WPC Met Watch |
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Mesoscale Precipitation Discussion: #0851 (2021) |
(Issued at 811 PM EDT Sat Aug 21 2021
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MPD Selection |
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Mesoscale Precipitation Discussion 0851
NWS Weather Prediction Center College Park MD
811 PM EDT Sat Aug 21 2021
Areas affected...Middle Tennessee...Far Northern Alabama
Concerning...Heavy rainfall...Flash flooding likely
Valid 220010Z - 220510Z
Summary...Isolated instances of flash flooding are likely as
1-3"/hr rainfall rates allow for additional localized totals of
2-5 inches.
Discussion...Northwest flow aloft continues across the Tennessee
Valley with impressive convection ongoing west of Nashville, as
evidenced by GOES-16 imagery. Highly textured overshooting tops on
visible channels, impressive cooling on longwave infrared
channels, and the identification of above anvil cirrus plumes
(AACP) on both. These are all signatures of intense convective
activity, and radar imagery from KOHX bears this out with KDP
values as high as 2-4 deg/km (indicating hourly rates as high as
2-3 inches). These storms are traveling towards the southeast
along an instability gradient brought about by earlier storms,
with SB CAPE values ranging from 1500-3500 J/kg along the
gradient. Precipitable water values are highly elevated at 2.1 to
2.3 inches, and effective bulk shear of 20-25 kts is organizing
and prolonging the convection. Low-level flow has also finally
shifted more towards the northwest, but Corfidi vectors are still
hanging around only 5-10 kts. While instability should wane over
the next several hours, some backbuilding is possible in the
meantime amid a highly potent environment for heavy rainfall.
As if the meteorological conditions weren't enough, hydrologically
the highlighted area remains tremendously sensitive from earlier
rainfall. Northern portions of Middle Tennessee saw as much as
9-18 inches of rainfall since late last night, and farther south
into Middle Tennessee and far northern Alabama antecedent
conditions are also problematic with NASA SPoRT-LIS 0-40 cm soil
moisture anomalies above the 95th percentile. This has resulted in
3-hr FFG of less than 2 inches for the bulk of the area, so heavy
rainfall rates of 1-3"/hr will not take long to produce additional
flash flooding. FLASH data are one more component signaling the
flash flood risk, as CREST maximum unit streamflow values remain
elevated above 200 cfs/smi for a good chunk of the northwestern
quarter of the highlighted region, while CREST soil moisture
indicates 50-80% saturation over the southern half.
Churchill
ATTN...WFO...HUN...MEG...OHX...
ATTN...RFC...LMRFC...OHRFC...NWC...
LAT...LON 36628702 36578623 35968593 35018592 34588615
34448708 34468773 34648809 35308825 36258818
36518797
Last Updated: 811 PM EDT Sat Aug 21 2021
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