Mesoscale Precipitation Discussion 0792
NWS Weather Prediction Center College Park MD
259 PM EDT Fri Sep 07 2018
Areas affected...Portions of the Mid Atlantic
Concerning...Heavy rainfall...Flash flooding possible
Valid 071858Z - 080050Z
Summary...Scattered thunderstorms are expected to develop across
the Mid Atlantic in a moist and unstable air mass. Some of the
storms could be slow-moving and produce over an hour of heavy rain
in localized areas. Rain rates may exceed 2 in/hr in the strongest
storms, and this could lead to flash flooding, particularly if
heavy rain rates occur over urban areas.
Discussion...The air mass in place across the Mid Atlantic region
early this afternoon was very unstable, and by 21Z the strongest
instability was forecast by the RAP to be concentrated in the
vicinity of Chesapeake Bay and the Delmarva Peninsula. Scattered
convection was beginning to develop across the region as localized
convergence boundaries produce sufficient focus for development,
and some areas reach their convective temperature. Over time, the
scattered convection should gradually build from the mountainous
regions and along the ocean and bay breezes, into the remainder of
the region including portions of the I-95 corridor. This process
of convection becoming increasingly concentrated in northern
Virginia and Maryland should peak after 21Z, and this is supported
by probabilistic guidance from the 12Z HREF model. Precipitable
water values were around 1.8 inches on average (slightly lower in
the mountains), which was above the 90th percentile for this time
of year. The strong instability suggests deeper updrafts, which
may be able to entrain some slightly drier air in the mid-levels.
Additionally, mixed-phase and ice layers are expected in these
thunderstorms. Therefore, the precipitation efficiency may not be
maximized, but given the anomalously high precipitable water
values, 1-2 in/hr rain rates should still be relatively common in
the strongest convection. Rain rates could exceed 2 in/hr given
storm-scale interactions, or temporary stalling of convection
along mesoscale/outflow boundaries. Flash flood guidance is
relatively high over most of the region, so the greatest threat of
flash flooding should occur if slow-moving thunderstorms affect
urban areas. However, FFG is slightly lower in the mountains, so
some smaller basins with steeper terrain gradients could also be
favored in those areas.
Lamers
ATTN...WFO...AKQ...CTP...LWX...PBZ...PHI...RLX...RNK...
ATTN...RFC...MARFC...OHRFC...SERFC...
LAT...LON 40637841 40397744 40047660 39927522 39737420
39407421 38787482 38277510 37927549 38307691
38387765 38047842 37587903 37407998 37378068
37598145 37878184 38198133 38628019 39147956
39517933 40067918
Last Updated: 259 PM EDT Fri Sep 07 2018