Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 212337
AFDARX

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

.UPDATED...
Issued at 630 PM CDT Wed Jun 21 2017

Storm threat continues to shift a bit more northeast and later into
the evening/overnight per latest HRRR/Hopwrf model runs. Current
convection tied to the daytime instability and warm front just south
of the local area. This should weaken/die out with loss of that
instability. Expected convection later tonight should be driven by
the nose of the LLJ/850 mb moisture transport, working across the
advancing northeast warm front. Some instability above the
stabilizing boundary layer to play with, and hail/heavy rains still
a possiblity. But...this threat may be more confined to the I-94
corridor northeastward...and after midnight.

Have been refining pcpn chances tonight, moving toward this trend.
Will continue to refine as the evolution becomes more apparent.

In addition, some of the meso models hint that the western portions
of the front and/or another weak sfc boundary will hold across
northeast IA/southwest WI early Thu morning, and with persisting
moisture transport along it, could spark a few showers/storms. Going
to continue some small pops in those areas with that in mind.

.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 00Z TAFS through 00Z Thursday evening)
Issued at 630 PM CDT Wed Jun 21 2017

Cigs: Scattered high clouds should/may become a bkn mid level deck
later this evening, with expected convection developing mostly north-
east of the TAF sites. Could end up just being scattered if
convection is delayed and/or farther northeast. Eitherway, looks VFR
into Thu morning.

Cold front slated to drop across the region late morning/afternoon
Thu. 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: shra/ts threat continues to shift more northeast of the TAF
sites per latest meso model runs, and will keep KRST/KLSE dry for
now. SHRA/TS looking more likely by Thu afternoon as front pushes in
from the west. Expect some vsby restrictions with any mod/strong
storms - all will be capable of heavy rain. Going to go with VCTS
for now, refining forecast as timing becomes clearer.

Winds:  some LLWS concerns for a few hours tonight, mostly in the 05-
10z window, as the low level jet pushes overhead. Sfc direction
will stay generally southerly until the passage of the cold front
Thu afternoon - going northwest to north.

&&

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

$$

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



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