Mesoscale Precipitation Discussion 0574
NWS Weather Prediction Center College Park MD
1119 AM EDT Wed Aug 01 2018

Areas affected...NE Alabama and Nrn Georgia into Srn Appalachians

Concerning...Heavy rainfall...Flash flooding likely

Valid 011518Z - 012115Z

Summary...Persistent moderate to heavy rain is expected from the
I-20 corridor in Alabama and Georgia up into the southern
Appalachians into the afternoon hours. This same area received
heavy rainfall in the past 24 hours, and additional heavy rain
will likely cause flooding in some areas. Some locations will
likely receive 2-3 inches of rainfall by 5 PM EDT, and a few
isolated spots could see more than that.

Discussion...Regional radars at 15Z showed an arc of heavier rain
extending from central Alabama and the Birmingham metro area
northeast into the southern Appalachians. This was occurring along
a band of increasing low-level convergence, and on the nose of a
surge of 2+ inch PWATs (per GPS-PW observations, satellite
estimates and RAP analysis) from the south. Although instability
is somewhat limited, enough exists for some convective banding.
RAP forecast soundings indicate around 500 j/kg of MUCAPE, with
most of the CAPE profile situated below the -10C layer. The
expectation of shallow convective bands is being borne out in
reality, with KBMX and KTX radar showing the vast majority of
higher reflectivity values (35-40 dBZ or more) concentrated below
the melting layer. This is suggestive of dominant warm rain
processes and this should mean more efficient rainfall production
relative to the modest reflectivity values currently detected by
radar. Despite the lack of strong instability, increasing
low-level convergence in the presence of a very moist atmosphere
should be able to yield 1-2 in/hr rain rates in this environment
in some of the stronger rain bands. Embedded within a broader area
of light to moderate rain, this is expected to yield some swaths
of 2-3 inches of rainfall by 21Z, with locally higher amounts.

There is a fair amount of overlap between the current position of
the axis of highest radar reflectivity, and the RAP forecast 18Z
boundary layer moisture convergence axis. Combined with broad
southwesterly low-mid level flow, this is a recipe for training of
shallow convective rain bands and the potential for flooding. The
area most favored for training rain bands would be from far
eastern Alabama (near Talladega) northeast to the far western tip
of North Carolina (around Franklin). Given the antecedent
conditions with heavy rain in the past 24 hours, flooding appears
likely given the all the aforementioned factors. This could occur
either via sustained rainfall over a longer duration (4-6 hours),
or a quick burst of heavier rainfall in 1-2 hours from a more
focused convective band.

Lamers

ATTN...WFO...BMX...FFC...GSP...HUN...MRX...

ATTN...RFC...LMRFC...SERFC...

LAT...LON   36488250 35878147 34648291 33488439 32618590
            33028687 34378586 35688433