Mesoscale Precipitation Discussion 0807
NWS Weather Prediction Center College Park MD
714 PM EDT Sat Sep 08 2018

Areas affected...Srn Indiana, Nrn Kentucky, Ctrl/Srn Ohio; Wrn
West Virginia

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

Valid 082311Z - 090500Z

Summary...Widespread flooding is expected to continue into the
evening in the Lower Ohio River Valley, and should begin to expand
east through northern Kentucky and into the southern half of Ohio.
Moderate to occasionally heavy rain is expected over the entire
region, with locally very heavy rain rates and flash flooding
expected in the vicinity of the Ohio River border between Indiana
and Kentucky.

Discussion...The area that has received storm total rainfall in
excess of 3 inches continues to expand, and now extends from south
of St. Louis, east through southern Illinois, and into much of
southwest Indiana. The highest amounts in that region are more
than double that, per radar estimates and reports. Widespread
areal flooding is expected to continue in these regions, and
similar lingering flooding is expected even after rain ends in the
rest of southern Indiana, and perhaps small portions of
north-central Kentucky.

A surface low was situated in far southwest Indiana, and
convective rain bands and thunderstorms were ongoing up to 120
miles to the east and east-northeast near a surface front. This
convection had access to increasing boundary layer inflow, and was
likely beginning to intercept this inflow and northward moisture
flux. Therefore, the heaviest rain during the evening hours could
actually end up south of where most of the latest hi-res models
place it (including recent HRRR runs and the 18Z HREF). Many
hi-res models want to place the heaviest rain further north into
central Indiana along the 850mb front and deformation zone, but
that seems less likely given the current radar trends. At 23Z,
KLVX radar showed a supercell structure over Meade County, KY that
was producing extremely high estimated rain rates (dual pol and
MRMS) of 2-3 in/hr. ZDR was relatively low in the supercell other
than in the immediate vicinity of the mesocyclone (due to drop
size sorting), and this is suggestive of very efficient rain
rates. The supercell was headed toward Louisville metro area, and
trailing convective bands were becoming increasingly channeled
into a training pattern to the rear of the supercell. These
convective bands also showed ZDR values indicative of numerous
small water droplets. Therefore, the concern is for a training
band of convection with highly efficient rainfall pushing into a
large metro area. This could produce severe flash flooding with
extreme rain rates and over an inch of rain already today.

The greatest concern for flash flooding, therefore, is from
northeast of Owensboro, into Louisville Metro, and continuing into
north-central Kentucky during the evening hours. The High Risk on
the Day 1 Excessive Rainfall Outlook has been adjusted to fit
these expectations. Further to the north, steadier rain is
expected. Portions of central Indiana and southwest Ohio have
already received rainfall around 2-3 inches of rain in the past 24
hours, so even lower rain rates could lead to development or
expansion of ongoing flooding issues. Therefore, widespread
flooding is anticipated over the southern half of Indiana,
northern Kentucky, and into southwest Ohio. Additional flooding
may develop further east into the remainder of southern Ohio and
parts of West Virginia as the 850mb southerly flow increases and
expands further east during the evening hours. Additional
isentropic ascent and stronger low-level convergence could lead to
an increase in rain coverage and intensity in those areas,
especially after 00-01Z.

Lamers

ATTN...WFO...CLE...ILN...ILX...IND...IWX...JKL...LMK...PAH...
PBZ...RLX...

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

LAT...LON   40468468 40468293 40398079 39128057 38558173
            38118305 37598406 37818527 37478662 37728779
            38368809 39668741 40218634