Severe Center
Mesoscale Discussion WPC #1339 for Heavy Rainfall
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Area of Concern - Severe T-Storm
Area of Concern - Winter
Area of Concern - Heavy Rainfall
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Mesoscale Precipitation Discussion 1339
NWS Weather Prediction Center College Park MD
105 AM EDT Sun Sep 20 2026
Areas affected...Central and Eastern IA...Northern IL...Northern
IN
Concerning...Heavy rainfall...Flash flooding likely
Valid 200505Z - 201105Z
SUMMARY...Expanding and locally training clusters of heavy showers
and thunderstorms will drive a major increase in the threat for
flash flooding overnight, and some of this may be locally
significant.
DISCUSSION...The latest GOES-E IR satellite imagery shows
cold-topped convective clusters expanding in coverage across
southern and eastern IA along with portions of north-central IL as
a wave of low pressure and associated shortwave impulse lifts
east-northeast across the region.
Increasing warm air advection and moisture transport ahead of this
energy will set the stage for a more expansive convective
footprint over the next several hours as convection expands in
coverage. Much of the activity should tend to be focused along and
north of a quasi-stationary front extending east from the low
center across eastern IA and down through central IL and southern
IN.
However, this front will attempt to lift north in time as a warm
front as the nose of a southwest low-level jet strengthens to 30
to 40+ kts, and this will allow for convection in time to impact
areas of northern IL and northern IN in conjunction with the
corridor of strongest isentropic ascent.
MUCAPE values of 1000 to 2000 J/kg are already in place across the
region, and with PWs of 1.75 to 2.0 inches coinciding with a
strengthening low-level jet, the rainfall rates are likely to be
quite high. Some rainfall rates based on the 00Z HREF/REFS suite
of guidance are forecast to increase to 1.5 to 2.5 inches/hour.
Gradually there will be concerns for cell-training as the
convection becomes aligned in a more west to east fashion near and
to the north of the front. Cell-training concerns will drive
potential for localized swaths of 3 to 5+ inch rainfall totals
going through dawn and that is supported by the 00Z hires CAM
consensus.
The 3-hour FFG exceedance probabilities from the 00Z HREF/REFS
guidance are very high across eastern IA through northern IL. This
is an area that has seen heavy rainfall recently and has a very
moist antecedent footprint with elevated soil moisture and high
streamflows. This along with low FFGs overall suggest a high
likelihood for flash flooding overnight and extending through
early Sunday morning. Locally significant flash flooding impacts
will be possible and this will include concerns for the more
sensitive urban environments.
Orrison
ATTN...WFO...ARX...DMX...DVN...ILX...IND...IWX...LOT...MKX...
ATTN...RFC...MSR...TIR...NWC...
LAT...LON 42578945 42438788 42208674 41908588 41538506
40958487 40578514 40498564 40498685 40528864
40639090 40879267 41359352 41909343 42269274
42509138
NWS Weather Prediction Center College Park MD
105 AM EDT Sun Sep 20 2026
Areas affected...Central and Eastern IA...Northern IL...Northern
IN
Concerning...Heavy rainfall...Flash flooding likely
Valid 200505Z - 201105Z
SUMMARY...Expanding and locally training clusters of heavy showers
and thunderstorms will drive a major increase in the threat for
flash flooding overnight, and some of this may be locally
significant.
DISCUSSION...The latest GOES-E IR satellite imagery shows
cold-topped convective clusters expanding in coverage across
southern and eastern IA along with portions of north-central IL as
a wave of low pressure and associated shortwave impulse lifts
east-northeast across the region.
Increasing warm air advection and moisture transport ahead of this
energy will set the stage for a more expansive convective
footprint over the next several hours as convection expands in
coverage. Much of the activity should tend to be focused along and
north of a quasi-stationary front extending east from the low
center across eastern IA and down through central IL and southern
IN.
However, this front will attempt to lift north in time as a warm
front as the nose of a southwest low-level jet strengthens to 30
to 40+ kts, and this will allow for convection in time to impact
areas of northern IL and northern IN in conjunction with the
corridor of strongest isentropic ascent.
MUCAPE values of 1000 to 2000 J/kg are already in place across the
region, and with PWs of 1.75 to 2.0 inches coinciding with a
strengthening low-level jet, the rainfall rates are likely to be
quite high. Some rainfall rates based on the 00Z HREF/REFS suite
of guidance are forecast to increase to 1.5 to 2.5 inches/hour.
Gradually there will be concerns for cell-training as the
convection becomes aligned in a more west to east fashion near and
to the north of the front. Cell-training concerns will drive
potential for localized swaths of 3 to 5+ inch rainfall totals
going through dawn and that is supported by the 00Z hires CAM
consensus.
The 3-hour FFG exceedance probabilities from the 00Z HREF/REFS
guidance are very high across eastern IA through northern IL. This
is an area that has seen heavy rainfall recently and has a very
moist antecedent footprint with elevated soil moisture and high
streamflows. This along with low FFGs overall suggest a high
likelihood for flash flooding overnight and extending through
early Sunday morning. Locally significant flash flooding impacts
will be possible and this will include concerns for the more
sensitive urban environments.
Orrison
ATTN...WFO...ARX...DMX...DVN...ILX...IND...IWX...LOT...MKX...
ATTN...RFC...MSR...TIR...NWC...
LAT...LON 42578945 42438788 42208674 41908588 41538506
40958487 40578514 40498564 40498685 40528864
40639090 40879267 41359352 41909343 42269274
42509138