Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211133

Area Forecast Discussion
National Weather Service La Crosse WI
633 AM 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 12Z TAFS through 12Z Tuesday morning)
Issued at 633 AM CDT Mon Aug 21 2017

At 1130Z, radar imagery showed scattered showers and
thunderstorms across the area. This activity will continue through
mid morning, but confidence is low that anything will reach either
TAF site. For late morning through at least mid-afternoon, expect
VFR ceilings and light southerly winds. Widespread showers and
storms are then expected this evening. While confidence is high
that TSRA will occur at both TAF sites, there remains some
uncertainty regarding the exact timing of TS. Therefore,
introduced a period of SHRA with VCTS for now. Once the threat for
showers and storms exits the area early tonight, a period of MVFR
ceilings and light winds shifting to the northwest are expected
with a cold frontal passage. Ceilings should improve by 12Z




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