upright convection above a stable boundary layer into
northeast IA/southwest WI with the intrusion of impressive negative
theta-e lapse rates. In fact, NAM/GFS most unstable CAPE profiles
suggest around 150 J/kg working south/east of La Crosse during
Tuesday evening. Not every day you see that in January.

All told, this system no doubt still has some tricks up its sleeve,
though the overall trend toward higher precip amounts looks really
good given recent trends, available moisture, and the strength of
the system. Exact placement of the heaviest precip still remains
just a bit in question, but given the forcing and potential for
convective-induced heavier rates at times, a band of 4-8 inches of
snow (possibly more?) looks in line either side of the I-90
corridor, with slightly lower amounts to the south where mixing
issues may come into play. With that said, just a slight shift south
in the expected storm track could easily result in higher amounts
into much of northeast IA and lower ones to the north or vice
versa.

Precip rates will diminish later Tuesday night into Wednesday, but
do expect we`ll keep some lighter accumulating snow around into
Wednesday evening as the upper wave and associated deeper cyclonic
flow hold steady overhead. That setup actually doesn`t change all
that much into Thursday either as a secondary weaker shortwave
arrives, with periods of flurries and snow showers likely to
continue but with only minimal additional accumulation of perhaps an
inch in spots. Based on increasing confidence, have decided to
upgrade areas west of the Mississippi River to a Winter Storm
Warning 18Z Tuesday-00Z Wednesday, while holding Clayton/Fayette
Counties in Iowa in an advisory given more mixing issues
anticipated (could even be just rain for a time). Will keep the
Winter Storm Watch east of the river with a later start time of
precipitation expected, but would anticipate an upgrade for
some/most of the watch area tonight.

.LONG TERM...(Thursday Night through Monday)
Issued at 310 PM CST Mon Jan 23 2017

Through the weekend, models continue to show general longwave
troughing over the eastern CONUS and longwave ridging over the
western CONUS, which places the Upper Mississippi River Valley under
cyclonic flow. Given steep low-level lapse rates and modest low to
mid-level negative omega values associated with various upper-level
disturbances passing through, do think that periodic flurries/
scattered light snow showers will be possible through Sunday.
However, details regarding snow chances are far from certain, so
will not add to the forecast at this time. As for temperatures, a
return to near normal looks to be in store. Both the GFS and ECMWF
show brief ridging Monday ahead of a clipper system coming down from
southern Canada.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1132 PM CST Mon Jan 23 2017

MVFR ceilings will be the rule for much of tonight at both
KRST/KLSE, although could see a reduction to IFR at KRST for a few
hours around sunrise. With no dense fog tonight, any reductions in
visibility should be brief and within the 3 to 5SM range. Bigger
impact to aviation operations will be moderate to heavy snow late
in the period as a strong winter storm moves across TAF airfields.
Most recent forecast guidance suggests a slower onset to snow, but
once it starts, conditions will rapidly deteriorate to IFR/LIFR.
For now, will bring the heavier snow into KRST at 25.01Z and into
KLSE at 25.02Z. These times will need to be fine-tuned as details
become clearer with time. Light/variable wind tonight will
increase slightly Tuesday morning/afternoon from the east-
southeast.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from Tuesday afternoon through Wednesday
     afternoon for WIZ032>034-041>044-053>055.

MN...Winter Storm Warning from noon Tuesday to 6 PM CST Wednesday for
     MNZ079-086>088-094>096.

IA...Winter Storm Warning from noon Tuesday to 6 PM CST Wednesday for
     IAZ008>011-018-019.

     Winter Weather Advisory from noon Tuesday to 6 PM CST Wednesday
     for IAZ029-030.

&&

$$

SHORT TERM...Lawrence
LONG TERM...Hollan
AVIATION...Rogers


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