Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 280452
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1152 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 06Z TAFS through 06Z Wednesday night)
Issued at 1152 PM CDT Tue Jun 27 2017

Convection has developed this evening over the Dakotas and
Nebraska and was moving into the Missouri River Valley. The 28.03Z
HRRR seems to be doing the best with this and continues to show
this will move into the area late tonight and across both airports
through the middle of Wednesday morning. It will be weakening so
should primarily come through with VFR conditions but cannot rule
out some MVFR/IFR if a stronger cell moves over one of the
forecast points. There remains considerable uncertainty on where
and how much convection will redevelop Wednesday afternoon. The
short wave trough, currently moving onto the High Plains, will
provide the support for the storms. The hi-res models suggest the
activity late tonight into Wednesday morning will put out an
outflow boundary that should be the focus for new storms but they
are also showing activity along the cold front as it comes in. For
now, will just continue with VCSH/VCTS through the afternoon until
it become clearer where/if this activity will form. Will also
continue the low level wind shear for both airports tonight until
the convective activity from the west rolls in. VWP wind profiles
are already showing 35 to 40 knots in the low level flow ahead of
the convection.

&&

.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.

&&

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

$$

SHORT TERM...Wetenkamp
LONG TERM...Wetenkamp
AVIATION...04
HYDROLOGY...Wetenkamp


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