380
FXUS63 KARX 041834
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
134 PM CDT Thu Jul 4 2024

.KEY MESSAGES...

- Moderate to High Confidence for Strong to Severe Storms Later
  This Afternoon & Early Evening.

- Scattered to Isolated Pop-Up Shower & Storm Chances Friday &
  Saturday.

- More Widespread Storm Chances For Sunday, Returning Monday
  Night and Tuesday Night.


&&

.DISCUSSION...
Issued at 1223 PM CDT Thu Jul 4 2024

Ongoing Storms & Precipitation This Morning & Afternoon:

Have been monitoring multiple rounds of storms through this
morning into the early afternoon with a quasi-closed upper-mid
level area of lower heights in the Northern Plains and multiple
surface frontal boundaries draped across the Upper Midwest. This
area of lower heights is also embedded in a negatively tilted,
amplified upper level longwave trough on GOES water vapor
imagery early this afternoon. As a result, return flow has been
advecting a more moist, more unstable airmass through the
Plains into the Upper Mississippi River Valley. Initial,
original synoptic forcing lifted through the Upper Mississippi
River Valley early this morning, leaving an occluded surface low
early this morning, placing the triple point to our west.
Further occlusion and closing of the tightened warm sector has
been firing off storms through western Minnesota early this
afternoon. In the wake of morning precipitation, clearing skies
seen on visible satellite imagery and are helping to build
diurnal instability.

Subsequent Forcing This Afternoon-Evening:

With a formation of a subsequent low along the triple point
with better synoptic forcing, quick-hitting deepening is
expected, causing subsequent storm and precipitation chances
later this afternoon and evening. The low is expected to trudge
east-southeastward along the baroclinic boundary from west to
east from central Minnesota into central Wisconsin, bringing the
once stationary cold/dry surface frontal boundary with it. The
passage of this frontal boundary will provide sufficient forcing
for strong to severe storms. Timing of peak diurnal heating
will be primary influence for location of highest impacts.

Strong To Severe Storms This Evening:

Confidence for strong to severe storms is moderate to high as
the low center impacts the local area and the cold/dry surface
frontal boundary provides sufficient subsequent forcing to
conditionally unstable, nearly straight 7.0C/km soundings in
upstream 04.12Z RAOBs. Overcoming any low level PBL inversion
has been observed within the upstream clearing skies, shifting
northeast early this afternoon through northern Iowa and
southern Minnesota. Sufficient multi level speed and directional
shear expected to sustain storms through the evening hours.
High resolution forecast model soundings and associated
hodographs suggest the potential for an initially discrete
storm mode, albeit transiently, quickly congealing cold pools
eventually making things messy before the mode becomes linear
along the frontal boundary as 850-300 mean wind is weak, 10 to
20 kts. Taking the expected cold/dry frontal boundary motion
into consideration, storm motion would be 20 to 30 kts to the
east.

Expected & Possible Hazards Into The Evening:

All in all, primary hazard of large hail with strong to
damaging winds possible. A quick spin up tornado cannot be ruled
out where the low level shear vector best aligns (normal to
line). Limiting factor will be initial storms potentially
consuming initial low level instability.

Shower & Storm Chances Friday:

As the low shifts east of the local forecast area, wrap around cold
air advection and tightened surface pressure gradient will result in
perpetual shower and storm chances through Friday. Increased
steepening of low level lapse rates concurrent with peak diurnal
heating will allow parcels to tap into a long, skinny SBCAPE
profile. The combination of transient saturation and little to
no shear will result any storms quickly popping up and down.
Could see some very limited opportunity for small hail given the
cooler sounding but no shear quickly terminates any updraft
sustenance, limiting overall chances and extent.

Shower & Storm Chances This Weekend:

The active pattern is expected to continue through the weekend.
Upper level heights are never able to build, persisting
synoptic troughing, perturbations, and precipitation chances.
Saturday storm chances seem similar to Friday with waves of low
level cold air advection within the upper level cold tongue,
wobbles in the upper level low, and peak diurnal heating will
cause scattered showers and isolated storms. A spatially
limiting factor will be a narrow tongue of increased
temperatures, increasing low level lapse rates and weakening any
incoming storms.

Through Start Of The New Week:

Slightly different story for Sunday as diurnal heating advects a
more moist airmass farther north into the Northern Plains. In turn,
a widespread area of diurnally building instability affects
much of Minnesota with long term global ensemble certainty for
500-1000 J/kg of SBCAPE taking shape. Have continued with
National Blend which has Likely chances, for Sunday Night,
Monday Night, and Tuesday Night.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1100 AM CDT Thu Jul 4 2024

CIGS: mid level cigs will lower with an area of convection expected
to sweep east across the area from mid afternoon into early evening.
Short term guidance further lowers the cigs into IFR/MVFR with the
passage of the cold front, holding there through Friday morning.

WX/vsby: shra/ts set to shift west to east across the region roughly
from 21-02z time frame. A variety of different forcers could adjust
this main threat time, some of which may not show their hand until
an hour before storms spark. Messy environment. Will sketch pcpn
chances out with where the higher confidence lies and adjust
forecast as trends necessitate. Looking for a break later this
evening before scattered to areas of -shra rotate in from the
northwest, post the outgoing storm system. This could linger through
the better part of Friday.

WINDS: mostly light southerly today but look for enhanced westerly
winds with gusts into the mid/upper 20 kts with any TS later this
afternoon. Winds will swing west/northwest with the passage of a
cold front this evening, picking up as we move into late morning
Fri.

&&

.HYDROLOGY...
Issued at 400 AM CDT Wed Jul 3 2024

We continue to monitor river trends with the recent heavy rains
with most rivers continuing to fall, however flood warnings
continue for the Mississippi River and the Kickapoo at Viola.
General rains of .5 to 1.5" are forecast with the greater
confidence along and north of I90. Some spots per the 04.00Z HREF
could see 2"+ with a 20-40% chance for Winona/Trempealeau Co.
6-hr flash flood guidance is generally 2 to 3 inches for flash
flooding. Cannot rule out localized flash flooding with the
storms later today and tonight if storms repeat over the same
area. Be weather aware with outdoor activities and if
traveling, be aware of your surroundings.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Rieck
HYDROLOGY...Zapotocny