Mesoscale Precipitation Discussion 1027
NWS Weather Prediction Center College Park MD
321 AM EST Thu Nov 29 2018

Areas affected...Coastal Central and Southern California

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

Valid 290821Z - 291900Z

Summary...A round of heavy rain will move into Southern California
early this morning, with the potential for rain rates exceeding
0.5 in/hr and flash flooding near burn scar areas. Further north,
additional heavy rain showers may push into coastal central
California closer to sunrise, with potential for localized heavy
rainfall and runoff issues.

Discussion...Bands of heavy rain are likely to push into the
Transverse Ranges of southern California in Santa Barbara,
Ventura, and western Los Angeles Counties in the next several
hours (08-12Z) and then may spread into other portions of southern
California thereafter. This is associated with an atmospheric
river, with a well defined plume of PW in excess of 1 inch.
Several GPS-PW observations along the coast from Santa Barbara
County north to San Luis Obispo County were around 1.2 inches,
which is around the 99th percentile climatologically for this time
of year. This deep moisture is also likely accompanied by some
marginal instability, with RAP analysis indicating around 100-300
j/kg of MUCAPE. This combination should favor bands of heavy rain,
with rates in excess of 0.5 in/hr. Most hi-res models indeed show
rates that high affecting favored upslope regions of the ranges in
southern California early this morning. An AMSU 89 GHZ pass around
05Z showed a significant enhancement about 100mi west of Santa
Barbara, likely associated with an area of heavier rain. This
should be affecting southern California now and over the next
several hours, and matches well with the timing indicated by
hi-res models. Any small convective rain bands with rain rates in
excess of 0.5 in/hr could cause runoff issues across the entire
area, with debris flows, landslides and flash flooding possible
near burn scar areas.

Further north, several reports of flooding have been received from
the Bay Area, and some rain gauges in the Bay Area and locations
just south have measured rain rates exceeding 0.5 in/hr. The
threat of flooding will continue over the next several hours with
any heavy rain bands across the area. GPS-PW observations indicate
values around 1 inch extending up the California coast toward San
Francisco. Closer to 12Z, another round of more scattered
convection will be possible on the central California coast. IR
satellite indicates some deeper convection offshore underneath a
strong upper level low, and steeper lapse rates were likely
contributing to sufficient instability to support the stronger
convection. This convection could reach the coast in several
hours, and provide a more localized threat of heavy rain that may
continue later in the morning.

Lamers

ATTN...WFO...HNX...LOX...MTR...SGX...STO...

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

LAT...LON   37892190 37462122 36742086 35621998 35051949
            34841873 34641805 34421755 34231684 33671668
            32981655 32801739 33371799 33811906 34172003
            34402069 35312113 36122198 36732224 37482267
            37802247