145
FXUS63 KARX 020001
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
701 PM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Two waves of showers and storms are expected overnight and
  again later Tuesday into Tuesday evening. It still looks like
  the main threat is heavy rain of 1-2" total /with localized
  3"/ across the area on swollen waterways and wet soils.

- July 4th has shower and thunderstorm chances increasing from
  west to east as the day progresses with evening fireworks in
  question. The timing and coverage can still change, but it
  looks like most will get wet in the afternoon to evening.

- 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 258 PM CDT Mon Jul 1 2024

Two Waves of Rainfall into Tuesday Night and Renewed Flooding
Risk

GOES water vapor at 1830Z is showing the trough axis from the
morning IA convection now on an FSD-DVN line with radar echos
diminishing as they head east but the core remaining just west
of DSM. This also matches the low-level moisture gradient with
very dry air to the northeast. The second more vigorous
shortwave trough is really visible coming through WY which will
act to really fire up the low-level return flow in concert with
the low-level ridge shifting east at ~1025mb. The air mass is
still quite dry over the area with dewpoints in the 40s, while
the 70 dewpoint line is still in KS. GOES precipitable water is
depicting an incredible 1.8-2.2 inch tongue through eastern KS
with the 01.12Z Topeka KS soundings showing 2.14 inches. WOW.

This evening the low-level jet ramps up with 45-50 kts at 850 mb
with strong moisture transport convergence ramping upward
isentropically along its trajectory from DSM->EAU. MUCAPEs
increase into the 600-900 J/Kg range near and south of I-90 in
concert with the forcing, so SHRA/TSRA should erupt in the
later evening as the airmass transitions to more moist. The very
tropical air mass previously described to the south doesnt
fully engage over the area until later in the forcing period
tonight toward pre-sunrise. Thus, while it does look favorable
for heavy rain with warm cloud depths just below 4km, the very
high precipitable water is in place for wave #2 for Tuesday and
Tuesday evening. Amounts tonight look to be near an inch with
only a 5-20% chance of 2"+ from the 01.12Z HREF as it remains
progressive shifting east in the morning.

After a mid-day break which the CAMS are all in great agreement
on, stronger forcing again shifts in ahead of another Pacific
NW trough shortwave trough ejecting out of WY toward the area
in the afternoon. While questions remain on exactly the recovery
that occurs and where the CAPE pool will evolve northward too,
the air mass in place ahead of the wave is now tropical with
near record precipitable water. With preconditioning tonight,
soils will be primed for flash flooding with super-efficient
rain rates. By mid-afternoon, a cold front should be on about
I-35 with the main CAPE pool south of the area but 750-1000
J/Kg suggested across swrn WI and nern IA and along the cold
front. This would bump rates and cause some flash flood concern
as moisture transport convergence ramps up and another round of
convective storms erupts in the afternoon. The most intense
rainfall rates would be closer to the instability pool which the
CAMS currently disagree on for placement. The latest HRRR runs
suggest swrn WI would be in play for strong storms, while others
suggest the strongest storms are south of the area.

Have considered a Flood Watch internally and with NWS neighbors,
but think tonights rain will be tolerable and a pre-
conditioning for Tuesdays second wave. Plan at this time is to
wait on a watch and continue to assess the Tuesday ingredients.
There could be some strong storms around with maybe a gust to
40-50 mph and small hail, but rain is the main threat with
soils/rivers. See Hydrology section below for more. Also of note
is the strong low-level jet in place tonight which could cause
wind gusts of 30-40 mph west and north of La Crosse.

July 4th Looking Wet

After a dry and mostly sunny day Wednesday, yet another weather
system is shifting in Thursday. While some differences on the timing
details exist, the big picture is that shower and storm chances
increase through the day as a moderately strong trough over the
Dakotas moves in with moderate 300-500mb QVector convergence
shifting over the area later afternoon and then evening into WI.
There is still quite a bit of spread on the details of how the near-
surface trough and instability evolve Thursday with a wide range of
SBCAPE suggested by 7 pm Thursday /200-1000 J/Kg 25-75th
percentile/. Also, the trough placement and orientation is
critical to the wind shear profile. Should a more NW->SE trough
occur, southeast low-level winds would favor a bit better wind
shear. But the majority of guidance would suggest the lowest 3
km of wind shear appears to be weak in the 01.00Z Grand
Ensemble, with instability leveraged into showers and generic
storms spreading in from west to east across the area during the
late afternoon and evening. 12-hour rain amounts through
midnight look to average around 0.25". Rain chances have been
increased with this forecast.

Unsettled into the Weekend

There is excellent agreement on modest amplitude, longwave troughing
over the northcentral U.S from Friday through Monday. Friday would
feature a lingering low-level trough axis across the area which,
should there be any clearing and instability increase, would be the
focal point for storms. 01.00Z LREF suggests a 25% chance of
SBCAPE values above 700 J/Kg so there isnt a great deal of
instability to work with. While the 01.00Z Grand Ensemble is in
agreement that high rain chances exist on that trough axis,
noticeable gradients exist on its north and south sides
suggesting the location will drive precipitation chances. Should
that location of the trough shift, Friday could turn out drier
for some. Shortwave ridging builds in overnight into Saturday
morning.

An unsettled northwest flow with less predictable timing and
shortwave trough energy takes over through Monday. Each day looks to
have some chance of showers or thunder. Heavy rainfall is not
favored in this pattern and shower and storm chances are tied to
both trough passage and diurnal heating. With details hard to
extract, the forecast for the Saturday through Monday is splashed
with 30-50% rain chances. These days should have plenty of dry hours
though. Temperatures through the period should be slightly
below normal.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 643 PM CDT Mon Jul 1 2024

First round of storms will develop and should move into the TAF
sites towards 03-05Z with widespread TSRA expected overnight as
a low level jet transports abundant moisture into the region.
Kept TEMPO MVFR to IFR visibility and ceilings as the storm
complex pushes eastward through daybreak. Thereafter, confidence
is increasing for more widespread/persistent MVFR to IFR
ceilings over the TAF sites through Tuesday morning, though daytime
heating is likely to raise ceilings slightly in the afternoon.

Second round of convection on Tuesday will be more scattered and
remains trickier to pin down trends and location. There should
be a brief lull once the overnight convection moves out, but
spotty showers/storms may be present at any point in the day.
Some additional instability will work into the region by midday
to support more storm development in the afternoon. Have just
gone VCTS for this given lower confidence.

Winds will increase out of the southeast tonight as the pressure
gradient tightens with gusts overnight into Tuesday morning of
22-27 knots. Winds relax a bit and become southerly in the
afternoon.

&&

.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...None.

&&

$$

DISCUSSION...Baumgardt
AVIATION...Kurz
HYDROLOGY...JAW