Mesoscale Precipitation Discussion 1018
NWS Weather Prediction Center College Park MD
155 PM EST Wed Nov 21 2018

Areas affected...California...Southwest Oregon

Concerning...Heavy rainfall...Flash flooding likely

Valid 211855Z - 220600Z

Summary...Some flash flooding and debris flows can be expected
this afternoon and evening across portions of California and
southwest Oregon with a particular emphasis on the area burn scars.

Discussion...The latest GOES-W satellite imagery shows a
well-defined upper trough and associated closed low advancing east
toward central and northern CA. Already a band of locally heavy
showers is focused along and just ahead of an associated cold
front that is very quickly approaching the coast. In fact, a 1415Z
AMSU pass indicated rainfall rates of as much as 0.30 to 0.50
inches/hr within several convective clusters along the cold front
and also offshore underneath the upper low where lapse rates are
steeper and more instability is established.

Over the next several hours, the cold front will advance inland
across southwest OR and central/northern CA with at least a broken
axis of heavy showers and perhaps a few thunderstorms. The latest
hires model consensus, including recent runs of the HRRR, favor
some expansion of convective activity and an uptick in rainfall
intensity. This will in part be aided by improving upper level jet
support and increasing low level south to southwest flow ahead of
the front which should reach as high as 40 kts. This will result
in enhanced rainfall at least briefly over the higher terrain with
stronger orographic forcing.

Expect hourly rainfall amounts of as much as 0.50 to 0.75 inches
within the stronger convective cores as the activity advances east
and inland through the afternoon and evening hours. Unfortunately,
these rainfall rates will enhance concerns for runoff problems,
and with an elevated threat of flash flooding and debris flows in
and around the area burn scars. This will include the most recent
Camp Fire burn scar in Butte County, CA. Storm total rainfall
amounts across the coastal ranges and across the foothills of the
Sierra-Nevada will be as much as 2 to 3 inches through 06Z. Will
continue to closely monitor.

Orrison

ATTN...WFO...EKA...HNX...LOX...MFR...MTR...STO...

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

LAT...LON   43652397 43612345 43112306 41912248 40832188
            40342167 39792125 39312092 38782063 38142005
            37281941 35971921 35191865 34301840 33901885
            34021957 34132035 34382086 35082102 35612158
            35942175 36642236 37722276 38212314 38732365
            39982428 40862423 41702423 42952449