Mesoscale Precipitation Discussion 1078
NWS Weather Prediction Center College Park MD
203 PM EDT Fri Sep 27 2024

Areas affected...Lower Ohio Valley into the Appalachians of
WV/VA/NC

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

Valid 271802Z - 272300Z

Summary...Showers and thunderstorms developing around Tropical
Storm Helene will train to the NW through the afternoon. Rainfall
rates as high as 1.5"/hr are likely, which through this training
could produce 1-3" of rain. Additional flash flooding is possible.

Discussion...The regional radar mosaic this afternoon shows an
expansive area of rainfall from the MS VLY eastward into the
coastal Mid-Atlantic states. This precipitation shield is
associated with the interaction of Tropical Storm Helene and an
upper low centered near western TN. Drier air noted in WV imagery
being ingested into Helene is impinging into an axis of higher
boundary layer theta-e air over NC, driving local ascent to help
to reinvigorate convection within this moist plume. PWs measured
by GPS are generally 1.9 to 2.4 inches, nearing daily records,
with U/A soundings featuring deep moist-adiabatic lapse rates to
support efficient warm-rain processes. Instability is severely
limited west of the Appalachians as noted via the SPC RAP
analysis, but a ribbon of MLCAPE approaching 1000 J/kg over NC is
slowly advecting westward on impressive 850mb SE inflow of 40-50
kts.

The evolution the next few hours is somewhat uncertain across this
region due to CAM disagreement, but the models appear to be
under-analyzing the current convective regeneration along the
theta-e gradient across NC. This suggests that the overlap of
ascent and thermodynamics is still robust, and sufficient to drive
new convection to the NW through the aftn. The pronounced 850mb
inflow should help advect the higher instability NW as well, which
will support an expansion of rain rates which the HREF
neighborhood probabilities suggest have a 20-40% chance of
exceeding 1"hr at times. Mean storm motions will remain
progressive on 0-6km winds of around 40 kts, but the accompanying
850mb wind evolution will also result in collapsing and veering
Corfidi vectors to become more anti-parallel to the flow
indicating an increased potential for backbuilding and training.
While the CAMs disagree on the greatest risk area for heavy
rainfall this aftn, there is a 90% chance for 1"+ and 10-30%
chance for 3+" of rain in both the HREF and REFS ensembles where
the most pronounced training occurs.

Much of this area has seen heavy rainfall the past 24-hrs reaching
1-2" in the lower OH VLY and as much as 4-6" in SW VA. This
suggests that the most vulnerable soils will remain in the terrain
of the Appalachians of WV/VA and far eastern KY, but even into the
OH VLY 3-hr FFG has fallen to around 1.5", and HREF FFG
probabilities indicate a 20-50% chance of exceeding these values.
While the greatest risk for flash flood impacts will be in the
Appalachians atop the most saturated soils, any training could
cause flash flooding into the evening.


Weiss

ATTN...WFO...GSP...ILN...JKL...LMK...MRX...RAH...RLX...RNK...

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

LAT...LON   39508400 39448269 39058161 38338063 37347964
            36577943 36027944 35767998 35838043 36218101
            36748170 37198252 37818367 38168437 38278454
            38588491 39098472