Mesoscale Precipitation Discussion 0271
NWS Weather Prediction Center College Park MD
153 PM EDT Wed May 15 2024

Areas affected...Portions of the Upper Ohio Valley and
Central/Southern Appalachians

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

Valid 151745Z - 152345Z

SUMMARY...Developing slow-moving and isolated to widely scattered
cells could contain rainfall rates of 1-2 in/hr over a region with
relatively low flash flood guidance. Localized flash flooding may
result where isolated 1-3 inch rainfall totals occur.

DISCUSSION...A stacked sfc-500mb low pressure system analyzed via
GOES-East water vapor imagery over the Upper Ohio Valley is
sliding eastward this afternoon while diurnal heating increases
surface instability into the region. However, the SBCAPE is only
expected to reach towards 750-1000 J/kg per the RAP and recent 12z
CAMs due to plenty of cloud cover, which should limit the updraft
potential somewhat. This heating also increases the thermodynamic
gradient between areas east of the Appalachians and north of a
stationary front located over North Carolina, where a wedge of
very stable air is in place. Any developing thunderstorms across
the upper Ohio Valley and central Appalachians will remain west of
the central Appalachian crest due to this boundary and weak
mid-level flow. The weak flow throughout the column was noted by
ILN's 12z sounding, which depicted 5-10kt winds from the surface
to 300 mb. Precipitable water values of 1-1.3" are also found in
this region and per ILN's 12z sounding was near the 90th
climatological percentile on SPC's sounding climatology web page.
Better instability exists farther south near a triple point in
southwest North Carolina and should lead to the potential for
rainfall rates up to 2" as storms form along the southern
Appalachians and slide east into the western half of North
Carolina, but these storms will likely exhibit faster forward
propagation due to westerly flow through the column. Otherwise,
most storms are expected to be highly localized with rainfall
rates of 1-2"/hr covering even less real estate.

CAMs depict a scenario into the early evening with scattered
storms continue to develop and maintain through the early evening
hours. Most storms won't produce flash flooding concerns and
exhibit a pulse nature due to the weak flow aloft and low
instability, but certain clusters of thunderstorms containing
1-2"/hr rates could linger over areas long enough to exceed the
1-2.5"/3-hr flash flood guidance. Long skinny CAPE profiles in
forecast soundings are a feature that could support the
maintenance of updrafts. The 14z HRRR highlights localized totals
of 1-3" within 3 hours and 12z HREF neighborhood probabilities of
30-60% for at least 2" in 6 hours by 00z tonight. Additionally,
it's worth noting that areas of northern Kentucky and Indiana
early this morning over preformed with the associated system as
localized totals exceeded 3".

Snell

ATTN...WFO...GSP...ILN...JKL...LMK...LWX...MRX...PBZ...RAH...
RLX...RNK...

ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...NWC...

LAT...LON   40598119 40498041 39408083 38968064 38677993
            37977962 36647994 35528078 35038163 35198253
            36528161 37388194 38078379 38908404 39638315
            40208215