Mesoscale Precipitation Discussion 1025
NWS Weather Prediction Center College Park MD
503 PM EST Wed Nov 28 2018

Areas affected...Central California Coast to Southern Sierra
Nevada

Concerning...Heavy rainfall

Valid 282202Z - 291002Z

Summary...An atmospheric river will continue to impact central and
later tonight into southern California, with heavy rainfall
expected in favored upslope regions of the coastal ranges and
southern Sierra Nevada. Rainfall rates of up to half an inch per
hour are possible at times through 09Z, and that may cause some
localized runoff and flooding issues, particularly for areas of
recent burn scars.

Discussion...low level westerly flow of 20 to 25 kt will increase
overnight along the central California coast as low pressure
approaches from the northwest. This flow is embedded within a
ribbon of precipitable water values of 1 to 1.2 inches (around 150
percent of normal per blended TPW products). Two GPS-PW
observations on the central California coast have exceeded 1 inch
this afternoon. As flow increases, the enhanced moisture will
extend further inland toward the Sierra Nevada.

Continuous light rain with rates around a tenth inch per hour will
continue this evening. MUCAPE of 100-250 j/kg is forecast to
spread into this area late tonight per the GFS/ECMWF/RAP which
should allow an increase in rain rates to 0.25-0.5 inches per hour
in embedded activity after 06Z which is supported by HREF mean 1hr
forecasts.

While rain rates like these are normally marginal for flooding,
several burn scars since 2016 exist in the coastal areas including
the Soberanes (2016) scar south of Monterey and the Thomas (2017)
and Woolsey (2018) scars in southern California. Mudslides and
debris flows are at a much greater risk in these burned areas.

Jackson

ATTN...WFO...HNX...LOX...MTR...

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

LAT...LON   36841950 36151907 35891871 35501846 35681905
            34941935 34731895 34631863 34361839 34091851
            34051932 34422038 34422066 34792076 35232102
            35532123 36262209 36512201 36562181 36682055