Small Craft Advisory 236, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1025 PM PST Wed Nov 28 2018

.SYNOPSIS...A potent storm system will push inland tonight into
Thursday, producing widespread moderate to locally heavy rainfall,
isolated to scattered thunderstorms, and strong, gusty winds.
Lingering showers will be possible into Thursday night. Dry
weather is forecast for most of Friday before another system
produces additional precipitation from Friday evening through
Saturday morning. Dry conditions are forecast for Sunday and
Monday, but precipitation chances return by Tuesday of next week.

&&

.DISCUSSION...As of 8:30 PM PST Wednesday...Precipitation has
been increasing across our area early this evening as an
approaching upper trough and associated frontal boundary begin to
interact with the moist airmass over our area. Thus far, the
heaviest rain has been in the Santa Lucia Mountains above Big Sur
where rain rates have been as high as 0.35" per hour. Radar
indicates that rainfall will become widespread farther north
across the SF Bay Area within the next few hours. According to the
latest HRRR, the most intense precipitation will occur between
about 11 pm and 4 am. Rain rates during this time could reach
0.50" per hour if convective bands develop as expected. Rain rates
could be briefly higher if thunderstorms develop. Thus far,
lightning detection networks have been picking up lightning
strikes well to our north and northwest. But thunderstorms are
definitely possible in our area later tonight when the frontal
boundary pushes onshore and when the 00Z NAM forecasts surface-
based CAPE values reaching as high as 500 J/kg. Thunderstorm
chances are even higher on Thursday morning when the surface low
approaches the Sonoma County Coast and when both instability and
low level shear are projected to be at their peak. Will need to
monitor for development of convective bands and thunderstorms
later tonight and Thursday morning as very heavy rain rates could
result in debris flows over burn scars. Could also see some
landslides in areas of steep terrain along the Big Sur coast where
overall precipitation totals are expected to be highest.

According to the 00Z NAM, we may see a bit of a lull in
precipitation late tonight after the frontal boundary pushes
through. But then, post-frontal shower activity is forecast to
increase between about sunrise and late Thursday morning as the
most unstable airmass pushes inland. Showers are expected to
gradually taper off Thursday afternoon, with a rapid decrease in
shower activity forecast by all the models on Thursday evening.
Rainfall totals forecast from tonight through Thursday range from
1-2 inches in most urban and low elevation locations and from 2-4
inches in the coastal mountains. The exceptions will be up to 5
inches of rain in the Santa Lucia Mountains above Big Sur and less
than an inch in some of the rain-shadowed valleys such as the
Santa Clara Valley.

Winds have not yet been much of a factor with this system.
However, south winds are expected to increase later this evening
as the frontal boundary draws closer and become locally strong and
gusty. After frontal passage, gusty southwest winds will persist
through the rest of the night and then turn to the west and remain
gusty through most of Thursday as a 992 mb surface low moves
inland across the North Bay. Strongest winds tonight and Thursday
will be at the coast and in the hills where gusts of 40 to 50 mph
are likely. A Wind Advisory is in effect for these areas from 10
pm this evening until 4 pm Thursday.

Most of Friday is forecast to be dry and seasonably cool with
brisk northwest winds. A fast-moving system is then forecast to
sweep in from the northwest and spread rain across our region from
Friday evening through Saturday morning. Because this system will
be moving fast, it is not expected to produce more than about a
half inch of rain, although a brief period of heavy rain could
occur along the cold front late Friday night and early on
Saturday. Also, the Friday Night/Saturday system will be quite a
bit cooler than the current system and we could see snow levels
drop as low as 3500 feet by Saturday morning. Thus, there could be
a dusting of snow on some of our higher mountain peaks by Saturday
morning. Saturday is not expected to be a complete wash-out, as
showers are forecast to quickly taper off by midday with just
widely scattered showers expected in the afternoon.

Sunday and Monday are forecast to be dry and cool days. But the
longer range models indicate a continued active weather pattern
with more rainfall possible by late Tuesday and Wednesday of next
week.

&&

.AVIATION...As of 10:00 PM PST Wednesday...The heaviest rain will
be moving through between 08-12Z. Winds increasing with gusts up
to 25-30 kt. Models consistent in bringing the front through the
SFO Bay Area around 09Z with winds shifting from southeast to
southwest. Rain will turn to showers. Gusty winds will continue
behind the front lasting through the day Thursday. An upper low
is about 300 miles west of Cape Mendocino and moving east at 25
kt. This low will move into the North Bay Thursday. A few
lightning strikes have been firing off over the waters around the
low and in the Sac Valley so there is a possibility of
thunderstorms over the CWA through the afternoon hours.

Vicinity of KSFO...Moderate to heavy rains will be moving into the
area the next 3-4 hours. Southeast winds gusting to 25 kt
switching to southwest after 09Z as the front passes. Rain will
turn to showers and decrease in intensity after frontal passage.
Cigs will remain mostly in the MVFR range through Thursday
afternoon with occasional breaks. The threat of thunderstorms
will be around through the day Thursday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Steady light rain is expected through 10Z
as the frontal system approaches. South to southeast winds
switching to southwest gusting to 25-30 kt by Thursday morning.
Scattered thunderstorms will be possible overnight through
Thursday.

&&

.MARINE...as of 09:45 PM PST Wednesday...A 991 mb low centered 400
miles west of Point Arena is moving east at 25 kt. This system
will move across the northern waters Thursday morning. Locally
heavy rain along with gale force wind gusts and possible
thunderstorms can be expected the rest of  tonight into Thursday.
Southeast winds will shift to westerly  early Thursday morning.
Additionally a large longer period west to northwest swell will
continue through Thursday before gradually  subsiding by Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...High Surf Advisory...CAZ006-505-509-529-530
             Wind Advisory...CAZ006-505-507>509-511-512-517-529-530
             SCA...Mry Bay until 12 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 12 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 12 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 12 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 12 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 12 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 12 AM
             GLW...Mry Bay from 12 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 12 AM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 12 AM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm from 12 AM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 12 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 12 AM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 12 AM
             SCA...Rough Bar Advisory for SF Bar
             SCA...SF Bay

&&

PUBLIC FORECAST: Dykema
AVIATION: W Pi
MARINE: W Pi

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
935 PM PST WED NOV 28 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0927 PM     DEBRIS FLOW      3 NNE SCOTTS VALLEY     37.10N 121.98W
11/28/2018                   SANTA CRUZ         CA   LAW ENFORCEMENT

            SB HIGHWAY 17 AT SUGARLOAF RD, 6 INCHES ACCUMULATING INTO
            NUMBER 2 LANE

0924 PM     DEBRIS FLOW      3 NNE SCOTTS VALLEY     37.10N 121.98W
11/28/2018                   SANTA CRUZ         CA   LAW ENFORCEMENT

            SB 17 AT SUGARLOAF RD 6 INCHES ACCUMULATING INTO NUMBER 2
            LANE

&&

WPI

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1037 PM PST WED NOV 28 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1028 PM     FLOOD            5 NE DALY CITY          37.74N 122.40W
11/28/2018                   SAN FRANCISCO      CA   LAW ENFORCEMENT

1024 PM     FLOOD            2 N DALY CITY           37.71N 122.47W
11/28/2018                   SAN FRANCISCO      CA   LAW ENFORCEMENT

            2 FT OF WATER ACROSS ROAD

1019 PM     FLOOD            1 ESE SAN FRANCISCO     37.77N 122.41W
11/28/2018                   SAN FRANCISCO      CA   LAW ENFORCEMENT

            WATER ON ALL LANES

&&

MMEHLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1113 PM PST WED NOV 28 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1050 PM     FLOOD            1 SW SAN FRANCISCO      37.77N 122.43W
11/28/2018                   SAN FRANCISCO      CA   PUBLIC

            MARKET AND CHURCH ST FLOODED INTERSECTION

&&

MMEHLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1208 AM PST THU NOV 29 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1203 AM     FLOOD            1 WNW SONOMA            38.29N 122.47W
11/29/2018                   SONOMA             CA   LAW ENFORCEMENT

&&

MMEHLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1232 AM PST THU NOV 29 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1207 AM     NON-TSTM WND DMG 5 S SAN RAFAEL          37.90N 122.52W
11/29/2018                   MARIN              CA   LAW ENFORCEMENT

            LARGE TREE FELL ON PICKUP TRUCK

&&

MMEHLE

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1240 AM PST THU NOV 29 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1145 PM     NON-TSTM WND GST 4 S CLAYTON             37.88N 121.91W
11/28/2018  M62 MPH          CONTRA COSTA       CA   MESONET

            MESONET STATION SJS02, MT DIABLO.

1030 PM     NON-TSTM WND GST 2 W MILL VALLEY         37.91N 122.58W
11/28/2018  M59 MPH          MARIN              CA   MESONET

            MESONET STATION PG006, 2 W MILL VALLEY.

0930 PM     NON-TSTM WND GST 7 NNW CALISTOGA         38.67N 122.63W
11/28/2018  M60 MPH          SONOMA             CA   MESONET

            MESONET STATION PG132, 5 SSW MIDDLETOWN.

&&