Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211937
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
237 PM CDT Wed Jun 21 2017

.SHORT TERM...(Tonight through Thursday Night)
Issued at 237 PM CDT Wed Jun 21 2017

Strong to possibly severe thunderstorms and heavy rainfall potential
are primary concerns tonight and Thursday.

Current water vapor/RAP 500mb analysis/radar showing a weak
shortwave trough and associated shower thunderstorm activity moving
southeast across northeastern IA and northern IL. Visible satellite
shows convective debris clouds affecting areas along/south of I-90.
Otherwise, mostly sunny and pleasant across the rest of the area
with temperatures in the 70s.

For tonight, we will be watching a warm front lift out of IA and
situate itself along between the I-90/94 corridors by 06z. Strong
push of 850-700mb moisture transport across the frontal boundary is
expected to kick off showers and thunderstorms mainly northeast of I-
94 later this evening per latest deterministic models and most CAMs.
Bufkit soundings and NAM 2-7km layer showing parcel LFCs above
800mb, so all of this convection is expected to be elevated in
nature. NAM shows 2-7km MUCAPE in the 2000-3500J/kg range with
Effective Bulk Shear around 30 kt. As such, expecting a few of these
storms to be strong to possibly severe at times with large hail and
localized heavy rainfall (as Precipitable Water values jump into the
1.6-1.9 inch range) being the main threats. Then expecting a
diminishing trend late tonight/toward morning as bulk of the thrust
of moisture transport shunts east of the area.

A bit of a break in the action expected to last through mid-Thursday
morning, but then with cold front sinking southward across the area
and impinging moisture transport will re-fire convection by later in
the morning through the afternoon. NAM has been very consistent from
run to run with strong 850-700mb frontogenesis along southeast MN
into central WI during the afternoon, likely setting up a focus for
convection/localized heavy rainfall. Severe threat will be
borderline as this convection is again expected to be mainly
elevated in nature. NAM 1-7km MUCAPE in the 1500-2000J/kg range with
effective bulk shear of 20-30kt, so thinking highest threat would be
heavy rain followed by hail, then wind. Look for this convection to
rapidly diminish/push off to the southeast of the area after
midnight.

.LONG TERM...(Friday through Wednesday)
Issued at 237 PM CDT Wed Jun 21 2017

A longwave trough digs across the central CONUS Friday through
Monday with a couple weak mid-level waves rippling through. This
will produce periodic shower/isolated thunder chances through the
period along with cooler temperatures. Look for highs Friday in the
upper 60s to upper 70s, cooling mainly into the 60s/lower 70s for
Saturday/Sunday/Monday.

Warm air advection will produce a chance of shower/thunderstorms
Tuesday and Wednesday. Otherwise, temperatures look to stay a few
degrees below seasonal normals with highs in the middle/upper 70s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1230 PM CDT Wed Jun 21 2017

Good VFR expected this afternoon/evening as dry-ish lower level air
remains over the area. Moisture/instability increases into/over the
area tonight, but this generally above a rather strong inversion
around 850mb. Clouds to gradually thicken/lower tonight, but remain
VFR until around 12z Thu when a cold front pushes into the area.
Some lower clouds/potential MVFR cigs then expected Thu morning with
moisture pooled near the front. Ahead of the front for a few hours
either side of 06z, winds aloft expected to increase to around 40kts
near the 2K ft level. A generally quiet time at the airfields, but
included a LLWS mention at both sites in roughly the 04-08z time-
frame.

Main forcing/lift with the returning moisture now looks to move
rather quickly NE of the taf sites this evening, with a reduced
chance of SHRA/TSRA at the taf sites tonight as warming mid level
temps right behind the moisture keep rather strong capping over the
area. Did leave a VCTS/CB mention at KLSE 01-04z when it appears
SHRA/TSRA could fire near the MS river before moving into central WI.

&&

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

$$

SHORT TERM...DAS
LONG TERM....DAS
AVIATION.....RRS



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