Mesoscale Precipitation Discussion 0770
NWS Weather Prediction Center College Park MD
245 PM EDT Wed Jul 31 2024

Areas affected...southern Indiana, central Kentucky, eastern
Tennessee

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

Valid 311844Z - 010030Z

Summary...Showers and thunderstorms will increase ahead of a
shortwave and train to the southeast into this evening. Rainfall
rates of at least 2"/hr are likely at times, resulting in
additional rainfall of 2-3" with locally higher amounts. Flash
flooding is likely.

Discussion...The GOES-E visible satellite imagery this afternoon
shows rapidly deepening Cu/TCu aligned NW to SE from far southeast
IN into southeast KY. This is reflective of deepening convection
ahead of a residual convective MCV noted in radar and satellite
dropping into southern IN, and the regional radar mosaic shows a
rapid expanse in reflectivity showing that these updrafts are
rapidly transitioning into showers and thunderstorms. The
environment downstream of the MCV is quite impressive and
favorable for thunderstorms, with PWs of 1.7-2.0 inches collocated
with MLCAPE as high as 3000 J/kg. There exists a tight instability
gradient from SW to NE as well, suggesting that convection will
dive along this boundary to the southeast as it organizes into
multicells or another MCS in response to 30-35 kts of bulk shear.

Rainfall rates so far have been somewhat limited to less than
1"/hr in current activity, but this is more a response to just
being in the early stages of development rather than being within
a limited environment. The HREF neighborhood probabilities suggest
a 15-20% chance for 2"/hr rates this evening, while the HRRR
15-min rainfall product indicates brief 3-4"/hr rates are
possible. A recent polar orbiter pass and NUCAPS sounding over
central KY indicates an impressive pre-MCS environment with PWs
around 1.7 inches and freezing levels of 14,000 ft with modest
mid-level lapse rates to support efficient warm-rain processes, so
these rainfall rate predictions seem reasonable. Mean 850-300mb
winds are forecast to be progressive at 15-25 kts to the
southeast, but aligned and collapsed Corfidi vectors suggest cells
will likely regenerate and train, at least until the MCS organizes
and sweeps through to the southeast. Areas that receive an overlap
of slow moving pre-MCS storms, and the MCS, could see 2-3" of rain
with locally higher amounts as shown by HREF 3"/6hr neighborhood
probabilities of 15-25%.

Most concerning about this setup is that these heavy rain rates,
even from individual cells, will be falling atop pre-conditioned
soils. MRMS 24-hr rainfall has been as high as 4 inches (highest
in southern KY and northeast TN), with 7-day rainfall from AHPS
generally 150-300% of normal. This has compromised FFG across most
of this area, with sensitive terrain features also increasing the
potential for rapid runoff. Any cells moving through this area,
especially where they occur multiple times to repeat heavy bouts
of rainfall, will likely result in instances of flash flooding.


Weiss

ATTN...WFO...GSP...ILN...ILX...IND...JKL...LMK...MRX...OHX...
PAH...

ATTN...RFC...LMRFC...OHRFC...NWC...

LAT...LON   39738720 39648611 39238495 38678428 37478362
            36478331 35728335 35278376 35268447 35518544
            35948582 36708592 37278633 38268770 38628820
            39028809 39338781