Mesoscale Precipitation Discussion 0581
NWS Weather Prediction Center College Park MD
1238 PM EDT Thu Aug 02 2018

Areas affected...Georgia and Northeast Alabama

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

Valid 021637Z - 022215Z

Summary...Rain will continue into the afternoon across portions of
far northeast Alabama and into northern Georgia, near the track of
a weak low pressure center. Further south in central and southern
Georgia, thunderstorms should become more widespread in the
afternoon. These factors will combine to increase the risk of
flash flooding in the next several hours in the aforementioned
areas. Localized rainfall totals through 6 PM EDT could exceed 4
inches.

Discussion...RAP analysis placed an 850mb low in east-central
Alabama as of 16Z, and the KBMX and KMXX radar VWPs (925-850mb)
placed a weak circulation near Coosa County, Alabama. This weak
low is projected to lift into northwest Georgia by late this
afternoon. An associated enhancement in the low-level flow existed
to the east of this low, with the LLJ likely to increase and shift
slightly to the east during the afternoon hours. Near the low, and
to the northwest of the stronger flow, KBMX and KMXX radars showed
slow-moving rain bands over east-central Alabama as of 16Z. These
were likely situated in a deformation zone, with limited access to
significant instability. Rather, they were being sustained by
persistent low-level forcing and abundant low-level moisture. This
may continue to the northeast into the afternoon near the track of
the 850mb low. The rain bands were low-topped, with most of the
35+ dBZ reflectivity values concentrated below the melting layer.
This is suggestive of dominant warm rain processes, and KMXX and
KBMX ZDR values in the heavier rain bands were near or less than
1, typical of numerous small rain droplets formed primarily
through the collision coalescence process. This may make rain
bands from east-central Alabama into northern Georgia more
efficient with rainfall production than reflectivity would imply,
or that radar rainfall estimates would suggest. Some of the area
that may be affected by this rain has already received heavy
rainfall in recent days, and this could pose a risk for flash
flooding.

Further to the southeast, most of central and southern Georgia was
within a patch of relative clearing per GOES-16 visible satellite
imagery, and thus instability was building. Stronger convection
can be expected in these areas, and some was already beginning to
develop as of 16Z. CIRA blended TPW product shows precipitable
water values exceeding 2 inches nosing into southern Georgia, and
RAP analysis puts the PWs slightly higher (around 2.2 to 2.3
inches). Given a GPS-PW observation of 2.24 inches near Dothan,
AL, the RAP values may be closer to reality with the plume of deep
moisture funneling into the area. The combination of moderate to
strong instability and tropical moisture should lead to very heavy
rain rates in convection that can develop. If the rain can be
sustained for at least an hour, rain rates of 2-3 in/hr would be
attainable. The primary uncertainty in this region is how long the
heavy rain will be sustained at any one location. The mesoscale
features driving the rainfall today will be slowly drifting east,
and a squall line from Cordele to Valdosta has shown some eastward
component to its motion. This may prevent storms from locking into
a particular area for an extended period of time. Nevertheless,
the overall environment, potential for very heavy rain rates, and
increasing signal in the 12Z HREF for heavy rainfall in the warm
sector this afternoon all argue for an increasing flash flood risk.

Lamers

ATTN...WFO...BMX...CAE...CHS...FFC...GSP...JAX...TAE...

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

LAT...LON   34658493 34268301 32838179 31378189 30568255
            30918360 31808402 32538469 32258576 32758626
            33838614