Mesoscale Precipitation Discussion 1313
NWS Weather Prediction Center College Park MD
823 AM EDT Thu Sep 17 2026

Areas affected...northern IL/IN into northwestern OH

Concerning...Heavy rainfall...Flash flooding possible

Valid 171222Z - 171730Z

SUMMARY...Some areas flash flooding may linger across northern
IL/IN into northwestern OH through 17Z with hourly rainfall of 1
to 2+ inches. Additional storm total rainfall is expected to peak
near 3 inches on an isolated basis.

DISCUSSION...1210Z radar imagery showed a WNW to ESE axis of
thunderstorms containing heavy rain extending across northern
IL/IN with MRMS-derived and gauge confirmed hourly rainfall
between 1 and 2 inches (locally higher). However, embedded
portions of this axis have produced 15-minute rainfall of 0.5 to
near 1.0 inches, owing to the efficient nature of the airmass with
precipitable water values of 2.0 to 2.3 inches. Additional, more
scattered thunderstorm coverage, was noted farther east from
eastern IN into western OH with occasional embedded repeating and
training. The areas of heavy rain were located north of a
quasi-stationary front that extended eastward across IL/IN/OH with
20-30 kt of 925 mb flow over northern MO/IL, helping to drive
isentropic ascent and an axis of elevated convergence to the north
of the boundary. Steering flow was oriented parallel to the low
level axis of convergence, allowing for training within a zone of
roughly 500-1000 J/kg MLCAPE (via SPC mesoanalysis data from 12Z).

Short term forecasts from the RAP show that as a 925-850 mb low
over the MN/IA border shifts east, and an eastern TN high
translates west through 18Z, the low level flow across the Midwest
will veer to more of a westerly direction while also weakening
with the typical diurnal cycle. This should reduce the amount of
overrunning of the stationary front and coincide with a reduction
in thunderstorm/heavy rainfall coverage across IL/IN/OH with the
axis of convection shifting east over time. Until then however,
the potential will remain for additional areas of training with
hourly rainfall of 1 to 2+ inches and additional storm totals near
3 inches on an isolated basis.

Otto

ATTN...WFO...CLE...DVN...ILN...ILX...IND...IWX...LOT...

ATTN...RFC...MSR...TIR...NWC...

LAT...LON 42318993 42198836 41838611 41318341 40738235
40188222 39918304 40368591 40598766 41208973
41779053