Mesoscale Precipitation Discussion 0960
NWS Weather Prediction Center College Park MD
535 PM EDT Sun Sep 01 2024
Areas affected...much of NC and adjacent portions of VA/TN/SC
Concerning...Heavy rainfall...Flash flooding likely
Valid 012130Z - 020330Z
Summary...Additional localized 1-2" of rainfall expected through
the early overnight hours, with isolated 2-4" totals possible
where storms repeat. Given prior rainfall (relatively wet
antecedent conditions) and urban sensitivities, isolated to widely
scattered instances of flash flooding are likely.
Discussion...Scattered convection has been ongoing for several
hours over north-central portions of NC and adjacent portions of
VA, producing localized hourly accumulations as high as 1-2". This
is occurring along and ahead of a pre-frontal trough, as the
associated cold front remains well behind with slow progress
towards the southwest (currently draped across across southeast OH
and through central KY). While the front is not forecast to be
even close to the region until dawn, the WSW-ENE orientation of
the pre-frontal trough AND the abundance of convection well ahead
of the front (clearly of the katafront variety, which speaks to
how anomalously strong the cold front and associated upper-level
trough are for early September). The mesoscale environment is
currently characterized by ample tropical tropospheric moisture
(precipitable water values of 2.0-2.3 inches, near or above record
levels, per GSO sounding climatology), sufficient instability (ML
CAPE 1000-2000 J/kg), and effective bulk wind shear of 20-25 kts
(providing some individual storm longevity and organization
potential).
Despite an overall favorable environment for at least localized
instances of excessive rainfall, hi-res CAMs generally depict
additional localized QPF of another 1-2" through 03z (generally
corresponding to 10-20% odds for 6-hr FFG exceedance, per 40-km
neighborhood method). That said, there is a small indication for
heavier totals, per HREF probability matched mean (PMM) QPF
depiction indicating potential for localized 3-4" amounts
(corresponding to 20-30% odds for 3" exceedance, per 40-km
neighborhood method). It is also worth nothing that the HRRR
(which was generally one of the drier solutions in earlier runs)
has come around to a solution closer to the HREF PMM QPF depiction
with the latest 19z and 20z runs. Given that the environment and
observational trends support this higher-end potential (along with
relatively wet antecedent conditions), isolated to widely
scattered instances of flash flooding are considered likely.
Churchill
ATTN...WFO...AKQ...CAE...GSP...ILM...MHX...MRX...RAH...RNK...
ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...NWC...
LAT...LON 37107767 36917625 36517575 35537547 35547678
35427766 34827956 34598130 35318358 35758362
36228221 36628111 36848022 37027916
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