Mesoscale Precipitation Discussion 0857
NWS Weather Prediction Center College Park MD
1120 AM EDT Mon Sep 17 2018

Areas affected...Ctrl/Ern North Carolina, Ctrl/Ern Virginia, Srn
Maryland, D.C.

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

Valid 171519Z - 172100Z

Summary...Training bands of heavy rainfall are expected to
continue across central and eastern portions of Virginia today,
which may produce rain rates to around 3 in/hr in the most intense
rain bands. This would be likely to lead to new and expanding
areas of flash flooding. Further south into North Carolina,
widespread areal and river flooding will continue. Scattered bands
of heavy rain may develop into the afternoon hours, and given the
saturated soils, this could lead to localized worsening of the
ongoing widespread and severe flooding issues.

Discussion...The strongest convective rain bands as of 15Z were
situated near the central portions of the Virginia-North Carolina
border, where numerous flash flood warnings were already in
effect. The Clarksville, VA (KW63) observation site measured 1.67
inches of rain in 40 minutes with one of these rain bands, and a
number of other mesonet or privately-owned weather stations in the
area have recorded 2-3 in/hr rain rates as well. These convective
bands were situated near the confluence of a number of mesoscale
features. First, they were on the edge of a dry slot in the
mid-upper levels of the troposphere, evident in the GOES-16
Channel 10 water vapor imagery and Simple Water Vapor RGB. This
was confirmed by 12Z soundings from RNK and GSO which showed
significantly reduced RH as low as 500-600mb with a corresponding
subsidence inversion. This may slowly press to the east through
the day, and limit convection to the west of the moisture gradient
as it does. Second, the convective bands were on the northwest
periphery of stronger instability, with RAP analysis indicating
around 500-1000 j/kg of MLCAPE in the inflow region. This is more
than sufficient to maintain intense rain rates in a tropical
moisture plume, and the instability axis is expected to nose into
eastern Virginia during the early afternoon. Finally, the
convective bands are embedded in a belt of strong southerly
low-level flow, which should favor continued backbuilding and
training.

These factors are largely expected to persist into the afternoon,
with the dry slot shifting slightly east, and the nose of the
stronger instability pushing north. This should support a gradual
trend toward the most intense rain rates being favored in
east-central Virginia, perhaps toward the I-85 and I-95 corridors
from the greater Richmond area south to the North Carolina border.
Rain rates of 2-3 in/hr would be likely in the strongest
convective bands, and could even locally exceed 3 in/hr given the
deep tropical moisture plume. Therefore, the flash flood threat
may expand and develop to the north and northeast during the
afternoon across the eastern half of Virginia. Even though flash
flood guidance values are more elevated, significant rain rates
may be the more important factor, especially if they align with
more urbanized areas. Furthermore, current soil moisture
percentile rankings in eastern Virginia are roughly in the 85th to
95th percentile, likely due to a wet summer overall.

Further to the south, in North Carolina, significant rainfall due
to Florence over the past several days has caused widespread areal
and river flooding, and that should continue today. The forcing
for additional convection further south may be more limited
despite a broadly confluent low-level flow pattern. Therefore,
convection across North Carolina is expected to be somewhat
scattered into the afternoon hours. Nevertheless, a deep tropical
moisture plume persists with the 12Z MHX sounding showing a
precipitable water value above 2.1 inches. Greater clearing (as
compared to areas further north on GOES-16 visible satellite)
should allow for stronger instability to build as well. Therefore,
despite the more scattered nature of convection into the
afternoon, intense rain rates of 2-3 in/hr will be possible as
well, just on a more localized basis.

Lamers

ATTN...WFO...AKQ...ILM...LWX...MHX...RAH...RNK...

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

LAT...LON   39027744 38937661 38107622 37397624 36007679
            34587656 33717805 33907888 34977907 35917995
            37537878 38387824