Mesoscale Precipitation Discussion 0040
NWS Weather Prediction Center College Park MD
1137 PM EST Sat Jan 27 2024

Areas affected...western NC into VA, WV Panhandle, D.C., and
central MD

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

Valid 280435Z - 281035Z

Summary...Localized flash flooding will be possible across
portions of the Mid-Atlantic region through 10Z. Steady rain with
peak rainfall rates of 0.5 to 1.0 in/hr and 1-2 inches in 3 hours
can be expected, which may lead to excess runoff across
hydrologically sensitive locations.

Discussion...04Z radar imagery showed steady rain across western
NC into much of VA, north of a slow moving warm front analyzed
through the Carolinas, connecting to a triple point in northern
TN. Poor mid-level lapse rates were limiting instability over the
Mid-Atlantic region with the 04Z SPC mesoanalysis only showing a
couple hundred J/kg MUCAPE in NC and less than 100 J/kg in VA.
Despite the limited instability, moisture was very anomalous for
January with GPS and 00Z soundings showing PWATs between 1.0 and
1.4 inches across NC and VA, with moisture being advected
northward via 30-40 kt mean flow in the 925-850 mb layer. In
addition, lift was occurring out ahead of an eastward approaching
closed mid-level low over the Ohio Valley and beneath
entrance/exit regions of the associated upper level jets to its
east.

Areas of heavier rainfall are expected to focus along the leading
edge of stronger low level flow, setting up a slow moving SW to NE
axis of low level convergence from western NC into central VA.
Mean steering flow from the southwest will act to support the
repeating of moderate to heavy rain along the low level
convergence axis, just north of the surface warm front which is
expected to edge northward tonight. As the upper trough over the
Ohio Valley moves east, some increase in instability is expected
after 06Z (up to 750 J/kg in southern locations) as steeper
mid-level lapse rates develop (up to ~6.5 C/km), helping to
support locally higher rainfall rates.

Rainfall rates are expected to peak near 1 in/hr at times which
could allow a few locations to pick up 1-2 inches of rain in 3
hours time or less. These locally higher rainfall rates could
support areas of flash flooding considering flash flood guidance
values are only about 2 inches in 3 hours for much of the region
and 1 inch in 3 hours along the WV/VA border and into the
D.C./Baltimore metro where urban flooding could become an issue
later this morning.

Otto

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

ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...NWC...

LAT...LON   39637733 39477675 39067662 38347682 37097740
            36417836 36027963 35438093 35228163 35388196
            35798194 36578149 37468063 38797944 39447824