212
FXUS63 KARX 021148
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
648 AM CDT Sun Jun 2 2024

.KEY MESSAGES...

- Dense fog this morning will erode quickly with the June sun
  angle. Then, a nice day!

- Two rounds of showers and storms will affect the area late
  tonight into Monday and Tuesday again. Rainfall amounts look
  to be in the 1-2" range for most places. River rises should
  be expected, with some minor flooding on rivers possible.

- Tuesday afternoon and evening still look on track to bring a
  stronger system through with at least a small chance of severe
  storms.

&&

.DISCUSSION...
Issued at 347 AM CDT Sun Jun 2 2024

This Morning`s Fog

GOES low cloud imagery showing fog continuing to expand through
WI mainly at 08Z. Development overnight has been more like a
classic river valley radiational fog event with the river
valleys preferred and other areas then follow. Will continue to
monitor the fog development for any changes to the advisory area this
morning.

Tonight and Monday`s Shower and Storm Chances

GOES water vapor loops upstream over the western CONUS show
trough energy shifting through the intermountain region but
quite disorganized without one hot area of concentration. Two
areas draw out slightly in the imagery...near Glacier Nat Park
in MT and another wave in NV. Both are players in the forecast
for tonight and although the northern wave had 250mb height
falls of 60m/12h in ern WA, the NV wave consistently is being
convectively enhanced on its way to the local area per CAM
guidance.

Instability is racing northward tonight with widespread 30-35kt
850mb winds in the radar VWPs over the Dakotas and Neb with
1000 J/Kg of MUCAPE now into swrn ND and ern MT. The 0 MUCAPE
line is near or west of I-35 at 07Z. Small convective areas are
present on the eastern 850 mb moisture transport convergence
gradient from ABR to west of OMA. As the northern MT shortwave
trough shifts east today, surface low pressure near the MT/WY
border will continue shift east, with the entire pattern, and
then weaken and dissipate per RAP forecasts. By 00Z, the MUCAPE
gradient is on about I-35 along with the eastern gradient of
the moisture transport convergence suggesting that activity
should be held west of the area or diminishing rapidly as it
shifts east of I-35. Thus, kept rain chances low toward I-35
late today. There are plenty of questions how the overnight
details in the storms/showers will evolve with the latest
02.06Z HRRR suggesting an early evening system affecting IA/MN.
Overnight the CAPE is slow to shift east and the consensus CAM
output suggests waning convection from the northwest, with a
convectively enhanced wave /current NV wave/ shifting across IA
and into WI for the main rain chances by Monday morning. This
wave in IA will enhance the warm advection and moisture
transport toward Monday morning, leveraging the moist pool over
IA and steered into WI by the mid-level flow. It appears the
wave will be into eastern WI by peak heating which could cause a
minor severe storm threat east of the area Monday. 02.00Z HREF
shows some updraft helicity tracks in srn WI. Will have to
monitor this timing closely as any slower timing would pull a
threat back into swrn or central WI.

Precipitable water values climb to about 1.5" so rainfall rates
will be good provided the intensity of the systems moving
through are decent. Coverage of storms is still of lower
confidence as the system shifts east with a discomfort hanging
my hat on a convectively enhanced wave for higher /80%/ rain
chances, thus have backed down a bit on those in WI.

Tuesday`s Severe Storm Potential

Not much has changed in the weather system forecast for Tuesday,
although a slight southern track to the dynamic lift and wave
track has been noted over the last 24 hours. The instability
pool present on Tuesday will be good with ~1500 J/KG of ML CAPE
to ~I-90. Even though a moderately strong trough is positioned
in the eastern Dakotas late Tuesday, the diffluent flow east of
the trough causes quite "meh" mid-level flow and deep layer
shear in the warm sector. Even the 0-3km shear is marginal
/10-20kts/. Thus, it appears storms will initiate on a stronger
cold front near I-35, quickly becoming a linear non- severe
system shifting east. Some pulse severe hail storms are possible
Tuesday later afternoon initially but current indications are a
low threat of a more organized severe weather event. SPC has
placed IA/MN and northwest WI in a marginal risk for Tuesday
which seems appropriate. There may be a small area of higher
risk right on the front during the initiation period. Better
storm chances appear to be over southern IA where the wind shear
may be better.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 700 AM CDT Sun Jun 2 2024

Fog, primarily in the river valleys, continues to lift with
diurnal heating. Taking slightly longer in the valleys given the
higher density overnight. Have opted for one additional hour of
1/4 SM at KLSE at 02.12Z TAF issuance. Upstream observations in
northeast Iowa and southeast Minnesota have mostly lifted to
MVFR visibilities so will be something to keep an eye on given
aforementioned VFR forecast at 02.13Z. Other challenge is
incoming stratus deck potentially limiting initial diurnal
influence. Therefore, low confidence in timing for valley fog
specifically at KLSE. May need to be extended 1 to 2 more hours.

VFR conditions will return later this morning with increased
winds primarily west of the Mississippi River. Flight
restrictions return early Monday morning with subsequent
precipitation and storm chances. With a west to east trajectory,
have included probability at KRST TAF without mention at KLSE.
Expect updates with increased confidence in exact timing with
future forecasts.

&&

.HYDROLOGY...
Issued at 347 AM CDT Sun Jun 2 2024

Inspection of some of the Hydrological Ensemble Forecast System
Output at the river forecast points in the area suggest that
about 3-4" of rainfall would be needed to reach minor flooding
on most of the area rivers. There are a few that are a bit more
susceptible to lower rainfall amounts to attain minor flooding:
the Turkey, Cedar, Lemonweir and the Yellow in WI (already
elevated).

Forecast rainfall will be refined as we get closer to the
Tuesday evening rainfall but at this time it seems 1-2" total
seems reasonable for these progressive systems. Thus, believe
these two systems will mainly lead to within bank rises.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 9 AM CDT this morning for WIZ017-029-
     032>034-041>044-053>055-061.
MN...Dense Fog Advisory until 9 AM CDT this morning for MNZ088-095-
     096.
IA...Dense Fog Advisory until 9 AM CDT this morning for IAZ010-011-
     029-030.

&&

$$

DISCUSSION...Baumgardt
AVIATION...JAR
HYDROLOGY...Baumgardt