AWUS01 KWNH 021833
FFGMPD
NCZ000-SCZ000-030030-

Mesoscale Precipitation Discussion 0963
NWS Weather Prediction Center College Park MD
232 PM EDT Mon Sep 02 2024

Areas affected...Coastal NC and adjacent Coastal SC

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

Valid 021830Z - 030030Z

Summary...Localized rainfall accumulations of 3-5" (and locally
even higher) are possible with efficient, training convection.
Scattered instances of flash flooding are possible, particularly
if these totals occur over sensitive areas and/or the higher (5"+
totals are realized).

Discussion...Convective coverage is gradually increasing this
afternoon in the vicinity of the coastal Carolinas, where several
fronts/boundaries are consolidating to allow for a short period of
an elevated flash flood risk. This is due to the presence of two
synoptic features that are consolidating, an approaching cold
front and a coastal trough/low, superimposed on mesoscale forcing
via the sea breeze circulation (evident via MRMS RALA imagery,
progressing inland and likely helping to anchor/train convection
along the coast an inland, as opposed to offshore). These factors
discussed are most pronounced over coastal NC, as the cold front
lags farther behind in SC. In this region, the mesoscale
environment is characterized by ML CAPE of 500-1500 J/kg,
precipitable water values of 2.1-2.3 inches (above the 90th
percentile and near the max moving average, per MHX sounding
climatology), and effective bulk shear of 25-35 kts. Perhaps most
concerningly, the 850-300 mb mean wind is also parallel with the
coast over NC (while it is pointed more offshore over SC).
Additionally, it is also worth noting that the the influence of
the right-entrance region of a 90+ kt jet streak off of the
Northeast coast, which is yet another factor that may help to
allow convection to proliferate and sustain itself.

Given this analysis of the environment and the latest
observational trends, the hi-res CAM output (12z HREF and
subsequent HRRR runs) seems to provide decent guidance going
forward. The HREF probability matched mean (PMM) QPF depicts
localized 3-5"+ totals through 00z, quite similar to the latest
HRRR (both 16z and 17z) depiction. This is notable as the HRRR had
been drier with prior runs (as had the 06z HREF suite), so in
addition to the observational trends being favorable, the guidance
trends have also been favorable. The post processed 12z HREF
indicates (with a 40-km neighborhood method) high chance for 3"
exceedance (40-90%) and some chance for 5" exceedance (10-30%).
When filtered through Flash Flood Guidance (FFG) thresholds, the
probability for exceeding 6-hr FFGs ranges from 10-40%
(corresponding most closely to a 10-year ARI, average recurrence
interval, as probabilities for a 100-year ARI are capped at 5%).
This speaks to the difficulty of flooding sandy soils, as this
region can typically take quite a bit of water before excessive
runoff occurs. As a result, scattered instances of flash flooding
are considered possible (rather than likely), as flooding will
largely be dependent on whether that higher-end totals can
manifest (localized 5"+) OR whether convection is able to train
over more sensitive, urbanized terrain along and near the coast.

Churchill

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!4ygLcQG-vecildtBRb8gXkP44e3xZ6Zx5e9JNbfmHxMxfvZKbt-Bw-fP1BZDPTU1AIUy=
-wr_bdhJ66CwOqqzQ-ioeEk$  for graphic product...

ATTN...WFO...ILM...MHX...RAH...

ATTN...RFC...SERFC...NWC...

LAT...LON   35847582 35627527 35057553 34477631 33747797=20
            33557891 33887925 34637828 35067755 35627649=20
=20=20=20=20=20=20=20=20=20=20=20=20

=3D =3D =3D
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