Gale Watch 9, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
854 PM PST Mon Feb 11 2019

.SYNOPSIS...An atmospheric river originating near Hawaii will
take aim at California on Wednesday with mild temperatures, heavy
rain, and gusty winds. Areas of flooding and wind damage will
become a concern as the system arrives on Wednesday. A return to
cooler, showery weather is expected over the weekend with snow
possible above 3000 feet.

&&

.DISCUSSION...as of 8:53 PM PST Monday...A mixed bag with respect
to temperatures today with some locations warming slightly more
than others as the tail end of Sundays system gradually lifted to
the northeast. This brought cool and unsettled weather over
Northern California. The North Bay saw a little to no warming this
afternoon while areas to the south warmed an average of 3 to 5
degrees over yesterday's highs. The warmest southern interior
portions of Monterey and San Benito counties saw upwards of 6 to 9
degrees of warming. Lingering moisture also brought a few showers
to the area through this afternoon. However all the shower
activity has now dissipated. Anticipate cool and dry weather
tonight into Tuesday with lows mainly in the 30s with lower 40s
around the Bays and along the coast. A very subtle warming trend
is forecast tonight through Wednesday with a few degrees of
warming expected each day as we transition from one weather system
to the next.

Our focus now turns to the Atmospheric River event that is
currently developing offshore. The forecast remains on track with
no update expected this evening.

From previous discussion...Now on to the upcoming storm. A trough
dropping south off the coast of the Pacific Northwest is forecast
to phase with a trough northeast of Hawaii. The trough near
Hawaii is associated with tropical moisture, which will be pulled
northeast as the troughs combine. A surface low northeast of
Hawaii will move northeast and deepen as the northern deep upper
trough begins to interact with it. By Wednesday morning, the low
center will be located about 100 miles off the northern California
coast. Southwesterly flow will increase over the area and a
region of high PWAT (1.1-1.3") will move overhead. The 925 mb flow
will be more southerly than southwesterly, which is not ideal for
prime orographic precipitation in our area but with speeds near
50-65 kt should still suffice to generate high precip rates over
the higher terrain Wednesday morning. The airmass, with roots near
Hawaii, will be mild and snow levels will rapidly increase to
8-9k feet by Wednesday morning. Thus, precipitation will fall as
rain in all areas. Snow contained in the higher terrain will melt
and add to the runoff. Heavy rainfall is likely for the Wednesday
morning commute, with some roadway flooding possible. At this
time, the most widespread rainfall is expected to occur Wednesday
morning with rain continuing into Wednesday afternoon under
southwesterly flow.

Models are struggling with a second wave in the atmospheric river,
which could move into the region Wednesday night into Thursday
morning. The 12Z NAM/ECMWF bring another surface low northeast
with the focus of additional heavy rainfall over the North Bay.
The cold front is then forecast to push through Thursday morning
with one last round of heavy rainfall possible in all areas as it
propagates. Meanwhile, the 12Z GFS pushes the upper trough
through earlier with precipitation tapering off by Wednesday
night. With the model discrepancies, have low confidence in the
precipitation forecast after 00Z Wednesday. A shift in the track
of the surface low will alter the axis of heaviest precipitation.
Have leaned the current forecast more towards the NAM/ECMWF
solutions with precipitation continuing through Thursday morning.
Storm total precipitation amounts through Thursday morning are
forecast to be 1-2" in the Bay Area and populated areas of the
Central Coast, 2-4" North Bay Valleys, and 4-6" (locally up to 8")
in the North Bay Mountains and Coastal Mountains. Soils are
already saturated from previous systems, leading to an increased
risk for widespread shallow landslides, rock slides, and debris
flows. A Flash Flood Watch has been issued from Tuesday evening
until Thursday morning. Additionally, mainstem rivers will rise
with some approaching monitor stage before precipitation ends.

Winds will increase with this system on Tuesday evening and
become gusty through Thursday morning. The strongest winds will
likely peak Wednesday morning and afternoon, with a secondary peak
Wednesday night as the second surface low approaches the area.
Gusts of 45 to 65 mph are expected and have issued a High Wind
Watch for much of the area. With saturated soils, these winds will
likely topple some trees and powerlines with power outages
possible.

After the cold front moves through, our area will remain at the
base of a cool upper trough Friday through early Sunday. Showers
will continue under this regime but will be much lighter than the
midweek atmospheric river. Colder air will also filter into the
area with snow levels dropping back down to near 3000 feet.
Temperatures for President's Day weekend will be below normal,
with highs in the mid 50s and lows in the upper 30s to mid 40s.

&&

.AVIATION...As of 3:55 PM PST Monday...Weak shortwave passing
through to the north. Cigs down to 3500-5000 ft over the North
Bay and SFO Bay Area with a few showers over the North Bay. Cloud
height will lift tonight as the system pulls NE. VFR expected for
Tuesday.

Vicinity of KSFO...Cigs may get down as low as 3500 ft before
rising up after 04Z. Light winds.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with clouds staying north of the MRY
Bay Area.

&&

.MARINE...as of 08:13 PM PST Monday...Locally breezy westerly
winds will back to become increasingly  southwesterly tonight and
into early Tuesday ahead of a strong  storm system. Gale force
winds will be likely by Tuesday to  evening with storm force gusts
winds by Wednesday morning. Strong winds will generate steep wind
waves creating hazardous  conditions over open waters and along
the coast. A series of  northwest swells will continue to impact
the waters through the  period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

PUBLIC FORECAST: ST/CW
AVIATION: W Pi
MARINE: DP

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