Tornadoes and Debris Flows in California, January 5 to 7, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
405 AM PST Sat Jan 5 2019

.SYNOPSIS...The storm door is open as several rounds of
precipitation and strong winds are expected this weekend into
next week. The first round will arrive today with gusty winds,
widespread rainfall, and isolated thunderstorms. The next system will
arrive on Sunday with widespread rain and gusty winds. A third
system will push through Tuesday through Wednesday with additional
precipitation. The pattern looks to remain active into next
weekend.

&&

.DISCUSSION...as of 03:45 AM PST Saturday...Clouds continue to
increase over the area this morning in advance of an upper level
trough. Radar returns are increasing offshore with widespread
rainfall expected later this morning. Moisture has been steadily
increasing and many areas have likely already hit their overnight
lows. An 990 mb surface low located approximately 300 miles west
of Pt Reyes is creating a strengthening pressure gradient over the
area as it moves northeast. In response southerly winds have
started to increase, especially in the higher terrain.

A moderate to heavy band of rain along a frontal boundary will
push east through the area this morning. As cooler air aloft moves
overhead, steady rain will transition to showers. Enough
instability will develop to support a slight chance of
thunderstorms late this morning through this evening for all
areas. Strong winds aloft could mix down in thunderstorms,
creating strong wind gusts at the surface. Small hail, brief
heavy rain, and occasional lighting are also possible with any
thunderstorms which form. Snow levels will drop to about 4000
feet by the evening, allowing peaks above 4000 feet to see a
chance of snow. Showers will taper off this evening with a lull
in precipitation before the next system arrives on Sunday.
Rainfall amounts through tonight will range from 0.25 to 1.00
inches in urban areas and valleys, with 1-2 inches in the coastal
mountains with locally up to 3 inches.

Expect winds to continue to increase this morning, with speeds
between 20 to 30 mph with gusts to 45 mph for the lower elevations
and populated areas. Stronger winds will be felt in the higher
elevations where gusts up to 60 mph are possible. Winds will
gradually subside this afternoon and evening. A Wind Advisory is
in effect from 4 am to 4 pm Saturday for most areas.

Another round of precipitation and gusty winds is expected on
Sunday. An impulse in the flow on the backside of the trough will
be accompanied by a surface low, which will move directly through
the area Sunday into Sunday night. This impulse will be
accompanied by an increase in precipitable water, with values
near 1 inch. Strong moist onshore flow will develop and lead to
widespread moderate precipitation. Rain will overspread the area
by early Sunday afternoon, with rain continuing into the evening.
Rain will taper off from north to south Sunday night, with a few
residual showers during the day on Monday. Rainfall totals will
range from 0.5 to 1.5 inches in the urban areas and valleys, with
2-3 inches in the higher terrain, with locally up to 4 inches.
Winds will again become breezy to gusty Sunday afternoon and
evening, with winds 20-30 mph and gusts up to 40 mph.

Scattered showers will continue into Tuesday under moist southwest
flow. Another upper level trough and frontal boundary will
move through and lead to more widespread rainfall Tuesday
afternoon through Wednesday morning. Scattered showers will be
possible during the day on Wednesday as colder air aloft arrives.
Stronger winds are again possible with this system, as well as an
additional 0.5-1.5" of rain in urban areas with 1-3" in the higher
terrain.

High temperatures through next Wednesday will be in the mid 50s to
lower 60s, with overnight lows in the 40s to low 50s. A brief drying
trend is likely on Thursday, but looks to be short-lived as
models are indicating another trough to move into the California
Coast on Friday next week. Another trough is possible later in
the weekend. Suffice it to say, the storm door is open once again
and people should be prepared for periods of wet weather this
week.

&&

.AVIATION...As of 4:00 AM PST Saturday...for 12Z TAF Package. As
an approaching frontal system approaches the coast, look for
ceilings to lower and southerly wind speeds to ramp up. Winds will
veer to the southwest by this afternoon, then switch to westerly
by tonight. Periods of moderate rain may also produce lower
visibilities at the terminals. Another concern will be
unidirectional wind shear at the low levels, but at this point
think the mixing through the low levels will be sufficient enough
to withhold from the TAF.

Vicinity of KSFO...VFR conditions will transition to MVFR
ceilings within the next couple of hours as the frontal system
approaches. In addition to the ceilings, winds increase out of
the southeast this morning and be gusty to 35 kt, and then veer
to the south and eventually southwest through the day, decreasing
in the afternoon. The wind will also be accompanied with periods
of rain.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR conditions are generally forecast to
prevail for most of the night ahead of the next approaching
system. Southeast winds will increase this morning, becoming gusty
through the day. 35 to 40 kt gusts are possible at SNS this
morning. Periods of rain and MVFR ceilings expected after about
16Z.

&&

.MARINE...as of 2:39 AM PST Saturday...Southerly winds will
strengthen today with approaching storm system. Gale-force gusts
are forecast across the waters through midday Saturday. Infrequent
storm-force gusts may be possible in the outer waters Saturday
morning. In addition, a slight chance of thunderstorms are
possible with the arrival of the storm. A larger long period west
to northwest swell is forecast to arrive Saturday night into
Sunday with hazardous squared seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ006-505>512-517-518-528>530
             GLW...SF Bay
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay

&&

PUBLIC FORECAST: ST
AVIATION: Sims
MARINE: Sims

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