Mesoscale Precipitation Discussion 0491
NWS Weather Prediction Center College Park MD
753 PM EDT Sat Jun 22 2024

Areas affected...southern WI...eastern IA...north-central IL

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

Valid 230000Z - 230600Z

Summary...Hourly accumulations of 1-3" likely to lead to numerous
instances of flash flooding, localized totals of 3-6". Some
significant, life threatening flash flooding is possible,
particularly in southern WI and surrounding portions of IA/IL
(where the greatest coverage of flash flooding is expected).

Discussion...A mesoscale convective system (MCS) is gaining
organization and intensity over southwest WI and northeast IA late
this afternoon. There has been a notable decrease in cloud top
temperatures (via GOES-East longwave infrared imagery) over the
past couple of hours, as overshooting tops have cooled an
additional -10deg C (from -65deg C to -75deg C). In tandem, MRMS
instantaneous rates have increased to the 4-6"/hr range, resulting
in hourly estimated totals of 1-2" (as convection is quite
progressive, reducing the residence time of these extreme
instantaneous rates). The mesoscale environment ahead of the
intensifying MCS is characterized by SBCAPE of 1000-3000 J/kg,
preciptiable water values of 1.9-2.2" (at or above the max moving
average and rivaling daily record values at DVN, per SPC sounding
climatology), and effective bulk shear of 35-45 kts (aided by the
right-entrance region of a 100 kt jet streak @ 250 mb over Lake
Michigan, providing large scale lift as well). This highly
favorable environment for deep, organized convection should
continue to support the MCS into the evening/overnight hours
(further supported by MCS maintenance probabilities of 60-80% and
derecho composite parameter values of 2-6).

While the progressive nature of the MCS should largely limit the
residence time of 1-3"/hr accumulations (as the 850-300 mb mean
flow is 40+ kts), the concern is that a 2-4 hour period of
training/repeating along the southwest flank of the MCS (where a
strong southwesterly low-level jet is providing strong moisture
transport/convergence for backbuilding) will result in localized
totals of 3-6" (as indicated by both the HREF PMM/neighborhood
exceedance probabilities and subsequent hourly runs of the HRRR).
While there is relatively good agreement between the CAMs in the
resultant QPF amounts, there is not as good agreement in the
placement of those amounts (anywhere from southeastern WI through
north-central IL and southwestern IA). Some recent HRRR runs (with
the 21z run being the best example) depict the highest swath
across southeastern WI, and this seems to be the most realistic
scenario based on the latest observational trends (largely due
easterly motion with the deep layer wind). That said, the HRRR
seems to be underdoing the overall intensity and coverage of the
convection, and a stronger, more consolidated cold pool would
likely allow for more significant upwind propagation (which would
result in a southerly component in the storm motion, bringing a
greater threat to northern IL). In addition, the upwind
propagation vector favors a slower storm motion (near 20 kts),
which combined with backbuilding could result in a more
significant flash flood scenario (more likely to achieve the
high-end 6" localized totals). Even without the
backbuilding/training, Flash Flood Guidance (FFG) mostly ranges
from 1-2" (over a 1-3 hour period), suggesting that numerous
instances of flash flooding are likely (with some localized
significant flash flooding possible).

Churchill

ATTN...WFO...ARX...DMX...DVN...ILX...LOT...MKX...

ATTN...RFC...NCRFC...NWC...

LAT...LON   43768840 43388752 41388765 40588865 40228987
            40469175 41029274 41979341 43089133 43458992