Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211940
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
240 PM CDT Mon Aug 21 2017

.SHORT TERM...(This evening through Tonight)
Issued at 240 PM CDT Mon Aug 21 2017

High Impact Weather Potential: All eyes on thunderstorm potential,
with some risk for severe storms, particularly during the 4-8pm
time frame and especially across northeast Iowa.

As of 18Z, not only is it darker than normal across the area (thanks
eclipse), but a warm front remains draped across central Iowa, while
a cold front is taking shape across Nebraska and through the
northern Plains, ahead of a stronger shortwave dropping through
North Dakota. Airmass destabilization continues south of the warm
front/cold front interface into much of northern and central Iowa,
while much less instability is in place farther north. On the radar
front, one complex of storms continues to work east out of South
Dakota, while additional storms continue to bubble across northern
Iowa, with some additional development northward into parts of
south-central Minnesota.

Heading through the afternoon and evening hours, will be watching
that South Dakota complex, which continues to gradually grow
upscale as it encounters better instability with time into
northwest Iowa. Meanwhile, additional convection should tend to
bubble along the northward advancing warm front into northern or
central Iowa. How far north said warm front advances remains very
low confidence, particularly with still-ongoing convection right
along it, but its placement will strongly dictate severe potential
up this way. Pretty decent lapse rates in combination with MLCAPE
pushing 3000 J/kg (or maybe even slightly higher) does suggest a
hail threat, with enhanced 0-1km shear right along the warm front
suggestive of a low end tornado threat. That threat looks to just
clip northeast Iowa per current trends, while better 0-3km shear
suggests more of a linear system with the potential for some
stronger wind gusts. At the moment, the highest severe risk looks
to be across northeast Iowa into maybe far southwest Wisconsin.

Thankfully this while setup appears progressive, and while PWAT
values do climb toward 2 inches, the overall risk for widespread
heavy rain appears low. With that said, will have to watch the
interface where convection develops along the warm front followed by
additional storms on the cold front, as a localized axis over
northern or central Iowa could receive some heavy rains. However, as
mentioned in this space yesterday, our risk for storms appears it
will really drop off by 06Z as the upper wave and cold front quickly
drop through the region.

.LONG TERM...(Tuesday through Monday)
Issued at 240 PM CDT Mon Aug 21 2017

Another great stretch of weather is on the way Tuesday through
Friday as upper troughing dominates while Canadian high pressure
drops through the region. That setup should deliver very comfortable
and mainly dry conditions, with the possible exception of a few
showers Wednesday night with hints of a stronger wave crossing the
area. Not much in the way of moisture with that feature but
something to watch. Bigger story is below normal temperatures once
again, with lows back in the 40s and lower 50s for everyone. Great
sleeping weather!

Otherwise, looking like precipitation chances will gradually return
to the area toward Saturday or Sunday as broad return flow gets
underway ahead of our next cold front slated currently to arrive
toward Monday. Still plenty of questions about specific timing that
far out, but even that far out, there remain pretty strong signals
that temps will remain near to below normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1227 PM CDT Mon Aug 21 2017

Scattered thunderstorms across western MN/northern IA expected to
push into vcnty of KRST after 20z and at KLSE after 22Z as a cold
front sweeps through the region. Expect these storms to last for
a few hours and push southeast of the area after 04z. Expecting
to remain mainly VFR but plan on intermittent MVFR as storms move
across the airports. Also, look for a period of MVFR
stratocumulus in the 05-10Z time frame. Cooler/drier/VFR
conditions expected for Tuesday as Canadian high pressure builds
in.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Lawrence
LONG TERM...Lawrence
AVIATION...DAS



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