Mesoscale Precipitation Discussion 0567
NWS Weather Prediction Center College Park MD
1114 PM EDT Sat Jul 23, 2022

Areas affected...Southeast MN...Southern WI...Northeast IA... Far
Northern IL...

Concerning...Heavy rainfall...Flash flooding likely

Valid 240315Z - 240900Z

SUMMARY...Increasingly more efficient heavy rainfall complex with
embedded training cells pose possible flash flooding through early
overnight.  Rates up to 2"/hr and broad swath of 2-4", perhaps an
isolated 5" amount is probable. 

DISCUSSION...GOES-WV suite depicts a weak shortwave ridge
associated with a 3H speed max and weakly diffluent flow
downstream crossing S MN into WI.  Thunderstorms are already
starting to blossom along and just north of an W-E oriented
surface warm front generally parallel to the MN/IA border before
crossing southern WI in the wake of the initial MCS. 
Strengthening but broad WSW LLJ is responding to the shortwave and
upper-level UVV providing strong advection and eventual speed
convergence through/along the front zone and downstream upglide. 
Strong convergence and axis of recharging/untapped 3000+ J/kg CAPE
axis will support strong updrafts and moisture flux convergence of
1.9-2" TPW to support thunderstorms with capability of sub-hourly
rates of .5-.75"/15 minutes; however, translation speeds will
limit duration of any specific cell as zonal flow is strong at
35-40kts.   As such, training/repeating cells will be critical for
producing excessive rainfall.   As mentioned, the orientation of
the front/convergence axis is expected to remain fairly flat to
the mean flow supporting a favorable axis for training and likely
to support a few hours along the axis for totals of 2-4" to occur.

Southward propagation due to cold pool generation and weak
500-1000 thickness divergence downstream into S WI/N IL remains
the greatest uncertainty at this time.  If southward propagation
is stronger, surface based/most efficient cells should track south
of due east and reduce duration for extreme totals... while best
forcing/convergence axis will slide northward with best upglide
over the front and present a training of elevated cells across
central WI from LSE to OSH.  As such, have broadened the eastern
side of the area of concern to encompass this uncertainty.  00z
Hi-Res CAMs denote this potential spread though there is some
increase in agreement toward less southward propagation.  Either
way, the rates/totals are in the range of the lowered FFG values
due to initial complex and as such scattered incidents of flash
flooding are considered likely through early overnight hours.


Boone-McHenry-Lake IL-De Kalb-Kane-DuPage-Kendall-Grundy-Kankakee-
Northern Cook-Central Cook-Southern Cook-Northern Will-Southern Will-
Eastern Will-Lake IN-Porter-
Including the cities of La Grange, Carol Stream, Calumet City,
Chicago, Portage, Kankakee, Crystal Lake, Wheaton, Woodstock,
Palatine, Beecher, Schaumburg, McHenry, Downers Grove, DeKalb,
Belvidere, Chesterton, Merrillville, Northbrook, Lombard, Oak Lawn,
Gurnee, Manhattan, Aurora, Hammond, Waukegan, Bourbonnais, Lemont,
Plano, Channahon, Buffalo Grove, Gary, Valparaiso, Bolingbrook,
Sycamore, Wilmington, Minooka, Morris, Algonquin, Evanston, Peotone,
Joliet, Oak Park, Cicero, Park Forest, Elgin, Des Plaines, Orland
Park, Mokena, Oswego, Oak Forest, Plainfield, Yorkville, Bradley,
Mundelein, Coal City, Crete, and Naperville
228 PM CDT Sat Jul 23, 2022

...FLOOD WATCH IN EFFECT FROM 7 PM CDT THIS EVENING THROUGH SUNDAY
MORNING...

* WHAT...Flash flooding caused by excessive rainfall is possible.

* WHERE...Portions of Illinois and northwest Indiana, including the
  following areas, in Illinois, Boone, Cook, De Kalb, DuPage,
  Grundy, Kane, Kankakee, Kendall, Lake, McHenry, and Will. In
  northwest Indiana, Lake and Porter.

* WHEN...From 7 PM CDT this evening through Sunday morning.

* IMPACTS...Excessive runoff may result in flooding of rivers,
  creeks, streams, and other low-lying and flood-prone locations.
  Creeks and streams may rise out of their banks. Flooding may occur
  in poor drainage and urban areas.

* ADDITIONAL DETAILS...
  - Thunderstorms with high rainfall rates are expected this
    evening, with additional thunderstorms lingering across much
    of the watch area into Sunday morning. The high rainfall
    rates over saturated soils will lead to localized street
    flooding, runoff into already elevated streams, creeks and
    rivers, and renewed inundation of low-lying areas.
  
PRECAUTIONARY/PREPAREDNESS ACTIONS...

A Flood Watch for flash flooding means rapid-onset flooding is
possible, but not yet certain, based upon the latest forecasts.
Flash flooding is a dangerous situation. Persons with interests
along area rivers, creeks, and other waterways should monitor the
latest forecasts and be prepared to take action should flooding
develop.