Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 271138

Area Forecast Discussion
National Weather Service La Crosse WI
638 AM CDT Tue Jun 27 2017

.SHORT TERM...(Today and tonight)
Issued at 339 AM CDT Tue Jun 27 2017

At 3 AM, a weak short wave trough was moving southeast across
Wisconsin. This system was producing 7-10K ceilings and a few
light rain showers across northern Wisconsin. One of these showers
just moved through the Medford area. Meanwhile further west, a
ridge of high pressure was located across the central and northern
Plains. The 27.00z models are in good agreement that this ridge
will build across the region today and then the ridge axis will
move quickly east through the region tonight.

Under mostly sunny skies, soundings continue to show that we will
mix up into the 850 to 800 mb range. This will result in the high
temperatures climbing into the mid and upper 70s. There may be
even a few low 80s in the Mississippi and Wisconsin river valleys.

For this evening, a low level jet will develop across the central
and northern Plains. As this occurs, the moisture transport will
increase into this area. Initially a cap will limit the
development of showers and storms. However as a short wave
approaches the region and ascent occurs this cap will gradually
weaken, this will result in the rapid development of showers and
storms. There will be sufficient shear for the development of
supercells. As their cold pools coalesce, these storms will evolve
into a line of storms. With the instability being weak across the
area, expect to see a diminishing trend in its intensity. This
should occur somewhere near Interstate 35.

.LONG TERM...(Wednesday through Monday)
Issued at 339 AM CDT Tue Jun 27 2017

On Wednesday, expect that a line of storms to move east across
the area during the morning. This should allow for the potential
for the instability to grow along and south of the Interstate 90
corridor during the afternoon. Like the past couple of days, the
NAM continues to have the dew points climbing into the lower to
mid 70s. This will allow the 0-1 km mean layer CAPES to climb into
the 1 to 3 K J/kg range. Meanwhile, the 0-1 km mean layer CAPES
are around 1K J/kg in the ECMWF and 1-2 K J/kg in the GFS. While
there are differences in instability, there is sufficient deep
shear for the development of supercell storms which will likely
transition to a line. The soundings show that there will be a dry
mid layer. This would support the development of large hail. In
addition, the strong winds aloft will result in the potential for
straight-line winds. SPC mentioned in their discussion that they
were concerned that outflow boundaries from the morning convection
may result in outflow boundaries that could potentially increase
the 0-1 km shear. If this does indeed occur, there will also be
the potential for isolated tornadoes. The greatest threat of
severe weather at this time looks to be from 3 PM to 9 PM.

On Thursday, the 925 mb temperatures will climb into the 17 to 19C
range. This should allow temperatures to warm into the mid and
upper 70s north of Interstate 90 and into the lower 80s across the
remainder of the area. However, the clouds may limit the warming
across the south.

From Thursday night into Friday night, a short wave trough will
bring additional showers and storms to the forecast area. With
0-1 km CAPES less than 1K J/kg and weak shear, not anticipating
any severe weather from this system.

On Saturday and Saturday night, a closed 550 mb low will produce
another round of showers and storms as it moves through the
region. While there would be favorable shear for supercell
thunderstorms across northeast Iowa and southwest Wisconsin, the
0-1 km mean layers CAPES are only to 500 J/kg, so not anticipating
any severe weather with this system.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 638 AM CDT Tue Jun 27 2017

Expect mostly sunny skies with some occasional cirrus and light
southerly winds through this evening at both KRST/KLSE as high
pressure dominates the regional weather pattern.

Late this evening and overnight, clouds will increase and begin
to lower ahead of the next low pressure system. At this time, will
keep cloud bases above 3000 ft agl, but will be watching the
potential for a line of showers and thunderstorms to move into
southeast MN/northeast IA prior to 28.12Z. If this occurs, at
least MVFR conditions are possible at KRST. For now, will
introduce some VCSH starting at 28.09Z.

Of greater concern, however, will be rapidly increasing winds
aloft tonight as a nocturnal low-level jet develops. Expect low-
level wind shear at both TAF airfields with winds at 2000 ft agl
increasing to between 40 and 50 kts after 28.05Z. While surface
winds will generally remain light, could see some gusts into the
lower 20 kt range at KRST prior to the end of the period.


.HYDROLOGY...Wednesday and Wednesday Evening
Issued at 339 AM CDT Tue Jun 27 2017

Precipitable water values will climb into the 1.5 to 1.8 inch
range on Wednesday morning and remain there into Wednesday evening.
At the same time, warm cloud depths will be climbing into the 3.5
to 4 km range. The combination of these two will make showers and
thunderstorms highly efficient rainfall producers. As a result,
rainfall will likely range from 1 to 2 inches. Locally higher
amounts of 3 inches will be possible.  With many soils wet and
many rivers and streams with higher than normal flows, this heavy
rain could potentially result in some flooding or flash flooding.




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