Heavy rain potential setting up for portions of northern Indiana, well east of the Chicago Metro area

Mesoscale Precipitation Discussion 0749
NWS Weather Prediction Center College Park MD
620 PM EDT Mon Aug 12 2019

Areas affected...Northern IN...Northwest OH...Far Southern Lower
MI

Concerning...Heavy rainfall...Flash flooding possible

Valid 122220Z - 130220Z

SUMMARY...Some brief training of heavy showers and thunderstorms
will be possible this evening which may pose a short-term flash
flood threat.



DISCUSSION...A narrow axis of heavy showers and a few
thunderstorms is expected this evening across portions of northern
IN and northwest OH. The convection is expected to form in
vicinity of a weak surface trough downstream of an approaching MCV
which is currently moving through northern IL. The thermodynamic
environment is modest with generally less than 1000 j/kg of MLCAPE
across the MPD threat area, but there is some relatively favorable
shear profiles with effective bulk shear on the order of 30 to 40
kts. Meanwhile, the column is quite moist with PWATs on the order
of 1.9 to 2.1 inches per the latest GPS-derived data sets.

An examination of the shower activity that has been developing
upstream across areas of northern IL and adjacent to Lake Michigan
over the last couple of hours suggests an environment of
warm-topped/shallow convection. Given this and the high PWAT
environment, warm-rain processes are likely in place and
suggestive of very efficient convection for enhanced rainfall
rates. The latest radar imagery is showing some subtle cooling of
the cloud tops with a west/east axis of showers across northern IN
currently which appears to be developing near a weak surface
trough.

The 12Z HREF suite of guidance generally appears to be too far
north with its axis of heavier rainfall as compared to the latest
radar trends. However, several runs of the experimental HRRR were
doing a bit better, and the last available run from 18Z supported
as much as 2 to 3 inches of rain through 03Z with isolated heavier
amounts. The general axis is expected to along and adjacent the
surface trough axis as mentioned across northern IN and over into
northwest OH. Far southern lower MI may also see some of this
activity.

Despite rather high FFG values, the warm-rain processes and
tropical moisture environment pose at least a localized concern
for some excessive rainfall. Additionally, there is some signal
for at least some episodic training/repeating of convective cells
which may result in enhanced rainfall totals. A couple instances
of flash flooding as a result cannot be ruled out.
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