Weather Prediction Center issues Mesoscale Precipitation Discussion #1082 concerning HEAVY RAINFALL...FLASH FLOODING POSSIBLE229
AWUS01 KWNH 292251
FFGMPD
MDZ000-VAZ000-WVZ000-300400-
Mesoscale Precipitation Discussion 1082
NWS Weather Prediction Center College Park MD
651 PM EDT Sun Sep 29 2024
Areas affected...Central Appalachians
Concerning...Heavy rainfall...Flash flooding possible
Valid 292250Z - 300400Z
Summary...Showers and thunderstorms are expected to increase in
coverage and intensity during the next few hours. Rainfall rates
within thunderstorms could reach 1-2"/hr, resulting in axes of
1-3" of rain with locally higher amounts. Flash flooding is
possible.
Discussion...The regional radar mosaic this evening indicates that
convective coverage is finally beginning to expand and intensify
across parts of Virginia. This is occurring in response to a slow
rise in favorable thermodynamics noted by the SPC RAP analysis of
SBCAPE reaching above 500 J/kg, coincident with PWs that are above
the 90th climatological percentile at 1.6-1.8 inches. Into this
overlapping moisture/instability, ascent is intensifying through
height falls and divergence downstream of an upper low positioned
over KY, topped by modest LFQ upper diffluence, and in the
presence of 15 kts of 850mb inflow out of the SE. Not only is this
inflow advecting the more robust thermodynamics northward into
VA/WV, it is also increasing ascent through convergence along the
nose of these higher wind speeds and through both isentropic and
orographic response to this wind trajectory. Rainfall rates within
the stronger convection have produced MRMS measured 1-hr rainfall
over 1 inch in southern VA.
Although the guidance has been a tad aggressive with convective
intensity this aftn, likely due to a slower increase in favorable
instability, the recent radar returns suggest this evolution is
beginning. The convection over southern VA should continue to
expand and then push N/NW on 0-6km winds that will gradually back
from SW to S at 15-20 kts through the evening. This evolution is
supported by most available high-res models, and despite the
generally progressive motion of cells, repeating rounds are likely
as storms regenerate into the stronger thermodynamics and pivot
northward. The HREF neighborhood probabilities for 1"/hr (2"/hr)
rain rates peak this evening above 60% (20%), greatest in the
higher terrain where orographic lift will enhance the rainfall
intensity. Where storms can regenerate and repeat, these rain
rates could result in 1-3" of rain, with locally higher amounts
possible as progged by 10-15% HREF/REFS neighborhood probabilities
for 5"/6hrs.
This area has been saturated with heavy rainfall that has been 3
to as much as 8 inches according to AHPS 7-day rainfall. This has
led to compromised FFG and 0-10cm RSM from NASA SPoRT that is
generally above 70%. Any heavy rain rates moving atop these
sensitive soils could quickly become runoff leading to instances
of flash flooding, but this appears most likely in the vulnerable
terrain from the Blue Ridge through the Allegheny Mountains.
Weiss
...Please see www.wpc.ncep.noaa.gov for graphic product...
ATTN...WFO...AKQ...LWX...PBZ...RLX...RNK...
ATTN...RFC...MARFC...OHRFC...SERFC...NWC...
LAT...LON 39497924 39307868 38947824 38087791 37337793
36797818 36617867 36577921 36697990 37368065
37578109 38098128 39198033
Source:
Weather Prediction Center issues Mesoscale Precipitation Discussion #1082 concerning HEAVY RAINFALL...FLASH FLOODING POSSIBLE---------------
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