Mesoscale Precipitation Discussion 0793
NWS Weather Prediction Center College Park MD
331 PM EDT Fri Sep 07 2018

Areas affected...Arkansas and small portions of adjacent states

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

Valid 071930Z - 080130Z

Summary...Rain bands were beginning to redevelop around the center
of Tropical Depression Gordon in Arkansas and nearby portions of
neighboring states. Narrow swaths with rain rates of 1-2 in/hr are
expected (locally higher rates possible), and this may produce
flash flooding if they align with urban areas or smaller basins
embedded within steeper terrain.

Discussion...Tropical Depression Gordon was continuing to slowly
move north across Arkansas. Skies around the center had
sufficiently cleared this morning to allow for some instability to
build, and between 17-19Z numerous spiral convective bands were
developing and becoming more organized. The center of Gordon was
still embedded in a bubble of higher precipitable water values, a
regional maxima of between 2.2 and 2.4 inches. Recent GPS-PW
observations in central Arkansas were around 2.3 inches, as was
the 12Z sounding from Little Rock. The combination of this deep
tropical moisture, with narrow, tall CAPE profiles and freezing
levels approaching 16,000 feet should lead to efficient warm rain
processes in the shallow convective bands. A potential limiting
factor for flash flooding would be the narrow nature of many of
the convective bands. Even a small drift of the bands could reduce
the residence time of heavy rain in a given location, and reduce
rainfall amounts. Nevertheless, the rain rates should be
sufficiently high in the tropical environment (1-2 in/hr common,
locally exceeding 2 in/hr) that flash flooding will be possible
should any of the convective bands persist for a sufficiently long
period of time. There are two areas this may be favored in the
immediate vicinity of Gordon. The first would be on the east side
of the circulation. The low-level flow is asymmetric, with the
highest wind speeds in the eastern semicircle. 925-850mb winds
were peaking around 20 knots or so in a southerly LLJ. The
northward motion of Gordon may also help reduce the drift of
convective bands in this southerly flow regime. The second favored
area would be in a region of enhanced low-level convergence from
southeast Oklahoma into Southwest Arkansas. Some convective bands
were developing in this area where northerly flow around the west
side of Gordon was meeting broad southerly flow in the sunny warm
sector over East Texas.

Lamers

ATTN...WFO...JAN...LZK...MEG...OUN...PAH...SGF...SHV...TSA...

ATTN...RFC...ABRFC...LMRFC...MBRFC...

LAT...LON   37149196 36609064 34529013 33299063 32359150
            33109259 33259387 33999489 34149642 35179577
            34929396 35449321 35949298 36829270