Mesoscale Discussion 0871
NWS Storm Prediction Center Norman OK
1232 PM CDT Tue May 21 2024
Areas affected...Central/Eastern IA...South-Central/Southeast
MN...Far Southwest WI...Far Northwest IL
Concerning...Severe potential...Tornado Watch likely
Valid 211732Z - 211930Z
Probability of Watch Issuance...95 percent
SUMMARY...Environmental conditions are expected to becoming
increasingly supportive of a significant severe thunderstorms
capable of all severe hazards, including very large hail up to 3"
inches, strong gusts to 75 mph, and long-track tornadoes. A Tornado
Watch will likely be needed around 18-19Z, and a PDS designation is
being considered.
DISCUSSION...Recent surface analysis places a low over southeast NE,
with an outflow-modified warm front extending east-northeast across
southern IA through the IA/IL/WI border vicinity. This warm front is
expected to rapidly move northward in response to strong mass
response ahead of the approaching shortwave trough, with upper 60s
dewpoints likely in place across much of IA and mid 60s possibly
reaching southern MN. As this occurs, steep mid-level lapse rates
will also advect into the region, resulting strong buoyancy (i.e.
MLCAPE 2500-3000 J/kg) by the early afternoon. Wind fields are
expected to strengthen during this time frame as well, result in in
very long hodographs and fast storm motion. Bunkers right motion for
much of the region will be around 50 kt by the early afternoon. The
result will be an environment very supportive of significant severe
thunderstorms.
An initially discrete mode is anticipated, with the fast storm
motions allowing storms to stay ahead of the cold front impinging on
the region from the west. Any discrete storms will likely obtain
supercellular characteristics quickly, with very large hail up to 3"
inches, strong gusts to 75 mph, and tornadoes all possible. This
includes the possibility of long-lived, long-track
supercells capable of strong tornadoes and intense wind damage.
Storm interactions with the warm front, as well as the presence of
the deepening surface low, suggest the significant severe potential
will likely extend into south-central/southeast MI and southwest MN,
despite being displaced just north of the better mid-level flow and
low-level moisture.
Some upscale growth is anticipated as storms continue eastward, with
the resulting convective line capable of significant wind gusts
around 75 mph. Given the continued strengthening of the low-level
flow anticipated, some embedded QLCS circulations are likely as
well.
All of these factors suggests a significant severe weather event is
probable, and a Tornado Watch will likely be needed around 18-19Z. A
PDS designation is being considered for this watch.
..Mosier/Guyer.. 05/21/2024
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...MKX...DVN...ARX...MPX...DMX...EAX...FSD...OAX...
LAT...LON 40969085 40619167 40579233 40589388 40649446 40959493
41799502 43169492 44119422 44189139 43209038 41869013
40969085