High Wind Watch 3, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1013 AM PST Mon Feb 11 2019

.SYNOPSIS...A few lingering showers possible early this morning
before drier weather develops this afternoon. An atmospheric river
will take aim at California on Wednesday with heavy rain and gusty
winds likely. 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.

&&

.DISCUSSION...as of 09:50 AM PST Monday...Radar this morning
still showing a few scattered showers, mainly off the coast. These
showers are slowly drifting east and a few isolated locations
could experience light rain this morning. Showers are expected to
continue to decrease in coverage as weak ridging moves overhead.
Temperatures this morning have been chilly, with 30s most
locations to lower 40s around San Francisco and parts of the Bay.
Cool high temperatures will continue today with low to mid 50s
expected in the lower elevations.

Focus this afternoon will be on the upcoming atmospheric river
expected late Tuesday through early Thursday. A trough currently
off the Pacific Northwest coast is forecast to phase with a trough
northeast of Hawaii on Tuesday. Moisture originating near Hawaii
will be drawn northeast into California as these two troughs
interact. As the trough and associated cold front to our northwest
moves southeast on Tuesday, light rain showers will be possible
mainly across the North Bay.

A surface low will form and deepen on Tuesday to our southwest.
This will quickly move northeast to just off the California coast
by Tuesday night. A plume of high PWAT will advect northeast into
our region under strong southwesterly flow. Moderate to heavy rain
will overspread the area Wednesday morning with the slug of
moisture, with impacts to the morning commute likely. A cold
front will approach but then stall out to our west before reaching
land. Southwesterly flow will continue into the afternoon with
more rain expected, although this could be less widespread and
steady than the morning. A second impulse is shown by the 12Z NAM
and 00Z ECMWF to move northeast Wednesday night with another round
of heavy rain. However, these models differ on the positioning of
the heavy rain. This feature is not shown by the 12Z GFS to move
into the area. As such uncertainty remains high with this possible
second impulse. In short, heavy precipitation is likely at times
across the area on Wednesday, with the areas seeing the heaviest
precipitation still uncertain. Confidence in precipitation
coverage and intensity on Wednesday night remains low. Areas of
flooding, landslides, and debris flows will be a concern as soils
are already saturated.

The surface trough will begin to move onshore Wednesday
afternoon. The pressure gradient will become very strong with 925
mb winds reaching 60 to 65 kt by midday Wednesday. Strong, gusty
winds are likely to reach the surface by Wednesday morning and
continue through Wednesday evening. Will wait for the 12Z WRF to
come in but likely will need to issue wind products for Wednesday.
For additional details, see the previous discussion.

&&

.PREV DISCUSSION...as of 3:48 AM PST Monday...The last gasp of
Sunday's storm system is still producing a few scattered showers
over the coastal waters and along the Central Coast. KMUX radar
imagery shows numerous echoes off the coast from near Pt Reyes
southward to Pt Sur. These showers are expected to continue
through mid- morning before gradually lifting to the NE. It should
be noted that snow levels still remain low and Mt Umunhum is
still sitting at 31 degrees. If showers move across the Santa Cruz
Mountains some snow would be possible. Outside of the shower
activity mostly clear skies prevail with cold temperatures. Lows
this morning will bottom out in the 20s/30s interior and mid
30s/40s urban/coastal areas. Given the recent rains and snow any
water runoff moving onto roadways has the potential to become
black ice. By this afternoon any lingering showers will be mainly
across the North Bay as ridging builds in from the south. Snow
levels will warm a little, but the highest peaks across the North
Bay could see some snow once again.

A cold front will approach NorCal late Monday into Tuesday as high
pressure remains across most of SoCal. Model consensus does show a
few showers will be possible ahead of the front during the day
Tuesday, but mainly north of the Golden Gate. By Tuesday afternoon
the area of high pressure to the south shifts eastward as a low
pressure develops along the cold front to the northwest. This
point in time is when the forecast starts to get real interesting.

For several days now medium range models have suggested an
atmospheric river midweek, but varied on details - timing and
strength. 00Z model suite is finally honing in on a solution that
will likely bring very gusty winds and heavy rain. Given the
tropical nature of the moisture plume snow levels will jump
dramatically to 7000-9000 feet in about 12 hours. For the most
part the ECMWF has been the most consistent with this potential AR
event and the GFS/NAM/Canadian are starting to follow. The first
big push of moisture is forecast to arrive over the North Bay
Tuesday night/early Wednesday before a secondary push across the
Big Sur Coast Wednesday with PWATs of 1-1.25" - pretty juicy
airmass. AR forecast models show IVT values approaching 750
kg/m/s...or a moderate/strong AR. When it comes to the make or
break part of the forecast it's all about where the AR makes
landfall. That's where one can go from Boom to Bust on rainfall
totals. WPC excessive rainfall forecast for Day 3 has much of the
Big Sur Coast in a slight risk of exceeding flash flood guidance.
Of concern as well is the current snow pack of a foot or more in
the Santa Lucias. Heavy rain and snow runoff would likely lead to
flooding concerns along the Central Coast. Other locations in the
Bay Area will also be of concern with flooding potential given
the copious amounts of rain in 12 hours. River Forecast Center
does show a rapid rise on mainstem rivers of 10-12 feet on
Wednesday. It should be noted that none of the mainstem are
projected to flood with the current guidance, but that could
change. Rainfall will gradually diminish through the day on
Thursday. Storm total rainfall from Tuesday through Thursday will
be: 2-3" North Bay - 1-2" elsewhere and 4-6" coastal mountains and
locally up to 8" Santa Lucias.

In addition to rain wind will be an impact starting Tuesday over
the coastal waters and higher terrain. Winds really crank up
early Wednesday with local WRF model showing sustained 30-40 mph
with gusts 60-70 mph in the hills. Given the saturated soils and
expected rainfall trees will likely come down. A wind advisory or
combination wind advisory/high wind warning will likely be
needed. The SW to S wind direction will also have some rain
shadowing effects during the onset of precip.

The heaviest precip will be done by Thursday afternoon, but medium
range models keep an unsettled weather into next week with
additional storm system.

Simply put, the midweek storm system could be a big impact event
with heavy rain, flooding and destructive wind. Please stay tuned
to the forecast as details become more clear and confidence
increases.

&&

.AVIATION...As of 10:10 AM PST Monday...For 18Z tafs. VFR
prevailing with SCT to BKN clouds 4,500 to 6,000 ft AGL. Radar
continues to show a few residual light, low-topped showers over
the coastal waters, moving onshore in the North Bay but generally
dissipating over land elsewhere. VFR conditions expected to
prevail through the day, aside from brief reduction to MVFR
vicinity of showers. Winds will generally be light.

Vicinity of KSFO...VFR. Varying SCT to BKN clouds 4,500 to 6,000
ft. Light offshore winds through early afternoon then light
onshore.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. E/SE winds into the early afternoon
then turning onshore through the evening. SCT clouds around 5,000ft.

&&

.MARINE...as of 08:06 AM PST Monday...Locally breezy westerly
winds this morning will back to become increasingly southwesterly
through the next 24 hours ahead of an arriving potent storm
system. Gale force winds will be likely by Tuesday afternoon to
evening with possible storm force 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...
     .Tday...NONE.

&&

PUBLIC FORECAST: ST
AVIATION: Blier
MARINE: DP

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