157
AWUS01 KWNH 250805
FFGMPD
TNZ000-KYZ000-251300-

Mesoscale Precipitation Discussion 0321
NWS Weather Prediction Center College Park MD
404 AM EDT Sat May 25 2024

Areas affected...south-central KY into Middle TN

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

Valid 250800Z - 251300Z

Summary...Locally heavy rainfall of 1-2"/hr may lead to short
totals of 2-4". Isolated to scattered instances of flash flooding
are possible.

Discussion...Convection has been gradually growing upscale into a
small MCS over portions of central KY into Middle TN early this
morning, after initial multicell clusters collided and merged into
a more linear single bow echo near the KY/IN/IL border region
several hours ago. As convection grows downstream ahead of this
bow, localized training of 1-2"/hr rates are beginning to occur
along the southwest flank (just as storms reach more vulnerable
terrain with 3-hr FFGs of 1.0-2.0"). The mesoscale environment
ahead of this compact MCS is characterized by MUCAPE of 1000-2000
J/kg, PWATs of 1.3-1.6" (near the 90th percentile, per BNA
sounding climatology), and effective bulk shear of 25-35 kts. In
addition, strengthening of the LLJ (at 850 mb) to 25-35 kts just
upstream (southwest) of the MCS may favor additional backbuilding
of convection perpendicular to the bow following the initial
passage of storms, occurring above the cold pool as warm air
isentropically ascends (assisted by directional and speed
convergence of the LLJ, fitting the conceptual model of a `bow and
arrow` MCS, which was also apparent with the MCS over AR earlier
in the night). To boot, the aforementioned downstream growth of
convection (along an effective warm front aloft preceding the MCS)
is oriented NNE to SSW, near parallel to the upwind propagation
vector (directed SSW at 10-20 kts). This may allow for additional
localized training (from the NNW to SSE, but in a more limited
fashion) ahead of the main bow echo.

This complex orientation of meso-beta storm structures was finally
depicted reasonably well by the latest (06z) HRRR, which suggests
a swath of 1.0-2.5" totals across much of central KY into Middle
TN (from NW to SE), generally just north and east of the Nashville
metro (but backbuilding along the western flank may directly
impact the metro). The orientation of the 06z HRRR QPF also
matches better with the other 00z HREF members, increasing
confidence in the general outcome. That said, these totals from
the HRRR (and HREF PMM for that matter) may be a bit underdone, as
suggested by 00z HREF 40-km neighborhood probabilities of 20-40%
and 10-20% for 2" and 3" thresholds, respectively. But even so,
the short-term totals from the HRRR are enough to meet or possibly
exceed the aforementioned FFGs locally, suggesting that isolated
to scattered instances of flash flooding are possible. Should a
more `worst case` scenario occur with the backbuilding convection,
the max totals may end up quite close to the Nashville metro
itself (as indicated by the HREF 3" exceedance probabilities of
10-20% located very near the metro), conditionally leading to
locally significant flash flooding.

Churchill

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

ATTN...WFO...JKL...LMK...MRX...OHX...PAH...

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

LAT...LON   37438545 36928485 36428452 35788456 35498520
            35808641 36578703 37178709 37418653