Mesoscale Precipitation Discussion 1047
NWS Weather Prediction Center College Park MD
138 PM EDT Tue Sep 24 2024
Areas affected...Southern to South-central Appalachians...
Concerning...Heavy rainfall...Flash flooding possible
Valid 241735Z - 242300Z
SUMMARY...Scattered cells ahead of main convective lines may
pre-soak rugged areas with spot of 1-2.5" resulting in scattered
possible flash flooding.
DISCUSSION...GOES-E visible imagery shows a fairly clear skies
over the Appalachians across S WV/SW VA into E TN/W NC and far
upstate SC. Temperatures have risen to upper 70s/low 80s with
well above average low level moisture having filtered through the
range with Tds in the mid to upper 60s. As such, the area has
become fairly unstable with 1250-1750 J/kg of MLCAPE. Solid
25-40kts of southwesterly 850-700mb flow has been advecting
increasing moisture through the lowest levels to further moisten
the lower column with total moisture reaching 1.5" even into the
higher slopes with some suggestion of 1.7" starting to reach
eastern TN through the Cumberland Plateau.
While main northern stream trough remains well west over the
Midwest digging into the Tri-Rivers area of W KY, a subtle
southern stream wave can be seen sliding through E TN moving
generally parallel to the slopes. This forcing combined with
low-level convergence from approaching pre-frontal convective line
out of KY, is starting to spark increasing TCu and incipient CBs
across the clearer skies downstream. More mature cells along the
pre-frontal trough as well as very near the shortwave lifting
north will likely continue to enhance with cells capable of
1.25-1.75"/hr rates. While cell motions will likely limit overall
totals to about 30-60 minutes, it is the pre-cursory cells that
given solid instability and updraft strength may be capable of a
similar 1-2"/hr total prior to the main forcing lines that may
result in spots of 1.5-2.5" totals over a 3 hour period as they
repeat or at time merge with the cells in the effective warm
sector ahead of the line.
Hydrologically, the rugged area has naturally lower FFG values at
1-1.5"/hr and 1-2"/3hr which suggest either slower moving
pre-cursory cells or the stronger forced lines will have a solid
potential of being exceeded. The scattered and transitory nature
is likely to result in localized scattered incidents of flash
flooding given 1.5-2.5" totals through 00z.