Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201843

Area Forecast Discussion
National Weather Service La Crosse WI
143 PM CDT Tue Jun 20 2017

.SHORT TERM...(Tonight through Wednesday Night)
Issued at 143 PM CDT Tue Jun 20 2017

Current water vapor imagery coupled with RAP 500mb analysis shows
the shortwave trough that gave the area morning convection now
sliding southeast toward Lake Michigan. High pressure now building
into our forecast area from the Dakotas/southern Canada. This high
will slide southeast across the area overnight for partly cloudy
skies. Given the rainfall that occurred today/moist ground, and
mostly clear skies/light winds tonight, should see areas of fog,
mainly over central WI where the ridge axis/lightest winds will
reside longest. Farther west, along and west of the Mississippi
River, there will be an increase in warm air advection mid-cloud
which should inhibit radiational cooling/fog formation. Otherwise,
looking for lows in the middle 40s in the Sand Country of central
WI, to the lower/middle 50s elsewhere.

For Wednesday, low pressure crossing the Dakotas will bring a push
of warm air advection and moisture transport into the area by
afternoon for a chance of elevated shower/thunderstorm activity.
Highest chances look to be along/west of the Mississippi River where
stronger push of moisture transport will exist. Otherwise, look for
highs in the middle 70s to around 80.

Showers and thunderstorms will become likely Wednesday night as
moisture transport increases ahead/along of a cold front pushing
into the area from MN. NAM/GFS showing 0-3km MUCAPE generally in the
1500-2000j/kg range with bulk shear values in the 25-40kt range.
Bufkit soundings showing this convection would be mainly elevated in
nature with LFCs generally above 800mb. At the same time, NAM is
pushing precipitable water values into the 1.5 to 1.7 inch range.
So, looking like a chance for a few strong to possibly severe storms
with hail, heavy rainfall being the main threats.

.LONG TERM...(Thursday through Tuesday)
Issued at 143 PM CDT Tue Jun 20 2017

The aforementioned cold front pushing into the area Wednesday night
looks to lay up across far southern WI/northern IL/IA by Thursday
afternoon as it becomes parallel to the mid-level flow. Increasing
moisture transport and lift over the front along with a fairly
strong mid-level frontogenetic feature will produce the likelihood
of showers/thunderstorms across our area. NAM/GFS showing MUCAPE
increasing into the 2000-4000j/kg range by afternoon mainly
along/south of I-90 with NAM displaying a whopping 50-60kt of Bulk
Shear in the 0-3km layer. Taking a look at Bufkit soundings shows
most of the thunderstorms over our area will have elevated LFCs
north of the front for a large hail threat with the more surface-
based storms/tornado threat across IL into IA. Of course, final
outcome will be the exact position of the front which is still yet
to be determined. In addition, with precipitable water values pumped
up into the 1.7-2.0 inch range, these storms will be efficient heavy
rain-makers.  Will therefore have to keep a vigilant eye on the
evolution of this scenario for severe potential.

A longwave trough digs over the region for Friday through Tuesday.
Shortwave troughs embedded within this cyclonic flow and steeper
lapse rates will trigger daytime cumulus and the possibility of a few
showers/isolated thunderstorms from time to time. Otherwise, look
for cooler temperatures, especially on Saturday and Sunday with
highs only expected to be in the 60s/lower 70s.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1244 PM CDT Tue Jun 20 2017

Rain showers/isolated thunderstorms have moved east of TAF
airfields, so expect dry conditions and gradually clearing skies
this afternoon into the evening as high pressure builds across the
region. Any cloud cover will be VFR with light northwest wind.

Will need to watch fog potential tonight, given recent rainfall
moistening the near surface layer, mostly clear skies and light
wind. Not enough confidence to introduce any reductions in
visibility at this time, especially given short nights, but
something to watch over the next 12-18 hours. Winds Wednesday will
shift to the south, but remain light with any cloud cover remaining
above 3000 ft agl.




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