Mesoscale Precipitation Discussion 0794
NWS Weather Prediction Center College Park MD
421 PM EDT Fri Sep 07 2018

Areas affected...NE MO, C IL, C/S IN, SW OH, N KY

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

Valid 071953Z - 080123Z

Summary...Bands of heavy rain are likely to continue into the
evening hours from northeast Missouri, east into Illinois,
Indiana, and southwest Ohio and northern Kentucky. Rain rates of
1-2 in/hr will be common in these heavy rain bands, and they may
be nearly stationary at times. This could produce locally heavy
rainfall in excess of 3 inches. Some areas of flooding are likely
to result from this heavy rain.

Discussion...A stationary front continues to be draped from the
Plains into the Ohio Valley, and has moved very little over the
course of the day. The front did show perhaps some slight slope to
the north with height, with the 850mb front (as suggested by RAP
analysis and radar VWP data) aligned with most of the heavy rain
bands and developing convection at 20Z. The environment across the
region continues to be characterized by deep moisture.
Precipitable water values are around 2 inches in most of the
highlighted area per CIRA blended TPW product, and these values
are generally above the 95th percentile for September in this
region. RAP soundings along the length of the front show saturated
or nearly saturated profiles through most of the depth of the
troposphere, and tall, narrow CAPE profiles. Both are
characteristic of an environment that will favor warm rain
processes in convective bands that are shallower in depth.

One such rain band was located from just southwest of Hannibal MO,
to near Springfield IL, to just south of Danville IL. Most of the
35-40+ dBZ reflectivity was concentrated below 15,000 feet, which
was around the freezing level per RAP soundings. The KILX dual pol
and MRMS rainfall estimates were substantially underestimating
observed rainfall. For example, KDEC reported 1.15 inches of rain
in an hour ending around 20Z, while KILX dual pol suggested around
0.6 inches, and MRMS suggested around 0.7 inches. Similar low bias
was observed at surrounding observation sites as well. Dual pol
radar parameters confirmed that warm rain processes were likely
dominating in this convective band, with ZDR near or less than 1
dB, indicative of numerous small, more spherical rain droplets.
This narrow band was showing a slight drift but was nearly
stationary, and could produce a significant swath of rainfall in
central Illinois with flash flooding likely. Recent runs of the
HRRR suggest this band could persist another few hours.

Further to the east, in portions of Indiana, Ohio and Kentucky,
convection was more scattered, but was also slow-moving, locked in
the low-mid level deformation zone along the front. Given the
shallow nature of the convection, it was being steered more by the
flow in the lowest 6km, which was generally less than 10 knots.
The 12Z HREF shows a notable increase in the probability of heavy
rain rates (1-2 in/hr) in the next few hours in south-central
Indiana. Some instances of flash flooding are also likely in this
region further to the east, but it may not necessarily manifest as
a continuous, focused rain band as is being observed in central
Illinois.

Lamers

ATTN...WFO...DVN...EAX...ILN...ILX...IND...JKL...LMK...LOT...
LSX...PAH...RLX...

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

LAT...LON   40498837 40158611 40258420 39568331 38368322
            38258501 38448694 38998827 39298945 39219102
            39029208 39559254 40309110