Mesoscale Precipitation Discussion 0923...Corrected
NWS Weather Prediction Center College Park MD
502 PM EDT Wed Oct 03 2018

Corrected for areas affected

Areas affected...Central Sierra Nevada Mountains into the
foothills

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

Valid 032038Z - 040235Z

Summary...Training convection from the San Joaquin Valley into the
central Sierra Nevada Mountains may lead to flash flooding over
the next 3-6 hours. Rainfall rates of 0.5 to 1.5 in/hr will be
possible.

Discussion...12Z RAOB data over CA showed many sites in excess of
the 90th percentile regarding precipitable water with values
ranging from about 1.0 to 1.3 inches. Filtered sunshine through
the early afternoon has allowed MLCAPE values to rise to near 500
J/kg in the northern San Joaquin Valley into the foothills of the
Sierra Nevada via the 20Z SPC mesoanalysis page where scattered
thunderstorms have increased in coverage over the past 1-2 hours.
RAP analysis soundings for these locations show high relative
humidities throughout the troposphere with tall/skinny CAPE
profiles and generally southerly flow throughout but with more of
a southeasterly component near 700 mb confirmed with the KHNX VAD
wind profile.

Water vapor imagery at 20Z showed a closed low centered roughly
100 miles west of Point Conception, advancing toward the east with
multiple smaller scale vorticity maxima revolving about the common
center. One of the embedded vorticity maxima was located on the
south side of the closed low, preceded by an area of colder cloud
tops on infrared satellite imagery crossing the southern
California coast. Enhanced mid-upper level diffluence will follow
into central CA ahead of this feature as the closed low edges
closer which is expected to allow for continued convective
generation well past 00Z.

While instability is expected to remain less than 1000 J/kg, the
anomalous moisture combined with sufficient instability and
generally unidirectional flow with lower levels parallel to the
spine of the Sierra Nevada range...training and repeating cells
into the evening are expected. With ideal training, rainfall rates
could approach 1.5 in/hr but are mainly expected to be between 0.5
and 1.0 in/hr which may cause flash flooding in a few locations,
especially if overlap occurs with sensitive burn scar locations.

Otto

ATTN...WFO...HNX...REV...STO...VEF...

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

LAT...LON   40302079 40272027 40052000 39681984 39231969
            38891956 38501931 37451875 37181861 36651885
            36611934 36871985 37692061 38672113 39442130
            39742125 40092109