Mesoscale Precipitation Discussion 0679
NWS Weather Prediction Center College Park MD
1027 PM EDT Thu Aug 16 2018
Areas affected...Northern Arkansas, Southern Missouri, Western
Tennessee
Concerning...Heavy rainfall...Flash flooding possible
Valid 170225Z - 170730Z
Summary...Thunderstorms were developing this evening across parts
of southern Missouri, northern Arkansas and western Tennessee.
These storms may move slowly at times, or repeatedly affect the
same areas, which would produce locally heavy rainfall and
possibly flash flooding. The areas most at risk for flash flooding
would be in far northeast Arkansas, the Missouri Bootheel, and far
western Tennessee -- areas that have already received over 3
inches of rain in the past 24 hours. Rain rates in excess of 2
in/hr will be possible with any slow-moving storms.
Discussion...A broken line of convection and associated outflow
boundary were advancing into northwest Arkansas and southwest
Missouri as of 02Z, and a weak cold pool and mesohigh were noted
near Joplin. This advancing boundary was beginning to lead to an
increase in convection just to the east, along a low-level
convergence maximum -- aligned in a ribbon from near Branson to
near Memphis. This was on the nose of a more coherent area of 5-10
knot surface winds, and also aligned with a substantial maximum in
surface thetae (around 370K). This should provide a favorable
environment for continued convective growth over the next several
hours. Deep layer mean winds were not perfectly aligned with the
convergence zone, but were at a sufficiently small relative angle
that some situations with a couple hours of training cells cannot
be ruled out. There is also the potential for supercells to move
more slowly and perhaps be anchored near the convergence zone.
Bunkers right-moving storm motion vectors are generally to the
southeast at around 10 knots, and a stalled supercell has already
been observed over Haywood County, TN on KNQA radar. Dual pol
estimates show rain rates exceeding 2 in/hr, which would not be
uncommon with a slow-moving HP supercell. It was showing a
tendency to drift or backbuild slowly into the west-southwest
low-level flow, and Corfidi vectors indicate the potential for
slow backbuilding storm motions (around 5 knots) this evening.
Therefore, although the mesoscale setup does not perfectly favor
training, there are enough factors that support brief periods of
repeated heavy rain, as well as backbuilding or stalling storms,
to support the threat of heavy rainfall and flash flooding. The
greatest threat would seem to be close to the Missouri Bootheel,
roughly from near Randolph County, AR to near Lauderdale County,
TN. These areas received significant rainfall in the past 24
hours, are very close to the low-level convergence zone and thetae
max, and have developing convection nearby already. However, flash
flooding would be possible over the entire outlined area given the
proper alignment of ingredients.
Lamers
ATTN...WFO...LSX...LZK...MEG...OHX...PAH...SGF...TSA...
ATTN...RFC...ABRFC...LMRFC...MBRFC...OHRFC...
LAT...LON 37399229 37229086 36528787 35488805 34918989
34819142 34949312 35549384 37069314
Last Updated: 1027 PM EDT Thu Aug 16 2018