Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 192314

Area Forecast Discussion
National Weather Service La Crosse WI
614 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 00Z TAFS through 00Z Thursday evening)
Issued at 614 PM CDT Wed Apr 19 2017

Late this afternoon, an area of low pressure over over southeast
Nebraska with a warm front extending east across southern Iowa.
Both of these features are expected to lift northeast through the
evening with the 19.21Z RAP suggesting the low will pass just to
the south of both airports during the overnight hours. As these
features approach, additional showers and storms are expected as
the forcing will be aided by a short wave trough that will come
out of the Dakotas and move across Minnesota. Through the evening
hours, expecting MVFR conditions to dominate with some occasional
IFR conditions in the heavier showers/storms. As the surface low
then passes by, ceilings should go down to IFR but most of the
rain should be done with VFR visibilities. Ceilings should then
improve back to MVFR late Thursday morning or early in the
afternoon as good low level lapse rates develop in the mixed
layer. This will also allow gusts of 20 to 25 knots to occur with
sustained west winds around 15 knots.


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.




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