621
FXUS63 KARX 120414
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1114 PM CDT Tue Jun 11 2024

.KEY MESSAGES...

- Isolated thunderstorms possible this afternoon, some of which
  could produce small hail and gusty winds.

- Complex severe weather setup for Wednesday afternoon and
  evening with the potential for multiple rounds of storms.
  Large hail, damaging winds, heavy rainfall and possibly a few
  tornadoes are all on the table depending on how storms unfold.

- Warmer weather on the docket for early next week with highs
  pushing into the upper 80s to low 90s from Sunday through
  Tuesday.

&&

.UPDATE...
Issued at 1004 PM CDT Tue Jun 11 2024

Challenging forecast for tomorrow on synoptics and mesoscale
evolution and the severe storm threat.

Things are still on track with the thinking of a moisture surge
into western MN tonight with storms erupting in the pre-dawn
hours near the ND/MN/SD triple point. Moisture transport
convergence and the instability is really keying on this area in
the RAP and latest 12.00Z CAM solutions. We are still firmly
entrenched in the pacific air mass overhead right now with low
dewpoints and stable conditions. During the morning hours,
moisture transport in the low-levels and instability continues
to surge northeast across MN, but the rate is in question.
Based on the timing of the pre-dawn storms and their southeast
motion, the instability would seem to favor a more southerly
track, with storms propagating into the instability. All of the
12.00Z CAMS want to track some convective system toward the
area arriving by late morning, with questionable instability,
mainly elevated, but a ton of warm air and moisture advection
into the area. But still, as the complex moves southeast,
possibly some severe, it is outrunning instability. And this
timing could kill the afternoon severe weather chances by
limiting heating. The 12.00Z HRRR delays the complex just an
hour or two into the forecast area from the northwest and has
the strongest outcome of afternoon storms. I think that speaks
to the sensitivity of when these morning storms/showers arrive
and if they continue to develop into severe storms or not. Later
favors severe storms. The earlier arrival seems to favor a low
severe weather chance overall with heating and moisture
advection impacted. Either way, the first system seems to really
seems to really cap off and stabilize the atmosphere. The CAMS
also suggest generally that there is probably only one
morning/afternoon system/shot.

Overall, the new 12.00Z CAMS seem to produce fewer waves of
storms from 18-09Z overall. One pattern we need to watch for is
a NW->SE oriented boundary setting up west of a strong
convective complex and cold pool. The orientation of the low-
level flow and moisture advection into that boundary orientation
would favor a training scenario with storms moving southeast
along the boundary.

So, still details to workout. Will be working to increase the
later morning storm chances in southeast MN after 15Z per the
accelerated CAMS into that area. Soundings and instability arent
the greatest for severe weather that early (as stated above per
later HRRR).

&&

.DISCUSSION...
Issued at 250 PM CDT Tue Jun 11 2024

This Afternoon and Evening: Isolated Storms

Current radar mosaic this early afternoon shows this morning`s
showers continuing to exit eastward out of the local forecast area.
GOES satellite imagery and RAP progs show an upper level trough
continuing to move towards the region, with surface observations
showing an approaching (currently draped down through central MN)
surface cold front. As the front moves across the area, model
guidance continues to suggest the potential for isolated to widely
scattered showers and storms to develop this afternoon/evening.
However, there still remains some uncertainty in just how much
instability will be able to build ahead of the front.

Hi-res guidance would suggest the potential for a very narrow
axis of instability with CAPE values generally around/under 1000
J/kg. If storms do develop, stronger storms may be capable of
producing small to marginally severe hail and gusty winds.
However, as we lose daytime heating the storm threat will
quickly decrease. There is still quite a bit of variability in
solutions on location/timing of initiation of storms, so will be
monitoring near-term satellite and radar trends closely through
the afternoon and evening.

Wednesday Afternoon and Evening: Severe Weather Details

A zonal pattern becomes established for tomorrow with a
seasonable strong 120-kt jet working over the Dakotas in the
wake of subtle perturbation in the flow. The surface boundary
sliding southeastward this evening quickly washes out overnight
with southerly flow becoming reestablished up through Minnesota
by sunrise tomorrow. Steepening mid-level lapse rates and
increasing surface theta-e values result in SB/MLCAPE values
approaching 2000 J/kg by the mid-afternoon while elongated
hodograph will be supportive of organized storm structures.

The main question revolves around convective mode and timing.
The latest 12Z HREF members are rather locked in on having
elevated convection develop on the nose of a weak low-level jet
towards sunrise in western Minnesota, with this complex and its
convective debris/cold pool serving as the focus for renewed
development during the early afternoon hours in central/eastern
Minnesota and western Wisconsin. The hodographs would be
supportive of supercellular structures and all storm hazards,
though the explicit composite reflectivity progs show rapid
upscale growth along the line into an MCS that propagates SSE
with the Corfidi vectors during the afternoon.

However, with the main synoptic forcing still off to the west,
renewed development is likely along the western flank of this
MCS cold pool that would result in multiple rounds of
thunderstorms training over the same area throughout the late
afternoon and evening hours along a northwest to southeast axis.
This would point to a potential heavy rain and flooding threat
somewhere in south-central Minnesota to northeast Iowa, but it
remains too early to pin down the exact area as such a threat
will rely heavily on how storms behave earlier in the day.

Hodographs become quite favorable for tornadoes as we move
through the evening hours, but the main question will revolve
around the surface airmass thermal characteristics given the
passage of the MCS earlier in the day. Any areas that do manage
to recover may realize the over 500 m2/s2 of 0-1-km SRH and pose
a tornado threat. At this stage, given the trajectory of the
afternoon MCS, such a threat might be limited to south-central
Minnesota and north-central Iowa.

Thursday and Friday: Cooler and Drier

Northwesterly flow settles in for the latter part of the week
in the wake of our midweek storms and holds temperatures near
seasonal norms for this time of year. With low level moisture
slow to depart during the day on Thursday, steepening low-level
lapse rates may result in diurnal showers developing during the
day, with the showers sliding southeastward as drier air filters
south during the afternoon. Friday is shaping up to be the
coolest and driest day of the forecast period as a 1020-mb
surface high pressure slides through, but highs should still top
out near 80 degrees.

Saturday and Sunday: Next Round of Stronger Storms

Return flow builds in behind the departing ridge early Saturday,
with surface dewpoints rebounding into the 60s by Saturday
afternoon. The medium range solutions are in modest agreement
with a sharp shortwave impinging on the backside of the
synoptic ridge and thus we should see strong to severe storms
somewhere across the Upper Midwest, but enough differences exist
in the placement of the surface baroclinic zone make it
difficult to nail down any further specifics on threats and
locations. How convection unfolds on Saturday and the timing of
the shortwave passage will play a role in the threat area for
Sunday, though the southwesterly flow is strong enough that the
warm sector may have time to recover on Sunday and result in
repeated rounds of storms each day. Stay tuned for more details.

Next Week: Warming Up

Upper ridging amplifies over the central and eastern CONUS for
much of next week, resulting in southwesterly flow across the
region and with it the first real surge of summertime heat.
There exists some modest synoptic differences with the various
ensemble members with respect to the strength of the ridge,
with the more amplified solutions resulting in warmer and drier
conditions whereas the flatter solutions bring periodic bouts of
thunderstorms and cooler temperatures as shortwave
perturbations ripple northeastward over the Dakotas. The
resultant precipitation forecast contains a smattering of 20 to
40 percent probabilities for the end of the forecast period, but
these will be honed in over the coming days as the strength of
the ridge is better analyzed. What is more certain is that we
will see above average temperatures with over 70 percent of the
available guidance pushing the mid-80s for highs and 20 to 30
percent of the members exceeding 90 degrees.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1115 PM CDT Tue Jun 11 2024

Forecast confidence after 15Z Wednesday is very low. A complex
weather scenario unfolds for Wednesday with abundant moisture
and instability, for thunderstorms, streaming into MN. This
should cause some later morning showers to the northwest of the
area, potentially a few storms depending on the instability
location. Since the previous forecast (00Z TAFS), high-res
models are suggesting a convective system approaches southeast
MN by late morning, shifting southeast. The forecast at the TAF
site is mainly VFR, with periods of storms causing the
potential for lower MVFR. With confidence lower, have decided to
keep the forecast about the same with an afternoon period of
storms in the forecast. However, this is likely to change as we
see thunderstorms form overnight near Aberdeen SD and move
southeast. Thus, a changeable forecast for Wednesday should be
expected with this lower predictability system.

&&

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

&&

$$

UPDATE...Baumgardt
DISCUSSION...Skow
AVIATION...Baumgardt