701
FXUS63 KARX 021144
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
644 AM CDT Tue Jul 2 2024

.KEY MESSAGES...

- The expected first round of efficient rain-producing showers
  and storms are ongoing overnight and expected to continue
  through this morning. A second wave of showers and storms,
  mainly focused on the southern half of the CWA, is expected
  this afternoon into this evening. Heavy rain and associated
  flooding remains the main threat with totals across both
  rounds of 1-2" total with localized 4"+ amounts across the
  area on swollen waterways and wet soils. Northeastern Iowa is
  the area of relatively higher concern this afternoon and
  evening for flooding.

- July 4th has shower and thunderstorm chances increasing from
  west to east as the day progresses, with most areas seeing
  rain at some point during the afternoon and evening. Showers
  and thunderstorms may affect Independence Day outdoor
  activities.

- Unsettled weather into the weekend with periodic rain chances
  and below normal temperatures. Heavy rain and severe weather
  chances look minimal at this time.

&&

.DISCUSSION...
Issued at 400 AM CDT Tue Jul 2 2024

Monday Night into Tuesday Night: Heavy Rain Threat, Potential Severe

07z WV satellite shows an upper high over the ArkLaTex and upper
troughing over the central/northern Rockies, with resulting flow
helping to pump Gulf and some E Pacific moisture northeastward to
the central Plains and Upper Midwest. RAP-based SPC mesoanalysis
suggests a plume of 2"+ PWATs is present centered around E NE and W
IA. Ahead of a shortwave currently advancing northeastward over
Sioux City, an area of showers and thunderstorms developed in north
central IA and swept eastward, with heaviest rain totals of 1-2.5" -
mostly falling within a single hour - focusing in Mower-Floyd and
Fillmore-Chickasaw counties before the convection outran axis of
strongest elevated instability and rain rates reduced accordingly.
Expect the ongoing moderate rainfall to briefly clear out this
morning as the shortwave exits to the east.

As the upper trough continues to advance eastward, expect the PWATs
described above to be advected northeast to our CWA overnight into
this afternoon. GEFS/ENS mean values continue to rise above 2",
approaching climatological maximums, with the operational runs of
those ensembles` parent models suggesting PWATs will reach 2.25" in
NE IA and SE WI. Thus, this afternoon, as our next shortwave trough,
currently located over the Rockies and Sangre de Cristos, ejects
northeastward, expect an additional round of showers and
thunderstorms capable of generating rain rates over 2"/hr. Remaining
uncertainty centers around how far north - if at all - appreciable
instability develops in our forecast area for those potential
blockbuster rates to be fully realized. Given the propensity for
heavy thunderstorms to outrun where guidance suggested appreciable
(500+ J/kg) MUCAPE was located Monday night and the robust moisture
transport expected, am leaning toward more aggressive NAMNest
guidance which would place areas southeast of a Charles City to
Mauston line most at risk. That said, with less aggressive HRRR
guidance suggesting Fayette/Clayton Counties will be on the edge of
the axis of instability, have issued a Flood Watch for these
counties covering this afternoon/evening, with later expansion
possible should other guidance trend toward that aggressive solution.

As for severe thunderstorms, whether or not enough surface
destabilization occurs remains the all important question as
guidance continues to suggest 50 kts of sfc-6km shear will be
present as winds aloft increase ahead of the upper trough. While
HRRR/NAMNest continue to point toward best surface-based CAPE being
located to our south, cannot rule out severe thunderstorms capable
of all hazards - sfc-500m SRH could top 100 m2/s2, sufficient for a
tornado risk - in our far south, particularly if greater than
expected clearing of skies occurs today. Given the high amount of
uncertainty, still think heavy rain and flooding are the easily the
primary concern this afternoon and evening.

Independence Day and Friday: Additional Rain Likely

Guidance remains in good agreement that an upper low should slowly
churn east over SD to the Great Lakes, bringing the potential for
periodic showers and a few thunderstorms Independence Day and
Friday. While there remains a good deal of uncertainty regarding
timing, latest trends perhaps suggest rain may hold off until the
evening of the 4th, keeping rain off parades but ruining fireworks.
LREF has trended upward with instability probabilities with now a 20
to 45 percent chance for at least 750 J/kg of SBCAPE, so there may
be a few atmospheric fireworks (thunderstorms) at least. In fact,
given the increasing southwesterly winds ahead of the upper low,
can`t totally write off the chance for a strong to severe storm, but
the probability appears low (LREF joint probabilities for sufficient
instability and shear are generally under 30%).

Saturday through Monday: More Rain Possible

With cyclonic flow over CA and the northern CONUS remaining in
place, expect an additional upper trough to advance over the Upper
Midwest Saturday into Sunday. This may kick off additional
convection. At this time, the probability for heavy rainfall and
severe thunderstorms appears low as robust low level moist advection
looks to be displaced to our south and east. That said,
thunderstorms could occur each afternoon given that the probability
of SBCAPE building to greater than 500 J/kg is around 50% or higher
each afternoon.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 644 AM CDT Tue Jul 2 2024

MVFR/IFR cigs will be the initial TAF concern this morning as a
lower stratus deck moving in this afternoon with the NBH probs
showing modest probabilities (40-60% chance) for IFR cigs across at
both KLSE and KRST late this morning with observations upstream in
north/central MN verifying this to some degree. These cigs are
expected to persist into the early afternoon ahead of an incoming
weather disturbance from the west. As it approaches, showers and
storms will overspread much of the region during the evening hours
with occasional dips to MVFR/IFR visbys in more intense storms.
Conditions will improve to VFR from west to east during the
overnight hours. Winds will begin the TAF period from the south at
around 10-15 kts before diminishing and to around 5-10 kts and
switching to westerly behind the aforementioned weather system.

&&

.HYDROLOGY...
Issued at 258 PM CDT Mon Jul 1 2024

he Mississippi River will remain in flood stage through at least
the next week and likely into mid-July as additional precipitation
is expected to fall across the Upper Mississippi River basin. The
bulk of this precipitation is expected to fall tonight through
Tuesday night and again Thursday evening into Friday. Exact crests
at each location are dependent on which Mississippi tributary
receives the rainfall. Tonights rainfall is expected to fall
along and near the Black, Root, Cedar, Upper Iowa, and Turkey
River basins. All of these rivers flow into the Mississippi
River at different locations, which brings uncertainty to
Mississippi River crests around Genoa southward.

With this additional precipitation, the Hydrologic Ensemble Forecast
Service (HEFS) suggests a roughly 20% chance of Minor Flooding at
Black River Falls, which would take roughly 2.25 to 2.5 inches of
rain through Tuesday night. The Cedar River at Charles City has a 10
and 15% chance of Major and Moderate Flooding respectively. The
rainfall amounts that drive these chances range from 3 inches
(Moderate) to 4 inches (Major); however, the latest HREF
probabilities show only an 11% chance of receiving 2.00 inches of
precipitation, so the HEFS probabilities may be a touch too high.

As for flash flooding, the current thinking is that the moisture
transport and precipitation tonight will prime the atmosphere and
soils for a higher flash flooding threat with the second round
of precipitation Tuesday/Tuesday night. The area with the most
precipitation forecast remains along the Clayton and Grant
county border across NE IA and SW WI. Communities whose
stormwater drains into the Mississippi will be more susceptible
to urban flash flooding as storm drains may have a reduced
capacity.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...Flood Watch from 3 PM CDT this afternoon through late tonight
     for IAZ029-030.

&&

$$

DISCUSSION...Ferguson
AVIATION...Naylor
HYDROLOGY...JAW