040
AWUS01 KWNH 162054
FFGMPD
VAZ000-NCZ000-SCZ000-GAZ000-TNZ000-KYZ000-ALZ000-170230-

Mesoscale Precipitation Discussion 0448
NWS Weather Prediction Center College Park MD
454 PM EDT Sun Jun 16 2024

Areas affected...TN Valley east through the Blue Ridge into
Upstate SC

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

Valid 162053Z - 170230Z

Summary...Slow moving thunderstorms with rain rates nearing 2"/hr
at times will expand in coverage through the afternoon. This could
result in locally more than 3" of rainfall and isolated instances
of flash flooding.

Discussion...The regional radar mosaic this afternoon depicts a
rapid expansion of showers and thunderstorms from central TN
eastward through northern GA and into Upstate SC. These storms are
developing around the periphery of an intensifying mid-level
ridge, and in the vicinity of a stationary front/warm front
analyzed by WPC to be waving around the southern Appalachians.
Thermodynamics across the region are impressive with PWs measured
by GPS of 1.5 to 1.7 inches, around the 90th percentile for the
date, overlapping extreme instability analyzed by the SPC RAP to
be 3000-4000 J/kg. Into these thermodynamics, weak ascent created
via modest upper diffluence, a weak impulse lifting northward, and
some upslope flow/isentropic ascent on the 10-15 kts low-level
flow is helping to create the ongoing convection. It is likely the
environment will remain favorable for additional development
through loss of heating several hours from now.

The CAMs are generally under-producing the current coverage of
convection, so confidence in the exact evolution is modest.
However, the robust thermodynamics and weak southerly flow are
progged to persist into the evening, and the ingredients suggest
that this will support additional convective development until
convective overturning or nocturnal stabilization occurs.
Thunderstorms will likely remain scattered and pulse in the
presence of weak shear, with outflow boundaries and storm mergers
contributing to additional development, and these storms have a
40-60% chance (10-20%) for 1+"/hr (2+"/hr) rain rates according to
the HREF neighborhood probabilities. These intense rates will also
be very slow moving as noted by propagation vectors that are
collapsed to just around 5 kts in the presence of weak flow, and
it is likely that terrain influences may also cause periods of
nearly-stationary cells. Limiting the flash flood risk is that
storm lifespans should be modest due the pulse nature, but where
storms can repeat or lock in place, even briefly, isolated amounts
of 3" or more are possible (20-40% chance) according to the HREF.

This area has been generally dry as noted by 7-day AHPS anomalies
that are less than 50% resulting in 0-10cm RSM from NASA SPoRT
that is only 30-50%. However, there are some wetter soils across
TN and in the vicinity of the Blue Ridge Mountains. Although the
risk for rapid runoff and flash flooding may be a bit higher in
the more saturated soils, or across any urban areas and sensitive
terrain features, the slow movement of these intense rates could
produce isolated instances of flash flooding anywhere across the
region through the evening.


Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...BMX...FFC...GSP...HUN...JKL...LMK...MEG...MRX...
OHX...PAH...RNK...

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

LAT...LON   37238507 37048360 36708256 36498205 36338159
            35768145 35068154 34518229 34208347 34298446
            34568587 35258766 35908824 36688796 37058706