Flood in California, December 18, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
449 AM PST Wed Dec 18 2019

.SYNOPSIS...Scattered showers will persist through the day as a
storm system still sweeps through the region. Although there may
be some light rain possible across North Bay Thursday into Friday,
more widespread rain will develop for this weekend as a cold
front moves through the region. Unsettled weather will continue
next likely impacting holiday travel.

&&

.DISCUSSION...as of 03:49 AM PST Wednesday...A line of moderate
to locally heavy rain pushed through the Bay Area earlier in the
night with a few embedded thunderstorms around Monterey Bay. As
the initial line moved through enough dry air was present to allow
for temps to wet bulb zero over the higher peaks. The temperature
trace at Chews Ridge in Monterey County showed a drop in temp and
then a rise thereafter to about 35 deg. Therefore, wet snow was
likely falling over the highest peaks, but then transitioned to
mostly rain.

KMUX radar imagery has remained active early this morning with
numerous showers over the coastal waters and land. The showers
are associated with another frontal boundary and the surface low
spinning off the North Bay Coast. Lightning activity has been non-
existent, but there definitely is a convective component. In
fact, a few cells over the coastal waters showed signs of some
small hail with weak rotation. Fortunately, as the stronger cells
approached land their intensity weakened.

For the rest of the day, showers will continue as low spinning off
the coast moves eastward dragging another front through the
region. Additional rainfall amounts will vary greatly given the
showery regime. Showers will finally taper off later this evening
and overnight for most locations as the system exits
eastward. Lingering showers will continue  Additional shower
activity will be possible this evening through Thursday mainly
north of the Golden Gate and coastal mountains as a weak warm
front lifts through the region. As the warm front moves northward
Thursday into Friday precip chances will also drift farther north.

A brief break in the action Friday evening and early Friday night
before another cold front arrives early Saturday. The next round
of steadier rainfall will arrive early Saturday with the front
and then move southward through Sunday. There are definitely some
timing differences between the GFS and ECM/CAN with the GFS being
the quickest. Will lean toward the slower ECM/CAN, which keeps
most of the area dry on Saturday afternoon. Sunday doesn't look
like a total washout, but precip lingers as the upper trough/low
sweeps through.

Another break in precip is possible early Monday, but unsettled
weather returns heading into Christmas. Medium range models drop
another low down the coast midweek with additional precip. Details
vary between the GFS and ECM, especially the track of the low. ECM
brings the low much farther west and ushers in colder air. If that
scenario pans out a white Christmas will be possible over the
higher peaks (above 4k ft) around the Bay Area. Stay tuned as this
low could have travel impacts around the state next week.

&&

.AVIATION...as of 4:20 AM PST Wednesday...Scattered showers will
persist across the region today as a cold front and associated low
pressure system move across the district. South to southeasterly
winds will prevail across most terminals with a west to southwest
component expected during the afternoon hours. Showers are
forecast to dissipate tonight.

Vicinity of KSFO...MVFR to VFR cigs with scattered showers and
wet runways expected through the day. Southeasterly winds will
turn south this afternoon. Southwesterly winds expected over KSFO
this evening. Showers are forecast to dissipate late tonight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR cigs with scattered showers and wet
runways expected through the day. Southeasterly winds will turn
southwest this afternoon. Southeasterly winds expected to return
this evening. Showers are forecast to dissipate late tonight.

&&

.MARINE...as of 3:20 AM PST Wednesday...A cold front will
continue to impact the coastal waters today with rain showers and
gusty west to southwest winds. There is a slight chance of
thunderstorms with the cold front this morning as the front moves
across the region. A building northwest swell will subside this
evening with a long period swell anticipated arrive Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
             SCA...SF Bay from 9 AM until 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 9 AM until 3
     PM

&&

PUBLIC FORECAST: MM
AVIATION: CW
MARINE: CW

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