Weather Prediction Center issues Mesoscale Precipitation Discussion #921 concerning HEAVY RAINFALL...FLASH FLOODING POSSIBLE652
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Mesoscale Precipitation Discussion 0921
NWS Weather Prediction Center College Park MD
442 AM EDT Wed Aug 13 2025
Areas affected...Northeast Arkansas through Central Tennessee
Concerning...Heavy rainfall...Flash flooding possible
Valid 130841Z - 131330Z
Summary...Showers and thunderstorms will drift across the region
through morning. Intense rainfall rates of 2-4"/hr will likely
cause pockets of 2-3" of rainfall in less than 1 hour. This may
cause instances of flash flooding, especially in urban areas.
Discussion...Showers and thunderstorms noted on the regional radar
mosaic early this morning have rapidly expanded across western and
central Tennessee and back into northeast Arkansas. This
convection is blossoming within a plume of elevated moisture
evident on the GOES-E WV imagery on SW flow emerging from the Gulf
Coast downstream of a mid-level trough axis that is still hung
back across MO/AR. Cloud top cooling is continuing to occur across
the area as well, indicating that convection is going to continue
to develop and intensify in response to deep layer ascent provided
through modest height falls, weak upper jet diffluence, and
low-level convergence along a surface trough. Together these are
driving sufficient lift for the rapid uptick in thunderstorm
coverage, as this ascent is working upon robust thermodynamics
characterized by PWs measured by GPS of more than 2 inches
coincident with a ribbon of 1000-2000 J/kg MUCAPE.
Storms that have developed already have produced MRMS measured
1-hr rainfall of more than 2 inches in some areas, and the
resulting FLASH unit streamflow response has reached 200-400
cfs/smi. This has prompted already at least one FFW SW of
Nashville, TN. The concern through the morning will be that these
storms will continue to produce excessive rainfall rates of
2-4"/hr, although the HREF and REFS neighborhood probabilities for
2"/1hr are modest at just 10-15%. However, the CAMs are really
struggling with the ongoing activity, so the ensemble
probabilities are suffering and are likely too low. Additionally,
with the primary 850mb inflow from the W/SW, propagation vectors
have dropped to 5 kts or less and are aligned with the weak mean
0-6km wind (around 10 kts) to the primary trough axis. This is
resulting in cells that at times have net motion near zero,
especially near mergers/outflow collisions driven by pulse storms
in the weak shear environment, lengthening the duration of this
heavy rainfall.
Although confidence in the evolution the next few hours is lower
than typical due to lack of model support, the recent HRRR has
started to capture the ongoing scenario and has exhibited a rapid
increase in coverage and amounts of QPF the next few hours. This
indicates that the environment will remain favorable for heavy
rainfall, and where storms train/stall, 2-3" of rainfall could
occur in less than 1 hour, with locally 4" possible by later this
morning. Despite elevated FFG from recent dryness, these rainfall
rates falling atop any more vulnerable soils or less-permeable
urban areas, could result in flash flooding.
Weiss
...Please see www.wpc.ncep.noaa.gov for graphic product...
ATTN...WFO...HUN...LMK...LZK...MEG...OHX...PAH...
ATTN...RFC...ORN...TIR...NWC...
LAT...LON 37008724 36908570 36288544 35738569 35128660
34838778 34848882 35069004 35549099 36289126
36628992
Source:
Weather Prediction Center issues Mesoscale Precipitation Discussion #921 concerning HEAVY RAINFALL...FLASH FLOODING POSSIBLE---------------
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