Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 271705
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1205 PM 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 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 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.

&&

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

$$

SHORT TERM...Boyne
LONG TERM....Boyne
AVIATION.....RRS
HYDROLOGY....Boyne



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