Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 180436
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1136 PM CDT Tue Apr 17 2018

.UPDATE...
Issued at 925 PM CDT Tue Apr 17 2018

Latest HRRR/RAP show an interesting evolution for tomorrow`s snow
event, keeping much of the precipitation south of the IA/MN
border through at least mid-afternoon. A closer look at soundings
suggest this is due to a lack of mid-upper level saturation.
Conversely, the 18Z GFS and the 00Z NAM bring snow much farther
north into and just north of the I-90 corridor by Wednesday
afternoon/evening. With these differences and noting some rather
large spreads in snow amounts on the 18Z GEFS plumes, will
maintain current snow forecast and associated headlines. Hope the
full 00Z model suite sheds some light on which solution will
verify, so some adjustments are still possible overnight.

That said, current guidance is in good agreement with slowing the
onset of snow tomorrow morning. Now looks like snow will first
move into northeast IA and parts of southeast MN after 12Z and
then not into the MS River Valley and southwest/west-central WI
until after 15Z. As a result, delayed the Winter Weather Advisory
start time for Buffalo, Trempealeau, La Crosse, Jackson, Monroe,
Vernon, and Richland Counties until 10 AM. The morning commute may
be impacted across parts of northeast IA with impacts becoming
more widespread for the evening commute.

&&

.SHORT TERM...(Tonight through Wednesday Night)
Issued at 322 PM CDT Tue Apr 17 2018

Focus is on incoming storm system which will bring snow to the area
from Wednesday morning through Wednesday evening. GOES-East water
vapor imagery shows the incoming storm currently crossing the MT/WY
Rockies. This wave will continue to chug east through the Plains
tonight and then along the IL/WI border during the day Wednesday.

Model diagnostics bring a potent lobe of PV-advection and increasing
upper level difluence into the area early to mid-morning along with
NAM showing a respectable 2-5km MUCAPE axis nosing into northeast
IA/far southwest WI. This will lead to a few thunderstorms down
across that area with perhaps some sleet mixing in with moderate to
heavy snow rates. This forcing shifts east of the area by afternoon
with the area slipping into the deformation area of the mid-level
low in the afternoon/evening.

Current total snowfall amount thinking is for 3 to 8 inches with the
highest amounts extending from northeast IA into central WI. As a
result, Winter Storm Warnings and Winter Weather Advisories have
been posted. See the Warnings, Watches, and Advisories section below
or see our internet homepage at www.weather.gov/lacrosse for latest
headline details.

.LONG TERM...(Thursday through Tuesday)
Issued at 322 PM CDT Tue Apr 17 2018

Things are looking up in the extended as a strong ridge of high
pressure builds across the region. Daytime temperatures warm from
the 40s on Thursday to well into the middle 50s/near 60 by Monday.
Nighttime lows look to be in the 20s/30s. With these warming
temperatures, look for significant melting of the snowpack in place
across the area. This will have an impact on runoff and hydrology
concerns. More detail on this is highlighted below in the Hydrology
section. A chance of rain enters the picture Monday night into
Tuesday as a mid-level trough and surface cold front push
southeastward into the region.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1136 PM CDT Tue Apr 17 2018

Expect increasing VFR clouds overnight at TAF airfields.

An area of snow will then move across the region by mid Wednesday
morning and through the afternoon. Some differences in forecast
guidance have emerged that show one of two solutions. The first
solution brings light snow northward into TAF airfields by
18.15Z-18.16Z with higher snowfall rates producing reductions in
visibility to 1 SM by early to mid afternoon. The second solution
keeps the majority of snow south of TAF airfields with impacts to
aviation operations limited to lower ceilings. Will follow the
first solution for now, but will need to monitor radar trends
closely and amend as necessary. Snow or not, MVFR/IFR ceilings are
expected to expand across the region late Wednesday morning and
continue for much of the period. Not until Wednesday evening are
conditions expected to improve.

Light northeast wind will be the rule overnight, increasing
slightly and turning to the north through Wednesday.

&&

.HYDROLOGY...
Issued at 322 PM CDT Tue Apr 17 2018

Warming temperatures will produce significant melting of the
snowpack in place across the area, leading to a good amount of
runoff into area streams and rivers. Besides the Trempealeau River
at Dodge possibly exceeding flood stage this weekend, the rest of
the area streams and rivers are expected to just see significant
within-bank rises. Those with interests or property close to
streams and rivers will want to stay up to date on latest river
forecasts.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 10 AM Wednesday to 1 AM CDT
     Thursday for WIZ032>034-041>044-053-055.

     Winter Weather Advisory from 7 AM Wednesday to 1 AM CDT Thursday
     for WIZ054-061.

MN...Winter Storm Warning from 7 AM Wednesday to 1 AM CDT Thursday
     for MNZ086-087-094-095.

     Winter Weather Advisory from 7 AM Wednesday to 1 AM CDT Thursday
     for MNZ079-088-096.

IA...Winter Storm Warning from 7 AM Wednesday to 1 AM CDT Thursday
     for IAZ008>011-018-019-029-030.

&&

$$

UPDATE...Rogers
SHORT TERM...DAS
LONG TERM....DAS
AVIATION...Rogers
HYDROLOGY...DAS



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