Mesoscale Precipitation Discussion 0696
NWS Weather Prediction Center College Park MD
1239 AM EDT Mon Aug 20 2018

Areas affected...Raleigh-Durham Metro and Surrounding Areas

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

Valid 200434Z - 200830Z

Summary...Thunderstorms are likely to continue affecting the
Triangle Region (Raleigh-Durham Metro Area) and adjacent locales
for at least a few hours tonight. The persistent storms could
produce local rainfall totals in excess of 5 inches in under 6
hours, which would be likely to lead to flash flooding. Amounts of
that magnitude could also lead to more significant flash flooding
(widespread and/or deeper inundation) given the location over an
urban area.

Discussion...A classic training convective pattern is setting up
in the Triangle Region of central North Carolina, and could lead
to some substantial rainfall totals in an urban corridor -- from
Durham and Chapel Hill to near Raleigh, and the greater
surrounding area. A lead convective cluster had pushed into
Raleigh as of 04Z, and was showing a general southeast motion.
KRAX radar showed an outflow boundary had pushed out to the south
and southwest, now trailing the convective cluster to the west to
KTTA and KEXX. New cells were developing rapidly to the north of
the outflow boundary, thanks to broad WSW low-level inflow
ascending over the shallow stable layer. Elevated CAPE in the
layer just above the surface was still substantial, around 1500
j/kg or so, and was leading to a WNW-ESE oriented band of
convection. Individual cell motions in this trailing band were
more easterly, and this was having the effect of sustaining a
nearly stationary training band of convection (with perhaps a very
slight southward drift). The general configuration of favorable
inflow for backbuilding and new cell growth into a training band
of convection seems likely to persist for at least another couple
hours, which could lead to substantial rainfall totals right in an
urban corridor. Several hi-res models, notably the past several
runs of the HRRR and the 00Z NCEP-ARW, produced a significant
localized swath of 5-8 inches of rain somewhere in central North
Carolina. The concern is that this will play out in reality, only
right over a metro area.

KRAX dual pol fields show the strongest convection with KDP
generally around 2 deg/km and ZDR closer to 1 dB, which implies
more numerous small droplets, an indicator of warm rain processes
and greater rainfall efficiency. This was especially true in the
convection trailing the lead convective cluster. Rain rates of 1-2
in/hr were being estimated in most of the convection by KRAX dual
pol and MRMS, with a few small pockets in excess of 2 in/hr. It
does seem plausible that some locations in the Triangle Region, or
perhaps just south if the drift of the line continues, could see
an average of around 1.5 inches of rain per hour or so for several
hours, and end up achieving the 5+ inch totals suggested by hi-res
models. This suggests the potential for significant flash
flooding, while the overall training pattern suggests flash
flooding will be likely in the highlighted region. Significant
flash flooding would encompass particularly widespread flooding in
a given area and/or greater inundation depth with more severe
impacts. The fact this will be occurring at night will increase
the hazard level a bit as well. It's worth noting that 5 inches of
rainfall in less than 6 hours has an annual exceedance probability
of less than 1 percent, so that would be a climatologically
significant event in this region.

Lamers

ATTN...WFO...RAH...RNK...

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

LAT...LON   36277951 36167879 35717793 35337849 35607949
            36067988