Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 192043
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
343 PM CDT Wed Apr 19 2017

.SHORT TERM...(This evening through Thursday)
Issued at 341 PM CDT Wed Apr 19 2017

At 3 PM, a surface low was developing across north-central Kansas.
A surface warm front extended east across southern Iowa. South of
this warm front, the surface based CAPES have climbed into the
1 to 2K j/kg range. Meanwhile north of this front, the most
unstable CAPES have climbed into the 500 to 1000 J/kg across the
remainder of Iowa. In addition, the effective shear has climbed
into the 40 to 50 knot range. SPC has just recently issued a
Tornado Watch for southeast Nebraska, southwest Iowa, and
northwest Missouri.

Further to the north, a band of showers was moving northwest
across the Upper Mississippi River Valley. This was associated
with 850 to 700 mb frontogenesis. The CAMs are in good agreement
that this forcing will continue to move northeast across the area
late this afternoon and evening.

The models continue to show that elevated supercells will develop
north of the warm front and ahead of the approaching short wave.
In our area, the greatest threat of severe weather will be mainly
along and south of the Interstate 90 corridor. The greatest severe
weather threat will be large hail from 7 PM through 2 AM tonight.
However, there are still some models that suggest that there will
be the potential that the warm sector may get into northeast Iowa
and southwest Wisconsin between 20.05z and 20.08z. The CIN at
this time is less than 50 J/kg, so cannot completely rule out the
possibility of damaging winds and isolated tornadoes in this area.
However with the warm front so far south, confidence is not high
that this will occur.

There will be also be a possibility of heavy rain south of
Interstate 90 tonight. More on this can be found in the Hydrology
Section below.

.LONG TERM...(Thursday night through Wednesday)
Issued at 341 PM CDT Wed Apr 19 2017

From Thursday night into Sunday, high pressure will provide dry
weather for the Upper Mississippi River Valley. High temperatures
will be in the mid and upper 50s on Friday, around 60 on Saturday,
and from the mid 50s to mid 60s on Sunday. Low temperatures will
be in the 30s. There will likely be some frost in western
Wisconsin on Friday and Saturday mornings. At this time, the only
place where vegetation may be at risk to frost and freezing
temperatures is along the Mississippi and Wisconsin Rivers.

From Monday afternoon into Monday night, a short wave trough will
move east through the Upper Mississippi River Valley. Like
yesterday, there is limited instability ahead of this system;
thus, only expecting showers with this system.

Another short wave trough will move through the region from
Tuesday night into Wednesday. Like its predecessor, there is
limited instability ahead of the wave; thus, only expecting
another round of showers with this system.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1232 PM CDT Wed Apr 19 2017

Main taf concerns are showers and thunderstorms late this afternoon
into this evening and IFR conditions at both taf sites tonight into
Thursday morning. Low pressure is expected to push into the region
through the taf period. Latest mosaic radar indicates a line of
showers/storms moving slowly north into southern Minnesota and
northern Iowa. Instability weakens as it lifts north...thunderstorm
chances diminish early this afternoon at both taf sites. For
now...left mention of VCTS 20z through 23z Wednesday at RST and
21z through 02z at LSE taf sites. Thunderstorm chances increase
this evening and become more widespread across the area. Included
tempo at both taf sites for thunderstorms after 23z Wednesday at
RST and 02z Thursday at LSE. Next concern are IFR conditions at
both taf sites this evening into Thursday morning. MVFR conditions
occurring across the area per latest metars...as daytime heating
occurs there will be a brief window of VFR conditions this
afternoon. Then...ceilings lower this evening into tonight and IFR
conditions expected at RST taf site by 03z Thursday and possibly
to LIFR after 08z Thursday...at LSE IFR conditions expected by 12z
Thursday.


&&

.HYDROLOGY...Tonight
Issued at 341 PM CDT Wed Apr 19 2017

The 19.12z models continue to show that the precipitable water
values will climb into the 1 to 1.4 inch range tonight and the
warm cloud layer depths will climb up to 3.5 km. In addition the
925 and 850 mb moisture transport will dramatically increase this
evening and then it veers quickly east into southeast Wisconsin
during the overnight. Overall, it looks like most areas will see
near an inch of rain. However, there will be the potential for a
narrow axis of locally heavier totals (maybe 2 to 3 inches) across
northeast Iowa and southwest Wisconsin on the northern edge of
the CAPE gradient and ahead of the surface low. The overall trend
is toward a more progressive system; thus, there is still some
uncertainty on rainfall amounts. Given this uncertainty, we will
reissue the ESF. If the details come into focus on where any heavy
rain is more likely, a flood or flash flood watch may yet be
needed. Continue to monitor rainfall forecasts closely.

&&

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

$$

SHORT TERM...Boyne
LONG TERM...Boyne
AVIATION...DTJ
HYDROLOGY...Boyne



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