Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 122325
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
625 PM CDT Thu Apr 12 2018

.SHORT TERM...(This evening through Sunday)
Issued at 313 PM CDT Thu Apr 12 2018

A complex spring storm system remains on target to impact the area
tonight through the weekend with multiple hazards and impacts
likely. Some of the details still need to be ironed out, but
confidence is high that this will be an impactful storm for the
area.

For tonight into Friday, surface low pressure organizes across the
central plains ahead of an upper trough coming out of the Rockies,
with a warm front trying to lift north across Iowa and Illinois. Mid-
level warm advection/frontogenesis north of the surface warm front
along with increasing low-level moisture transport will lead to
increasing rain coverage overnight tonight into Friday from south to
north. With steepening mid-level lapse rates, MUCAPE could
approach 1000 J/kg into northeast IA Friday. While the greater
threat for strong/severe storms will remain south closer to the
warm front, could not rule out some hail given the low freezing
heights with any elevated storms, especially over northeast Iowa.
Precip should mainly be rain through Friday, although will have
to watch surface temps in far north-central Wisconsin for a
possible mix. Winds will strengthen from east to northeast on
Friday gusting to 25 to 35 mph by afternoon. Most areas will
remain in the 30s to low 40s for highs, possibly a bit higher into
far southwest WI/northeast IA nearer the warm front. Rainfall
amounts of around 0.50 to 1.0 inches are expected through Friday.

Widespread precip is expected north of the elevated frontal zone
Friday night with some embedded convection, especially as convection
developing out to the south and west near the triple point lifts
northeast over the warm front. In addition to the precip, strong
E/NE winds will gust up to 45 mph. Precip will persist into
Saturday with some continued moisture transport wrapping into the
area. An additional 1 to locally 2 inches of precip is expected.
Colder air wrapping in from the north and east will lead to
increasing wintry precip chances from the north Friday night and
Saturday. In general, much of precip should be liquid into early
Saturday with no cloud ice present as the mid level dry slot works
northward. However, could not rule out some sleet, especially
with embedded convective elements. Freezing rain vs rain will
depend on surface temps, which will be near freezing for most
areas, although north-central Wisconsin should be cooler in the
mid to upper 20s. Icing amounts into Saturday are tricky, with
potential for more accumulation on elevated surfaces (especially
given the strong winds), while road conditions will depend on
air/road temps and whether or not the roads have been treated.

As the deformation zone begins to swing eastward later Saturday into
Sunday, remaining precip will trend towards snow as the warm nose
aloft weakens and deeper moisture is introduced. Snow amounts are
still somewhat uncertain, and impacts will depend on snow rate
and time of day, so that higher impacts may be during overnight
periods. Several inches are possible, though.

Coordinated with WFO Green Bay for a winter storm watch for
Taylor/Clark Counties Friday evening through Sunday where there
is the potential for over 6 inches of snow along with some icing.
Areas to the south will likely at least a winter weather advisory.
Determining impacts in these late season events is tough, and in
this case a long duration event.

.LONG TERM...(Sunday night through Thursday)
Issued at 313 PM CDT Thu Apr 12 2018

The slow-moving low pressure system finally will loosen its grasp on
the region on Sunday night and Monday as it tracks into the eastern
Great Lakes. Mid-level ridging along with surface high pressure
should provide a respite from precip for early week with highs in
the 30s and 40s Monday recovering into the 40s to low 50s on
Tuesday.

Another shortwave trough and associated surface low will eject out
of the plains Tuesday night into Wednesday with the next chance for
rain or snow, although models have not settled on a track of the
upper wave yet, so confidence is lower. Temps should moderate
through mid-week, but remain on the cooler side of normal for mid to
late April.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 625 PM CDT Thu Apr 12 2018

Rapidly changing conditions expected through the night as showers
develop pretty much overhead in response to developing low
pressure to our west. As that occurs, ceilings should also drop
into MVFR range for LSE and likely to IFR at RST, with a few
rumbles of thunder also possible at just about any time. Given the
uncertainty regarding exact timing of thunder potential, have
opted to just keep showery wording, but will likely need some
near-term amendments as things develop. Could also see some small
hail with any showers/storms overnight and into Friday, with
periods of rain/showers expected to last right on through Friday
evening as ceilings gradually continue to lower, and winds
increase from an easterly direction at 25-30 knots.

&&

.HYDROLOGY...
Issued at 313 PM CDT Thu Apr 12 2018

Rainfall amounts of 1 to locally over 2 inches remain likely
tonight through Saturday. With at least partially frozen
ground, much of the rainfall will likely runoff, producing
ponding of water in low spots and cause river rises. Flooding
could become an issue this weekend into early next week on some
area rivers and streams, especially if the rain falls heavier or
in a shorter period of time. Please continue to monitor forecasts
closely, especially if you have interests along or live near a
river or stream.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from Friday evening through Sunday evening
     for WIZ017-029.

MN...None.
IA...None.
&&

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...Lawrence
HYDROLOGY...DAS/JM



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