Mesoscale Precipitation Discussion 0120
NWS Weather Prediction Center College Park MD
1251 PM EDT Sun Mar 31 2024

Areas affected...Southern California...

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

Valid 311700Z - 312300Z

SUMMARY...Widely scattered thunderstorms capable of 1-1.5" hourly
totals continue to pose a risk for localized flash flooding late
this morning into the afternoon. 

DISCUSSION...Persistent narrow updraft, shallow thunderstorms have
persisted near a stubborn effective stationary front from
southeast of Santa Cruz Island to Santa Catalina bending southward
about 10-20 miles off the San Diego county coastline. CIRA LPW
continues to show enhanced moisture of .75" (for a non-AR plume)
pooled along the boundary with solid surface to boundary layer
confluence centered along and near Santa Catalina. With steep
lapse rates and cold air advection aloft maintaining above normal
unstable airmass with MUCAPE values of 750 J/kg.  The combination
has seen persist convective towers along the frontal zone
eventually shearing bringing light to moderate rainfall ashore
across SE LA, Orange and W San Diego counties.  However, limited
eastward progression of the boundary has limited the offshore more
intense rain rates of .5-1"/hr from coming ashore...still
conditions remain favorable for additional upstream development on
the front.

GOES-WV suggests the upstream vorticity center is a the base of
the longer wave trof axis but still appears to be digging south
rather than rotating northeast.  This continues to provide a
favorable upper-level divergence pattern and DPVA to maintain
thunderstorms but has not allow for westerly low level flow to
dislodge the stationary front.  However, there remain a majority
of Hi-Res CAMs suggesting this eastward advancement, which would
allow for isolated cells to produce .75-1.5" totals in short
duration and given already saturated soils with 0-40cm saturation
near 75-80%; well above normal (and impermeable urban conditions),
suggest any stronger rates than .5"/hr could result in rapid
run-off and localized flash flooding into the afternoon hours. 
While best convergence parameters remain just offshore, proximity
and potential for eastward moving cells into the area of concern,
will keep a threat of localized flash flooding possible through
23z.

Gallina

ATTN...WFO...LOX...SGX...

ATTN...RFC...CNRFC...NWC...

LAT...LON   34301826 34211778 33931722 33291662 32871648
            32581651 32571717 32801737 33381773 33471805
            33041813 32791844 32981864 33501863 34051866
            34241859