Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 280720
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
220 AM CDT Wed Jul 28 2021

.SHORT TERM...(Today into Thursday)
Issued at 220 AM CDT Wed Jul 28 2021

Very active day in store for the area...potentially dangerously
so...with heat and severe storm concerns. Significant threats that
should not be taken lightly.

MORNING STORM CHANCES: most meso models point to a weak ripple in
the upper level flow sliding across the local forecast area between
15-19z, coupled with some low/mid level warming. With decent
instability already in place to work on, scattered showers and
storms will have a decent shot to spark as the shortwave moves
through - mostly elevated in nature. Not expecting anything strong
at this time, let alone severe. Quick mover too, with not a lot of
qpf.

EVENING STORM CHANCES - SEVERE THREAT: stronger shortwave trough
dropping southeast from northern MN and across eastern WI this
evening. Associated sfc low follows suit with a warm front running
east of it and a cold front then slipping southeast across the
forecast area late evening/overnight. Low level jet ramps up by 00z,
pointing into northeast/eastern WI as the shortwaves sinks across
the region. Models focus most of the convection with theLONG
TERM...(Wednesday night through Tuesday) Issued at 302 PM CDT Tue
Jul 27 2021 shortwave, along/north of the warm front and where the
moisture transport peaks. This generally keeps convection across
northern/eastern parts of the Badger state - locally mostly east of
I-94 corridor. Not much convection suggested as you move westward
along the cold front (capping vs convergence battle).

Bottom-line upfront, there WILL be severe weather today. How much
occurs in the local forecast just depends on the track of the
shortwave/storm clusters. Very fat CAPE profiles in Bufkit soundings
with SBCAPES in excess of 4000 J/kg. Once the storms start, they
will escalate very quickly...zero to severe in 1/2 hour? Wind shear
is ample with 0-6km from 50 to 60 kts. Long hodographs with some
turning in the 0-1 km layer. Organization won`t be a problem. Expect
supercells to morph into line/bowing segments. With strong 0-3km
shear of +40 kts, the storms could eventually morph into a long
lived wind bringer as it drives across southeast WI, continuing
east/southeast. For further details, check out the latest Day 1
discussions from SPC.

Severe Threats: All severe types are on the table, and super sized.
Hail upwards of 2 inches should be expected. Winds in excess of 70
mph certainly possibly, although maybe more to the east. There will
be a tornadic threat, especially in the vicinity of that warm front.
STP values in the SPC HRRR peak around 4 for the evening while 0-1KM
SRH climb to around 200 m2/s2 for central/eastern WI.

Lastly, the atmosphere remains very condusive for heavy rain (warm
cloud depths around 4 kft and PWS upwards of 2 inches). Not
expecting a flash flooding threat at this time, but locally heavy
rains could result in urban street flooding with rapid rises in some
creeks/other low lying areas.

Timing/Location: pretty good agreement between the variety of short
term/CAMS models for development to start across northern MN by 18z,
dropping toward Taylor co, WI by 00z. The storms reach the I-90
corridor by 03z, exiting southeast by approx 06z. How far westward
storms would extend along the cold front is questionable with low
level cap remaining in place into mid evening. The I-94  corridor is
still the area of greatest concern locally.

HEAT: still looking like a hot/steamy day...dangerously so. RAP
925mb temps progged to creep above +30 C from the Mississippi river
westward this afternoon. GFS not as enthusiastic with a few degrees
cooler. The NAM lies somewhere in between. RAP looks a tad
aggressive, but the end result still looks to be highs in the low to
mid 90s. Caveat on cloud cover/potential showers for later this
morning, but should see a good amount of sun to the west for the
afternoon. A pool of mid 70s dewpoints already across southwest
MN/western IA, expected to spread eastward through the day. Heat
Indices into the lower 100s still look like a good bet, especially
in the west. Advisory already in effect and no changes currently
anticipated.

Cooler and less humid air works in post the cold front, bringing the
region some relief from the sticky conditions.

.LONG TERM...(Thursday night thru Tuesday)
Issued at 220 AM CDT Wed Jul 28 2021

Forecast period will see an amplification in upper flow patternLONG
TERM...(Wednesday night through Tuesday) Issued at 302 PM CDT Tue
Jul 27 2021 , especially ridge in western U.S. setting up more of a
northerly flow across region, including cooler and less humid
airmass.

With subtle height falls taking place main boundary will be shifted
well south and west of the area for later in the week but will still
need to watch for impulses in northwest flow, especially as mean
trough digs out in east.

Medium range guidance suggests stronger upper low and related short
wave troughs around it could impact western Great Lakes late
Saturday into Sunday with rain threat but this too will push even
drier and cooler air in suggesting dry trends going into early next
week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1125 PM CDT Tue Jul 27 2021

Cigs: mostly high based sct-bkn vfr expected through the period.

WX/vsby: current convection expected to be south of the TAF sites,
but a weak impulse a loft could spark shra/ts roughly between 15-19z
that could impact the TAF sites. Nothing vigorous per latest CAMS
models. Will add VCTS to cover potential. Strong/severe cluster of
storms will then develop across northern WI in the afternoon,
dropping southeast. Current trends in the meso models could keep
this activity east of the TAF sites. Something to keep a close eye
on though.

Winds: light southeast early picking up a tad and shifting to the
south in the morning. Looking to hold there into the evening,
shifting northwest overnight with the passage of a cold front.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Heat Advisory from 10 AM to 9 PM CDT today for WIZ032>034-
     041-042-053>055-061.

MN...Heat Advisory from 10 AM to 9 PM CDT today for MNZ079-
     086>088-094>096.

IA...Heat Advisory from 10 AM to 9 PM CDT today for IAZ008>011-
     018-019-029-030.

&&

$$

SHORT TERM...Rieck
LONG TERM....Shea
AVIATION.....Rieck


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