-- Discussion containing changed information from previous version are highlighted. --
540
FXUS63 KARX 202041
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
341 PM CDT Thu Jun 20 2024

.KEY MESSAGES...

-- Changed Discussion --
- Multiple rounds of heavy rain are expected Friday morning
  through at least Saturday morning. How the bands line up will
  determine the local flooding threat. Confidence is lower on
  exactly how the later storms Friday night into Saturday
  afternoon evolve.

- A few storms Friday and Saturday may be severe, with damaging
  winds the main threat.

- A brief respite from the active pattern for Sunday and Monday,
  but more storms loom for Monday night into Tuesday.
  Unfortunately, severe storms cannot be ruled out for Monday
  night into Tuesday.
-- End Changed Discussion --

&&

.DISCUSSION...
-- Changed Discussion --
Issued at 338 PM CDT Thu Jun 20 2024

This Afternoon into Tonight: Decreasing Potential for Thunderstorms

Largely as expected, widespread cloudiness across much of the CWA
has limited surface insolation, with latest RAP guidance suggesting
less than 250 J/kg of MLCAPE is present and, with winds out of the
east, am not expecting robust enough low level warm advection to
change this. Have therefore continued to limit mentions of thunder
to 30 percent or less. Additionally, with 18z WV satellite showing
the CWA is largely between two shortwaves - one southeast of the CWA
and the second looking to pass just to our northwest, much of the
area looks to remain free of precip entirely, with the exception of
areas along an Albert Lea MN to Medford WI axis, where the close
approach of the northwestern shortwave should be enough for at least
a few showers. In either case, am not too concerned about excessive
rainfall with any showers this afternoon and evening given PWAT
values are generally under 1.65" and 700/850mb moisture transport is
modest.

Friday through Saturday: Heavy Rain, Flooding, and Severe Storm
Potential

This evening, moisture transport increases markedly over the central
High Plains ahead of an advancing upper trough. As a shortwave
ejects over NE, an MCS should develop and translate east-
northeastward along a northward lifting warm front to SD and
eventually S MN by Friday morning. As for severe thunderstorms, this
morning MCS could pose an isolated damaging wind threat in S MN if
it remains sufficiently organized. Finally, flood threat with this
initial rounds appears to be low as 700/850mb moisture transport
remains modest for this initial round.

Friday afternoon and evening, low level moist advection should help
MLCAPE build to 1000-1500 J/kg as the front, forced southward by the
morning MCS, pushes back northward. While 0-6km bulk shear values
are generally 35 knots or less, this would be sufficient for perhaps
a bit of organized convection. While hodographs are not overly
impressive, if enough surface destabilization can occur, there would
be at least a small probability for all hazards with damaging winds
having the higher relative risk. The primary concern with this
activity will be heavy rainfall and flooding as 700/850mb moisture
transport will increase in time as the main upper trough advances
eastward over the central Plains and becomes oriented out of the
southwest, somewhat along the expected east-west axis of the front.
PWAT values look to top 2 inches, with these values exceeding the
99th percentile in both the NAEFS and ENS climatology. Thus,
expect efficient rain producers with some areas potentially affected
by training storms. Given this and the saturated soils from abundant
recent rainfall, have issued a Flood Watch beginning Friday
afternoon.

Main uncertainty Friday afternoon focuses on how far north the low
level warm advection pushes the front after it is forced southward
by outflow from the morning MCS. Non-CAM guidance suggests the
boundary may return to a position just northwest of our CWA while
many CAMs have trended toward a solution roughly around Interstate
90. While all guidance tends to struggle with the effects of cold
pools, am inclined to learn toward the CAM solutions as they tend to
do a bit better. Have therefore trended axis of highest QPF to the
south and this adjustment may not yet be enough. Second uncertainty
focuses on the timing of the morning MCS - should this arrive
sufficiently late, this round could carry a higher risk for severe
wind as the morning inversion breaks. Additionally, this would delay
airmass recovery and onset of the second round of storms.

Moving ahead to Saturday morning, our third round of convection
looks to occur as the main upper trough finally approaches. As moist
advection continues, PWAT values approach NAEFS/ENS climatological
maximums and training risk increases as 850mb moisture transport
becomes more oriented along the surface boundary. Have therefore
kept the Flood Watch going through midday Saturday.

The already high uncertainty in details increases markedly by
Saturday afternoon. Should the previous three rounds of convection
occur largely as envisioned, the boundary may very well be shunted
southeast of the CWA for good. Given this uncertainty, have ended
the Flood Watch early Saturday morning. If the boundary remains in
our CWA, would need to extend this Watch in time. Finally, if the
boundary remains in our CWA, destabilization to the south of this
feature Saturday afternoon would lead to another period with severe
thunderstorm potential. Given the modest increase in deep shear as
mid to upper level flow ramps up ahead of the upper trough, all
severe hazards could be in play in the southeastern third of the CWA
if this fourth round manifests itself.

Sunday through Wednesday: Brief Break from Storms, then Active
Weather Returns

In the wake of Saturday`s upper trough, northwesterly flow aloft
begins to dominate proceedings. With conditions at 700/850mb drying
out considerably, most areas should remain free of rain Sunday, with
the exception of north central Wisconsin due to a passing upper
wave. Next wave Monday night into Tuesday may bring additional
rainfall with CSU ML severe outlooks suggesting at least some risk
for severe thunderstorms during this period. Admittedly had little
time to closely examine this given the abundance of active weather
in the short term but will need to keep an eye on Monday and
Tuesday.
-- End Changed Discussion --

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1248 PM CDT Thu Jun 20 2024

Main taf concerns are MVFR conditions this afternoon...then
showers/storms and the potential for IFR conditions after 09z
Friday. Ceilings have lowered into MVFR late this morning at LSE
taf site and RST is expected to lower into MVFR by middle of the
this afternoon as low stratus slowly moves over the area. These
conditions will continue through tonight at both RST/LSE taf
sites. Then...next system will bring showers/storms into the
region and lower the ceilings into IFR...especially at RST...
after 09z Friday. Underneath the stronger
showers/storms...visibility could drop briefly to IFR at both
taf sites. Have introduced IFR conditions at RST after 09z
Friday and continued with MVFR at LSE through the taf period.

&&

.HYDROLOGY...
Issued at 400 AM CDT Thu Jun 20 2024

Multiple rounds of heavy rain are forecast for Friday morning
through Saturday afternoon. Total QPF values may locally exceed
5-6 inches with more widespread amounts of 2-4 inches becoming
increasingly likely. Given the breaks between each round of
storms, any major flash flood threat will occur where the axes
of heavy rain overlap. These rains on top of already saturated
soils will result in river flooding concerns along the
Mississippi River and its tributaries. Many locations along the
Mississippi River will exceed flood stage towards next weekend
(60-90% chance) if rainfall unfolds as forecast. Should the
heavy rain line up over the smaller river basins, there is a
10-20 percent chance that these basins could see periods of
flooding at moderate flood stage.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WI...Flood Watch from Friday afternoon through Saturday afternoon
     for WIZ032.
MN...Flood Watch from Friday afternoon through Saturday afternoon
     for MNZ079-086>088-094>096.
IA...Flood Watch from Friday afternoon through Saturday afternoon
     for IAZ008>011-018-019.
-- End Changed Discussion --

&&

$$

DISCUSSION...Ferguson
AVIATION...DTJ
HYDROLOGY...Skow