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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1442Z Apr 22, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1042 AM EDT Sun Apr 22 2018

VALID 15Z Sun Apr 22 2018 - 12Z Mon Apr 23 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 NNE MOB NBG 10 SSW BTR 35 S HEZ 15 WSW HKS 20 SSE GWO
25 S UTA 25 ESE M19 15 NNW M19 20 NW BVX 20 WSW FLP HRO
35 NE ROG 15 W SGF 35 N SGF 20 S AIZ 40 W FAM 20 ENE FAM
25 WSW MDH 20 SW MDH 15 NNE M30 35 SSE EHR 15 W BWG 25 E MQY
45 NW CHA 25 WSW CHA 20 NE DNN 15 ENE 1A5 15 NW GMU 15 SE 27A
20 E WRB 20 NNE BGE 25 N DHN 10 W MGM 35 NNE MOB.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SW BNA 10 ESE MDQ 15 SW RMG 10 SW GVL 15 NE WDR 35 NE OPN
25 E CSG 15 WNW LSF 25 E EET 20 WSW TCL 35 W TUP 20 NNW DYR
20 WSW HOP 35 SW BNA.


1500 UTC update

The only change of note to the previous day 1 Excessive Rainfall
Potential Outlook was to extend the marginal risk area
northwestward into far northeast Arkansas and southeast Missouri. 
This is in a region where slower moving precipitation areas are
possible in the pivoting region of the comma head/deformation
precip band to the north of the strong closed low pushing eastward
across central to northern Arkansas on Sunday.   The new 1200 utc
hi res guidance from the NAM conest...ARW and NMMB all show some
1-2"+ totals possible in this slower moving precip region to the
north of the closed low.

Oravec


...Southeast U.S...

A closed low of moderate strength and maintaining itself along its
slow eastward march - characteristically a very April system -
will spread rain and thunderstorms throughout areas from the lower
Ohio Valley to the Gulf Coast.
 
Model guidance is in decent agreement, and the notion of
widespread 1 to 3 inch rainfall during the 24-hour period. The
main question is whether rain rates will be intense enough or
persistent enough over any given location to yield much in the way
of flash flooding. Generally the most persistent forcing and
better alignment of lower and upper level flow should occur during
the day from northern Mississippi into western Tennessee. This is
perhaps where training will be most prevalent, but instability
will be more marginal. Areas farther south should see somewhat 
greater instability, but with mid level flow cutting across from
west to east, the mean 0-6 km winds are at a sharp angle to
Corfidi vectors, suggesting training will not be too common. Given
flash flood guidance values are also much higher, and will be
difficult to surpass with southward extent, WPC trimmed back a bit
on the inherited Slight Risk area for the 12z Sun- 12z Mon period.
Otherwise, Slight Risk is maintained in areas of lower FFG across
northern MS AL into southern TN, and including the Atlanta Metro
to the east, as well as the more favorable training environment in
western TN.

Throughout these areas areal average precipitation is forecast at
1.5 to around 3.0 inches for the 24-hour period, and judging by
moisture availability and hi-res model solutions, hourly rates
could peak around 1.50 inches, leading to at least some risk of
flash flooding, especially from the time of peak heating into the
evening hours.

Burke