Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 220436
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1130 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 06Z TAFS through 06Z Thursday night)
Issued at 1130 PM CDT Wed Jun 21 2017

Cigs: mid/high level clouds across taf sites, and should hold into
thu morning.

Cold front slated to drop across the region late Thu morning. Should
see lowing cloud deck with the boundary, and maybe a dip into mvfr
especially along and post of the front. Likely see a clearing out of
any low cigs by the overnight.

WX/vsby: first, convection firing off over northwest IA, making
progress eastward. Region of low level convergence and favorable
instability mostly responsible for the shra/ts, but also a hint of
ripple in the upper level flow lending a hand. Unclear whether these
storms will hold together as they move east. Radar trends currently
showing a decaying trend, but the influence of the upper level
shortwave could keep them going - and a potential impact for KRST.
For the moment, going to keep KRST dry, but will monitor closely.

A couple other focuses for shra/ts threat for KRST and KLSE. First
is a cold front slipping across the area late Thu morning/afternoon.
Meso models now developing convection a bit earlier than some
previous runs, and have an increased threat between 15-18z, which
likely continues into the early afternoon. Could see a few hour
break before the next focus - upper level shortwave and elevated
frontogenetic region - brings another band of shra/ts across the
area. Without a lot of confidence in timing and placement of the
main bands, keeping with vcts and won`t get too cute with timing.
Anticipate updates with tempo groups providing more clarity. There
will be vsby reductions with the showers/storms - all capable of
heavy rain.

Winds:  some LLWS concerns for the overnight, mostly through 10z, as
the low level jet pushes overhead. Sfc direction will stay generally
southerly until the passage of the cold front late Thu
morning/afternoon - going northwest to north.

&&

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

$$

SHORT TERM...DAS
LONG TERM....DAS
AVIATION.....Rieck



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