AWUS01 KWNH 271841
FFGMPD
CAZ000-280615-

Mesoscale Precipitation Discussion 0016
NWS Weather Prediction Center College Park MD
141 PM EST Wed Jan 27 2021

Areas affected...central CA coast

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

Valid 271837Z - 280615Z

SUMMARY...A narrow and slow moving axis of heavy rain will likely
lead to significant runoff with potential for flash flooding,
debris flows and mudslides along the Coastal Ranges of CA over the
next 6 to 12 hours. An additional 4-8 inches of rain is expected
along the central CA coast with rainfall rates in excess of 1
in/hr at times. Burn scar locations will be at greatest risk, but
impacts outside of burn scar locations are possible as well.

DISCUSSION...Regional radar imagery along the central coast of CA
has shown that the southward sinking axis of heavy rain from
overnight has stalled over Monterey and San Luis Obispo counties
since ~12Z. Precipitable water values along the coast were near or
just below 1 inch with southwesterly 850 mb winds peaking in the
50-60 kt range, supporting IVT values between 500 and 700 kg/m/s.
Despite a lack of instability and only modest precipitable water
values, several Wunderground.com observations showed rainfall
rates over 1 in/hr in northern San Luis Obispo County around 16Z
and 17Z. Orographics are certainly having an impact on rainfall
intensity given favorable southwest facing slopes of the Santa
Lucia Range.

Water vapor imagery through 18Z showed a shortwave had moved
inland across northern CA with an amplifying trough offshore,
about 800 miles west of the central CA coast. Amplification of the
mid-upper level trough over the eastern Pacific will support a
southward pivoting of the strongest low level moisture flux
offshore, with only slight north/south wavering of the moisture
axis along the CA coast. While slight variations in moisture
magnitude and low level wind speed are expected, the average
intensity of rainfall is expected to remain the same through the
day today and into the early overnight hours. RAP output suggests
some jet enhancement to lift as a 250 mb speed max near 130 kt
crosses the central CA coast just after 00Z, placing the central
to southern Coastal Ranges within the right-entrance region of the
jet streak.

Short term indications suggest the heaviest rain will overlap
southern portions of the Dolan Burn Scar and may impact the
Mineral Burn Area downstream across the Diablo Range. Rainfall
rates over 0.5 in/hr are likely, exceeding 1 in/hr at times,
combined with the slow moving axis of the heavy rainfall will
allow for exceptionally high rainfall totals over a relatively
short period of time through 06Z, with 4-8 inches likely from
northern San Luis Obispo into southern Monterey County. The heavy
rain is expected to lead to flash flooding and debris
flows/mudslides, not limited to just burn scar regions.

Farther south into the Transverse Ranges, low level moisture flux
will not be as strong compared to locations farther north, but 30
to 40 kt of 850 mb winds perpendicular to the terrain may support
some localized runoff concerns.

Otto

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

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

LAT...LON 36822098 36822076 36782054 36432039 36052036
35562032 35372023 35122012 34962001 34841982
34781962 34781945 34611933 34531933 34341945
34291976 34332023 34512070 34992108 35692160
36242175 36752153
#
--- SBBSecho 3.12-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)