Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211727

Area Forecast Discussion
National Weather Service La Crosse WI
1227 PM CDT Mon Aug 21 2017

.SHORT TERM...(Today and tonight)
Issued at 332 AM CDT Mon Aug 21 2017

At 3 AM, a mesoscale convective complex was located over western
Iowa. With 2 branches of 850 mb moisture transport, expect that
one band of showers and storms will dive south into southern Iowa
and northern Missouri. Another band will move initially north
into northeast Iowa and southeast Minnesota and then become
orientated north and south as it encounters the strong westerly
winds across the Upper Mississippi River Valley. These showers and
storms will then move east out of the area by mid to late
morning. Subsidence in the wake of this precipitation will result
in a lull of the rain chances from late morning into the mid
afternoon. Both the NAM and HRRR runs continue to suggest that
areas north of 90 could potentially see skies become partly to
mostly cloudy during the solar eclipse. Meanwhile, areas further
south will likely only see small breaks in the overcast during
this time period.

From mid afternoon into the early overnight, a short wave trough
will move southeast through the region. Ahead of this system,
mixed layer CAPES are expected to climb into the 2500 to 3500 J/kg
range. This will result in the rapid development of showers and
storms along the stalled front located near the Interstate 90
corridor and approaching cold front. Soundings continue to show
that much of the shear is located in the 0-3 km layer. This will
result in mainly linear convection. Backing flow ahead of this
wave will increase the 0-1 km shear to greater than 25 knots which
could potentially result in isolated tornadoes along the residual
boundary near Interstate 90. In addition, with the 0-1 km shear
will become perpendicular with the cold front this evening, so will
have to watch out for the potential for isolated quasi linear
convective tornadoes mainly south of Interstate 90. Besides the
isolated tornado threat, damaging winds and maybe some large hail
look to be the primary severe threats. The main threat time for
severe weather will be 3 PM to 10 PM.

.LONG TERM...(Tuesday through Sunday)
Issued at 332 AM CDT Mon Aug 21 2017

In the wake of the cold front on Tuesday, high pressure will
build across the area. This high will provide dry weather from
Tuesday into Wednesday. With dry dew point and light winds on
Tuesday night, lowered the low temperatures in central Wisconsin
below the MOS guidance by up to 5 degrees.

On Wednesday night and Thursday morning, a short wave trough will
move southeast through the region. Both the GFS and GEM produce
rain with its passage through the area. Meanwhile, the ECMWF and
NAM keep the area dry. Considering the the dry air over the
region, sided more toward the ECMWF and NAM solutions and kept the
forecast mainly dry.

In the wake of this front, high pressure will provide dry weather
from Thursday afternoon through Friday. With light winds and dry
dew points, this looks to be another cool night for central
Wisconsin, so lowered the low temperatures a few degrees to
account for this.

The next potential for rain looks to be for the weekend as the
area begins to see return flow on the back side of the departing
high pressure system and a slow moving upper level trough moves
into the region. Considered raising the rain chances during this
time period. However, due to timing differences amongst the GFS,
ECMWF, and GEM, opted not to do it at this time.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1227 PM CDT Mon Aug 21 2017

Scattered thunderstorms across western MN/northern IA expected to
push into vcnty of KRST after 20z and at KLSE after 22Z as a cold
front sweeps through the region. Expect these storms to last for
a few hours and push southeast of the area after 04z. Expecting
to remain mainly VFR but plan on intermittent MVFR as storms move
across the airports. Also, look for a period of MVFR
stratocumulus in the 05-10Z time frame. Cooler/drier/VFR
conditions expected for Tuesday as Canadian high pressure builds




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