Area Forecast Discussion
Issued by NWS La Crosse, WI

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309
FXUS63 KARX 281136
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
636 AM CDT Wed Jun 28 2017

.SHORT TERM...Tonight through Wednesday evening
Issued at 128 AM CDT Wed Jun 28 2017

At 1 AM, the CAMS are in good agreement that the line of storms,
currently across western Minnesota and northwest Iowa, will move
into southeast Minnesota and northeast Iowa around 4 AM this
morning. As this line moves east through our area, it should out
run the better instability to the west. This will likely result in
a weakening trend of this line. However, still cannot the rule
out the possibility of gusty winds, small hail, and very heavy
rain primarily west of the Mississippi River.

VAD wind profiles were showing a rapid increase in the
925 and 850 mb moisture transport across western Iowa and western
Minnesota. As the low level jet veers tonight, the precipitable
water values will increase into the 1.5 to 2 inch range and warm
cloud layer depths will increase into the 3.5 to 4 km range by
morning and remain that high into the early evening. This will
make any showers and storms efficient rain producers. More on the
heavy rain aspects of this system can be found in the HYDROLOGY
section below.

For this afternoon, the RAP, GFS, and NAM are in general agreement
that the 0-1 km CAPES will climb into the 1.5 to 3K J/kg ahead of
an approaching cold front. The CAM models are in general agreement
that convection will fire in two areas by mid afternoon.

The first area will be located along an outflow boundary which
will be generated from the morning convection. The CAMs are in
disagreement on how far north this boundary will be located.
The NAMNest and ARW would suggest that this outflow boundary will
be south of our area. Meanwhile, the HRRR and NMM suggest that
this outflow boundary will be located from northeast Iowa
northeast into central Wisconsin. A 45 to 55 knots mid-level jet
will enhance the deep shear across eastern Iowa and southwest
Wisconsin. This will result in the development of supercell
thunderstorms capable of producing large hail and damaging winds.
In addition this boundary will enhance the 0-1 km shear which will
result in the potential of isolated tornadoes.

The second area of convective development will be located along
the cold front. The deep shear will increase along the front after
28.21z. As a result, there could be potential for isolated
supercells capable of producing large hail and damaging winds
prior to the system becoming linear. Once the convection becomes
linear the threat will become more of a damaging wind threat.

In the Day 1 Outlook, the Storm Prediction Center added an
enhanced risk along and south of the Interstate 90 corridor. This
has been by the far the most consistent threat area for the past
several days and it looks reasonable. They expanded the slight
risk area northward to cover the remainder of the forecast area.
This was done to cover the potential of damaging winds along the
line of storms which will be located along the cold front.

.LONG TERM...Thursday through Tuesday
Issued at 128 AM CDT Wed Jun 28 2017

From Thursday night into Friday evening, the models continue to
struggle on the location of the low pressure area. The NAM
continue to keep this system southeast of our area. Meanwhile,
the GFS has its surface low closer to La Crosse WI. The ECMWF has
its surface low moving across our far eastern and southeast
counties. At this time, thinking that the cold front will remain
south of our area. This would keep any severe weather threat to
our south and southeast.

From Saturday into Monday, a series of short wave troughs will
move through the region. This would keep the threat for occasional
showers and storms going.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 636 AM CDT Wed Jun 28 2017

A weakening line of showers with embedded scattered thunderstorms
will continue to move eastward this morning. Brief reductions in
ceiling/visibility are possible with the strongest storms, but
shouldn`t drop below MVFR thresholds. Behind the line expect
scattered showers with a few hours of more predominant MVFR
ceilings before rising back above 3000 ft agl layer this
afternoon. Thunderstorms are expected to re-develop this afternoon
and evening, but are highly dependent on this morning`s activity.
Therefore, timing, location, and coverage of storms is still a bit
uncertain, so will keep with the idea of VCTS starting at 28.22Z
for KRST and at 28.23Z for KLSE. Lower ceilings/visibility/gusty
winds possible with any storms, but if they don`t develop or pass
by TAF airfields, VFR conditions will be the rule.

Winds away from thunderstorms through the period will be breezy
from the south, gusting frequently into the 20 to 30 kt range at
both TAF airfields. It`s not until later this evening that winds
diminish and shift slightly to the southwest for the latter half
of the period.

&&

.HYDROLOGY...Through This Evening
Issued at 128 AM CDT Wed Jun 28 2017

A 45 to 50 knot low level jet early this morning will rapidly
increase the precipitable water values into the 1.5 to 2 inch
range and warm cloud depths will climb into the 3.5 to 4 km
range. In this environment any thunderstorms will be efficient
rain producers. However, the speed of the propagation and
orientation of the line will likely limit the hourly rainfall
rates into the 0.75 to 1.5 inch range this morning.

For this afternoon, there still remains a lot of uncertainty on
where additional shower and thunderstorm activity may develop.
Many of the CAMs suggest that the atmosphere will recover enough
that convection will fire along the outflow boundary (from the
morning convection) south of the Interstate 90 corridor. The
orientation and slow southeast movement of this boundary will
result in the potential for training cells which could produce
hourly rainfall rates in the excess of 2 inches per hour. The
main question is how far north will this outflow boundary be
located. The NAMNest and ARW would suggest that this outflow
boundary will be south of our area. Meanwhile, the HRRR and NMM
suggest that this outflow boundary will be located from northeast
Iowa northeast into central Wisconsin. This line will have the
potential of producing some flash flooding.

Another line of storms will likely develop north along the cold
front. However, the forward nature of this line will likely limit
hourly rainfall rates into the 1 to 2 inch range.

During the evening, the instability and moisture will quickly
wane as the cold front slides through the region.

With the confidence on flash flooding being low to medium, will
not issue a Flash Flood Watch and just continue to highlight
locally heavy rainfall in our messaging.

&&

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

$$

SHORT TERM...Boyne
LONG TERM...Boyne
AVIATION...Rogers
HYDROLOGY...Boyne



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