Mesoscale Precipitation Discussion 0694
NWS Weather Prediction Center College Park MD
912 PM EDT Sun Aug 19 2018

Areas affected...Southern Virginia, North Carolina

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

Valid 200111Z - 200600Z

Summary...Scattered clusters of thunderstorms across portions of
Virginia and North Carolina may persist into the overnight hours,
and are likely to be slow-moving when they do occur. This should
create pockets of locally heavy rainfall, with rates of 2-3 in/hr
and totals over 4 inches possible. This may lead to flash flooding.

Discussion...Widely scattered convective clusters existed around
sunset across Virginia and North Carolina, concentrated near
small-scale convergence zones or along existing outflow
boundaries. The largest, organized clusters (as of 01Z) were
situated near the VA-NC border, and were exhibiting a proclivity
to backbuild into the broad WSW-SW low-level inflow. This was
leading to effectively stationary thunderstorms in some cases,
with KRAX and KFCX dual pol radar estimates and MRMS estimates of
rain rates as high as 3 in/hr. Cold pools, where they existed,
were not particularly strong. Over southern Halifax County, VA,
the temperature had only fallen into the low-70s, only a few
degrees different than the adjacent warm sector with almost no
pressure difference. This is probably due to moist profiles
through most of the tropopause. The 00Z GSO sounding was not
saturated, but the RH was fairly high in all layers and the PW was
around 1.8 inches. This was slightly lower than some GPS-PW
observations just to the east (1.9 to 2.0 inches), but overall
indicative of deep moisture. The result is that new cell growth
has been focused along weak convergence boundaries generated by
thunderstorm outflow, but with a lack of a significant cold pool,
the outflow boundaries have been restrained fairly close to the
original convection. The result is very little effective storm
motion. This may continue with any convective clusters that are
able to grow large enough into the evening and early overnight
hours, setting up the possibility of additional backbuilding,
locally heavy rainfall and flash flooding.

In fact, hi-res models do show continued, almost-random growth of
new convection over the next several hours from southern Virginia
into portions of North Carolina. These likely occur in areas where
low-level convergence is briefly maximized, but models are not
showing any large scale boundaries that would be a particular
focus. Nevertheless, the potential for localized heavy rainfall
with rain rates exceeding 2 in/hr should persist based on the
hi-res model forecasts, and the amount of instability available in
the region (2000-2500 j/kg MLCAPE, weakly capped). The heaviest
rain would be likely in areas where the convective clusters' major
axis is oriented W-E or WSW-ENE, such that new cell growth and
propagation would be aligned to produce a quasi-stationary
training effect. As the boundary layer stabilizes into the
evening, convective coverage could diminish contrary to the
expectations of hi-res models, but the observed boundary layer
profile (GSO sounding, Sfc 78/73) does not leave much room for a
substantial amount of CINH to develop.

Lamers

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

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

LAT...LON   38127967 37887911 37197920 36967835 37187701
            36827599 36177607 35477707 34967807 35177952
            35698046 36518094 37158080 37578022