Mesoscale Precipitation Discussion 1028...Corrected
NWS Weather Prediction Center College Park MD
336 PM EST Thu Nov 29 2018

Corrected for Expiration Time

Areas affected...NRN CA...CENTRAL CA

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

Valid 291652Z - 292352Z

Summary...As a cold upper low moves ashore, instability and deep
layer lift will support convective rainfall with local half to
three-quarter inch per hour rates. This will maintain a flash
flood risk, particularly near fresh burn scars.

Discussion...At 1630Z satellite and radar imagery depicted a
broken band of convectively driven showers and a few embedded
distinct convective cores over northern California from the Bay
area up to the northern end of the Sacramento Valley. This was
occurring immediately ahead of a cold upper low with 500-mb temps
about -25 C. The low will progress eastward, with a fairly strong
rise/fall couplet in the omega (lift) field per RAP forecasts and
also evidenced by subsidence / erosion of mid level clouds
offshore to the west of the trough axis in satellite imagery.

Upper difluence will be maximized just northeast of the compact
upper low center, and this setup, particularly in the N-S oriented
Sacramento Valley, will encourage strong low level convergence to
support concentrated moderate and locally heavy rainfall with some
potential for training - as was already seen in radar loops from
KBBX.

By 18Z the lobe of forcing ahead of the upper low should reach the
Central Valley and Sierra foothills farther south. Hi-res models
such as the HRRR and 12Z WRF-ARW are strongly suggestive of
relatively robust convection in these areas today as the cold air
aloft moves into areas characterized by lower 50s surface
dewpoints. Forecasts generally bring MUCAPE values up to at least
a few hundred J/kg and perhaps greater than 500 J/kg down over the
San Joaquin Valley and adjacent foothills. There are enough
favorable ingredients for flash flooding, such as slow cell
motions near the upper low and sustained ascent and terrain
influences farther south, such that moderate rain rates may be
repeated at a few locations during a 1 to 2 hour period. This will
lead to some potential for flash flooding, especially in
vulnerable areas like recent burn scars.

Burke

ATTN...WFO...EKA...HNX...MTR...STO...

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

LAT...LON   40652241 40182144 39222047 38061978 36921890
            35921839 35581870 36971963 37782053 38152119
            37782169 37432181 37842253 39442292 40452263