Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201949
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
248 PM CDT Thu Jul 20 2017

.SHORT TERM...(Tonight through Friday Night)
Issued at 248 PM CDT Thu Jul 20 2017

Focus squarely on severe/heavy rain potential Friday afternoon into
early Saturday morning.

Current surface map has a quasi-stationary frontal boundary
extending across far southern WI into northeast IA. Some enhanced
Altocumulus Castellanus showing up along this boundary per latest
GOES-VIS. Otherwise, partly cloudy/mostly sunny skies prevailing
this afternoon along with seasonably warm and humid conditions.
Temperatures were in the lower/middle 80s with dew points ranging
from the middle 60s across northern WI to the middle 70s across far
northeast IA/southwest WI.

Going into tonight, low pressure over the Plains will start to push
the aforementioned frontal boundary northward as a warm front along
with modestly increasing 850mb moisture transport. This should be
enough to force some showers and storms west of the Mississippi
overnight. Better CAPE and shear stay south of the area, so
expecting storms to be elevated/sub-severe with perhaps 1/4 inch of
rainfall expected.

Focus then turns to expected upscaling thunderstorms Friday
afternoon into Friday night/early Saturday morning as moisture
transport continues over the warm front shifting a bit further north
into northeast IA/southwest WI. Ample shear and CAPE dictates a
severe threat with damaging winds and torrential downpours being the
main threat. Hail threat will be minimal due very high Wet Bulb Zero
heights around 14kft or 4.5km. Will also have to watch for the
possiblity of an isolated tornado or two south of I-90 closer to the
surface warm front and increasing helicity. But again, very heavy
rainfall looks possible given high precipitable water values
approaching 2.5 inches. With recent heavy rainfall and damage from
flooding Wednesday night/early Thursday morning and expected
additional 1-3 inches/locally higher amounts, Flash Flood Watch in
effect for a good portion of the forecast area from noon Friday
through 9am Saturday.

.LONG TERM...(Saturday through Thursday)
Issued at 248 PM CDT Thu Jul 20 2017

Mid-level trough/weak cold front push east across the area on
Saturday for a chance of showers and storms. May have to watch for a
few stronger storms in the afternoon in peak heating and modest bulk
shear. Hail and gusty winds would be the main threat.

High pressure builds in for Sunday for slightly cooler/drier
conditions. This high remains in control through Tuesday for a
relatively quiet/pleasant period.

Next chance of showers and storms come along Tuesday night through
Thursday as a mid-level trough and surface cold front pushes
southeast through the region.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1228 PM CDT Thu Jul 20 2017

Other than some sct diurnal cumulus in the 2000-3000 ft range,
generally good VFR expected this afternoon into this evening with
the area between weather systems. The recent rains coupled with
light winds and mostly clear skies, introduced some late night early
morning MVFR BR at both KRST/KLSE. Will have to watch KLSE for
potential of IFR cigs/vsbys in some thicker BR/FG in the 10-13z time
frame. There may be more of a bkn mid cloud deck spreading across
the area after about 09z, so did not include denser BR/FG mention at
KLSE at this time. Mixed signal among models on convection potential
Fri morning, with some much more robust than others. The variability
lowers confidence for TSRA in the last 3 to 6 hrs of the taf period.
Included a -SHRA mention, but given the lesser confidence, left the
VCTS/CB mentions out for now. Great chances of TSRA impacting the
taf sites trending to be later Fri afternoon/night.

&&

.HYDROLOGY...
Issued at 248 PM CDT Thu Jul 20 2017

Very heavy rainfall last night over a good portion of the area with
a general swath of 4 to 8 inches. This obviously has had an impact
on a few area rivers such as the Trempealeau and Kickapoo Rivers
where flood warnings are in effect.

Additional locally heavy rainfall is expected Friday into Friday
night. This, combined with the very recent heavy rainfall will keep
rivers running high.

For latest river forecasts and other details, visit our homepage at
www.weather.gov/arx

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch from Friday afternoon through Saturday morning
     for WIZ032>034-041>044-053>055-061.

MN...Flash Flood Watch from Friday afternoon through Saturday morning
     for MNZ079-086>088-094>096.

IA...Flash Flood Watch from Friday afternoon through Saturday morning
     for IAZ009>011-030.

&&

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...RRS
HYDROLOGY...JM


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