Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 182332

Area Forecast Discussion
National Weather Service La Crosse WI
632 PM CDT Tue Apr 18 2017

.SHORT TERM...(This evening through Wednesday morning)
Issued at 325 PM CDT Tue Apr 18 2017

At 3 PM, a surface low was located near the western tip of Lake
Superior. A cold front extended southwest of this low into
southwest Minnesota. Temperatures in the wake of this front were
in the 40s and 50s. Meanwhile south of this front, temperatures
were in the 60s and lower 70s. With most unstable CAPES just over
100 J/kg, there is enough instability for showers to develop. With
the CAM models still suggest that there may be enough instability
for isolated thunderstorms to develop across parts of northeast
Iowa and southwest and central Wisconsin late this afternoon and
early evening. Due to this, kept a mention of isolated storms in
the forecast.

.LONG TERM...(Wednesday afternoon through Tuesday)
Issued at 325 PM CDT Tue Apr 18 2017

On Wednesday afternoon and evening, the 18.12z models differ on
how far north that the surface low will get. The deterministic
models have the surface low moving as far north as La Crosse. The
further northwest track has been a consistent signal in the
dprog/dt. Meanwhile,the CAM models trend further south with the
track of this surface low. This is due to the development of a
cold pool/ meso high from the showers and scattered thunderstorms.
Soundings suggest that there will be sufficient 1-6 km shear for
the potential development of splitting supercells south of
Interstate 90. The main severe weather threats will be large hail
and maybe damaging winds near the warm front. In addition with the
nose of the low level jet, a coupled jet overhead, and
precipitable water values up to 1.4 inches, there will be a threat
of heavy rain. This may result in some localized flooding along
and north of Interstate 90. The greatest threat of severe weather
4 PM and midnight.

After midnight on Wednesday night, the low level jet veers into
eastern Wisconsin. As this occurs, the threat of any flooding or
severe weather will quickly end across the region.

From Monday into Monday night, a strong short wave trough will
move east through the region. While there is 30 to 40 knots of
0-3 km shear, the instability ahead of this system is lacking.
As a result, only expecting showers with this system.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 632 PM CDT Tue Apr 18 2017

An area of low pressure was over the western tip of Lake Superior
early this evening. A cold front extended south from the low
across western Wisconsin into eastern Iowa. The winds have already
shifted to the northwest at KRST behind the front and will do the
same at KLSE this evening. In the cyclonic flow behind the surface
low and cold front, observations indicate there is a large area
of MVFR ceilings. Expecting these clouds to spread across both
airports with these ceilings then remaining into Wednesday morning
before they move off and allow VFR conditions to briefly return.
The cold front that moves through this evening will start to
return north as a warm front Wednesday as another area of low
pressure starts to move northeast out of Kansas along the front.
As this low approaches, look for showers to spread in late
Wednesday afternoon with a return of MVFR to IFR conditions.


.HYDROLOGY...Wednesday and Wednesday night
Issued at 325 PM CDT Tue Apr 18 2017

On Wednesday afternoon and evening, there remains potential for
widespread 1 to locally 3 inches of rainfall. At this time, the
heaviest rain is expected to fall along and north of Interstate
90. However, some of the CAM models suggest that this may be a
bit further south. Due to the potential for heavy rain and still
some uncertainty on where the heavy rain may fall, we will
reissue the ESF. As details become clearer and if confidence
increases on the placement of the heavy rain threat, hydrology
headlines may be needed. Continue to monitor forecasts closely.




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