AWUS01 KWNH 130952
FFGMPD
VAZ000-NCZ000-SCZ000-GAZ000-TNZ000-KYZ000-ALZ000-131500-

Mesoscale Precipitation Discussion 0922
NWS Weather Prediction Center College Park MD
552 AM EDT Wed Aug 13 2025

Areas affected...northern Alabama through far northeast Tennessee

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

Valid 130951Z - 131500Z

Summary...Showers and thunderstorms will expand and intensify
through the morning along a low-level convergence axis. Rainfall
rates within the stronger storms will reach 1-2"/hr, which through
training could produce 2-4" of rainfall. Flash flooding is possible.

Discussion...The GOES-E IR imagery early this morning shows a slow
uptick in coverage of rapidly cooling cloud tops along a linear
boundary from northern AL through northeast TN. The colder and
deeper clouds are associated with thunderstorms evident on the
regional radar mosaic, with 1-hr rainfall measured via MRMS as
much as 1.5" beneath the stronger cells.

These thunderstorms are developing in response to intensifying
ascent impinging across robust thermodynamics. A mid-level trough
axis remains positioned well west of the region, which is leaving
pronounced low-to-mid level SW flow emerging from the Gulf and
advecting PWs above 2 inches northeastward, which is above the
90th percentile according to the SPC sounding climatology. This is
overlapped with a ribbon of MUCAPE 1000-2000 J/kg to provide an
environment favorable for heavy rain. Acting upon this airmass is
pronounced ascent driven by multiple ripples/weak shortwaves
moving through the flow combined with weak RRQ diffluence in the
tail of a jet streak positioned just NW of TN. Additionally, the
850mb LLJ has been steady from the WSW at 15-25 kts on the
regional VWPs, with the convergent nose providing an additional
mechanism for lift.

The result of this has been the growth and expansion of convection
the past hour or so, which is likely to continue for the next
several hours, potentially enhanced by an outflow boundary
dropping southeast as well. The high-res CAMs are struggling to
resolve the current activity, although the trends in the recent
HRRR, as well as the ARW and HREF-PMM support the increasing flash
flood risk. Rainfall rates will likely continue to surge to
1-2"/hr, and storms should move slowly northeast on mean 0-6km
winds of 5-10 kts. Concerning, however, is that propagation
vectors are aligned to the mean wind and also just 5-10 kts,
suggesting that cells will continue to regenerate along this
convergent nose and track northeast, a clear training signature
for the next several hours. While eventually the LLJ should veer
more to the west to reduce the training threat, until this happens
some areas could receive 2-4" of rainfall with locally higher
amounts (HREF neighborhood probabilities for 5" up to 15%).

Additionally, this rain could fall atop vulnerable soils where
3-hr FFG is as low as 1.5-2 inches, and the HREF exceedance
probabilities rise to 15-30%. Any training to produce locally
heavier rainfall across these soils or in any urban areas/more
sensitive terrain could produce impacts from flash flooding.


Weiss

ATTN...WFO...BMX...FFC...GSP...HUN...JKL...MRX...OHX...RLX...RNK...

ATTN...RFC...ALR...ORN...TIR...NWC...

LAT...LON 37148163 36968119 36488125 36048173 35678219
34928316 34528412 34298496 34008606 33918704
34098759 34878735 35708625 36238530 36618398
37068243

$$
--- SBBSecho 3.28-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)