Mesoscale Precipitation Discussion 0905
NWS Weather Prediction Center College Park MD
318 PM EDT Fri Sep 27 2019

Areas affected...Central Illinois

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

Valid 271916Z - 272230Z

Summary...Ongoing flash flooding will continue. Thunderstorms have
concentrated in a corridor from Peoria, to Pontiac, to Kankakee in
central Illinois and are producing rain rates in excess of 2
inches per hour in some places. A band of 3-6 inches of rainfall
has already occurred from Peoria to Pontiac, and thunderstorms
with heavy rain continue. Flash flooding is occurring and likely
to continue this afternoon. Significant flash flooding will be
possible.

Discussion...A more focused band of thunderstorms has developed in
central Illinois, aligned closer to parallel with the deep layer
mean flow. This band of storms has produced a radar estimated
swath of 3-6 inches of rainfall per KILX estimates, and KPNT
reported 2.14 inches of rain in an hour in the 18-19Z period. KILX
dual pol KDP values are above 2 deg/km in many of the strongest
convective clusters, an indicator of large concentrations of
water. This is supported by the observed rainfall rates on the
ground. The thunderstorm intensity has generally increased in the
last hour or two as the storms have coalesced in a linear fashion,
and clear regions have developed just south on visible satellite.
Instability has been increasing in this inflow region, with
surface dewpoints close to 70 degrees and temperatures pushing
into the upper 80s. Increasingly unstable inflow with PW values
approaching 1.8 inches (per GPS estimates) has likely aided in
pushing maximum rain rates over 2 in/hr. The 850mb low-level jet
remains quite strong near the western flank of the convective line
and just upstream, and is at least in balance with (if not
stronger than) the deep layer mean flow. This continues to be a
setup that favors backbuilding, and may sustain new upshear
development for another hour or so, prolonging the heavy rainfall
in central Illinois into the mid-afternoon.

Given the potential for further significant rain rates near or in
excess of 2 in/hr on top of areas that have already experienced
several inches of rain, flash flooding is likely. The greatest
concern is in the corridor from Peoria to Pontiac and surrounding
communities, although this may build east toward Kankakee with
time. Significant flash flooding will be possible, with deeper
inundation in some areas, if the rain coincides with city and town
centers that have a greater percentage of impervious surfaces. The
CREST hydrologic model has shown a considerable runoff response as
of 19Z that is consistent with significant, life-threatening flash
flooding.

Lamers

ATTN...WFO...ILX...LOT...

ATTN...RFC...NCRFC...OHRFC...

LAT...LON   41248801 41008746 40678805 40448915 40379010
            40779030 41098909