Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 271949

Area Forecast Discussion
National Weather Service La Crosse WI
249 PM CDT Tue Jun 27 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 249 PM CDT Tue Jun 27 2017

Main impactful weather concerns are on the potential for strong
storms tonight with the possibility of heavy rainfall and gusty
winds. Focus then quickly turns to the potential for strong to
severe storms Wednesday afternoon and evening, mainly across
northeast Iowa into far southwest Wisconsin.

A cold front will move east across the Northern Plains and toward
the Upper Mississippi River Valley tonight. Thunderstorms will
develop ahead of the front from central North Dakota southward
across South Dakota into central Nebraska. These storms will then
march east eventually forming into line segments. These storms
then look to weaken as they approach the interstate 35 corridor,
outrunning the strongest instability. However, the warm cloud
depth increases to 3500-4000 meters with precipitable water values
climbing to 1.5 to 1.8 inches, potentially creating locally heavy
rainfall concerns. The system looks progressive so not expecting
flash flooding but could certainly see some ponding of water and
some river rises. Flooding may become more of a concern in the
afternoon with a possible second round of locally heavy rainfall.

The thunderstorm development Wednesday afternoon and evening will
depend heavily on whether or not any sunshine occurs and how much
CAPE can actually build after the morning storms roll through.
Forecast models have been overdoing the surface dew points for
Wednesday afternoon and evening, so the convection the models
generate is likely overdone. The convection will all hinge on how
far north the higher dew points can get. The most probable
scenario would be that the morning storms keep these higher dew
points south and this trend is happening from run to run on the
RAP. If storms can redevelop Wednesday afternoon and evening there
is the possibility that they could become supercells capable of
producing large hail and damaging winds. Will have to see if any
boundaries are lingering from the morning convection to determine
if there would be a threat for an isolated tornado or two. At this
time, it appears that redevelopment would occur across portions
of northeast Iowa into far southwest Wisconsin but again weighs
heavily on how the storms play out Wednesday morning into
Wednesday afternoon.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 249 PM CDT Tue Jun 27 2017

Another trough dives into the region from the northwest Thursday
into Friday. A warm front is expected to be situated just south of
the forecast area.  If the front meanders just a little further to
the north we could have a chance at seeing a couple of strong to
severe thunderstorms. Much of the area should at least see scattered
showers and storms with with any outside chance of severe weather
across northeast Iowa and far southwest Wisconsin.

Shortwaves will continue to move across the Upper Mississippi River
Valley Friday into Sunday with continued chances for showers and
possibly a few thunderstorms. Surface high pressure then attempts to
build across the region late Monday into Tuesday and if this pattern
would hold, maybe a dry fourth of July?  A little to early to say
for sure.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1205 PM CDT Tue Jun 27 2017

Good VFR to continue this afternoon and evening as dry high pressure
slowly drifts east of the area. Winds to remain on the light side
thru 06z tonight as well.

Later tonight and for Wed morning, moisture increases ahead of the
next low, with increasing/thickening and lowering clouds. By very
late tonight and for Wed morning, a round or two of SHRA/TSRA are
expected to move east across the area. These to be accompanied by
period of IFR vsbys in the SHRA/TSRA cores and MVFR cigs. Gusty west
winds to accompany the stronger TSRA as well. Behind the initial
bands of SHRA/TSRA, MVFR cigs expected to persist thru most of Wed
morning, even outside of the SHRA/TSRA.

Also, during the bulk of the overnight hours, LLWS expected at the
TAF sites as winds around 925mb increase to 40-50kts from the SW,
while sfc gradient winds are expected to S-SE at 10-15kts. These
conditions abate around sunrise Wed as sfc winds become S-SW and
increase to 15-20kt G25-30kt thru the morning.


.HYDROLOGY...Wednesday and Wednesday Evening
Issued at 249 PM CDT Tue Jun 27 2017

Locally heavy rainfall possible Wednesday into Wednesday evening.
Precipitable water values will climb into the 1.5 to 1.8 inch
range and warm cloud depths are expected to increase into the 3-4
KM range. In this environment any thunderstorms will be efficient
rain producers. Rainfall totals Wednesday morning could range from
1 to 1.5 inches with another 1 to 1.5 inches possible Wednesday
afternoon into Wednesday evening. Any flash flooding concerns will
depend on if we see the heavy rain Wednesday afternoon and
evening fall over the same areas that see heavier rain Wednesday
morning. Placement of these two round of heavy rain is uncertain
at this time so will not be issuing a flash flood watch.
Confidence may increase enough to do so after the first batch of
heavy rain falls Wednesday morning. Will continue to highlight
locally heavy rainfall in our messaging.




SHORT TERM...Wetenkamp
LONG TERM...Wetenkamp
HYDROLOGY...Wetenkamp is the U.S. government's official web portal to all federal, state and local government web resources and services.