Mesoscale Precipitation Discussion 0775
NWS Weather Prediction Center College Park MD
429 PM EDT Wed Sep 05 2018

Areas affected...North-Central Kentucky, Southwest Ohio

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

Valid 052028Z - 060000Z

Summary...A concentrated cluster of thunderstorms has developed in
north-central Kentucky just north of Lexington, and was nearly
stationary. This cluster of storms is likely to produce heavy
rainfall and flash flooding. Elsewhere across southwest Ohio and
northern Kentucky, slow-moving thunderstorms will be possible, but
may be more isolated. Flash flooding cannot be ruled out, but may
be less likely than the area immediately north of Lexington.

Discussion...A small cluster of thunderstorms has developed and
quickly consolidated in the area between Lexington and Maysville
in north-central Kentucky. These storms were developing in a
subtle axis of higher boundary layer mixing ratios and thetae
values, and thus an axis of higher PWATs as well. CIRA blended TPW
product indicated PWATs around 1.8 to 1.9 inches in the area, and
several GPS-PW observations near Louisville and Cincinnati showed
values approaching 2 inches. These were near record values for
early-mid September near the central Ohio River valley. Individual
cells in the thunderstorm cluster were showing chaotic motions,
but the common characteristic is that they were all moving slowly,
and a large collision was taking place around 2030Z near Harrison,
Nicholas and Robertson Counties. Flash flooding appears likely in
this area where KLVX dual pol estimates of rain rates are as high
as 2-3 in/hr and MRMS estimates are as high as 2 in/hr. Given the
slow storm motions, the heavy rain could persist over an hour, and
a quick 3-4 inches of rainfall appears possible.

There was a weak confluence to the boundary layer winds in the
PWAT axis, but overall there is not much strong forcing to focus
convective activity, or to sustain it for long periods of time.
Therefore, any convective organization should be driven
predominately by storm-scale processes and interactions, such as
backbuilding near an expanding cold pool. This could sustain the
convective cluster in north-central Kentucky for a couple hours.
However, the lack of strong forcing should generally keep
convection elsewhere isolated and less organized. Localized heavy
rainfall and flash flooding cannot be ruled out in other parts of
SW OH and NC KY, but it would be less likely than the portion of
KY just north of Lexington.

Lamers

ATTN...WFO...ILN...JKL...LMK...

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

LAT...LON   39698407 39388300 38558327 37808392 37618484
            38238511 38898488