Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 181724

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1024 AM PDT Sun Mar 18 2018

.SYNOPSIS...Dry weather conditions and a slight warming trend can be
expected Sunday into Monday ahead of our next storm system. This
system will bring widespread rainfall to our region from Tuesday
through Thursday, along with the potential for periods of heavy
rain and locally strong southerly winds.

&& of 08:45 AM PDT Sunday...The broad upper low
that brought several days of unsettled weather to the region has
mostly exited California. In the wake of this storm system, high
pressure is now building in from the eastern Pacific. Residual low
level moisture and mostly clear skies overnight allowed
temperatures to cool to their dew points which were generally in
the mid 30s to low 40s, generating areas of patchy dense fog in
most of the inland valleys around the San Francisco Bay area.
Temperatures are running a few to several degrees cooler than this
time yesterday morning, even in areas with the fog, primarily due
to the absence of insulating clouds in the mid levels that were
present yesterday morning.

For the rest of the day, expect light- mostly offshore winds with
afternoon temperatures that are a few to several degrees higher
than yesterday (upper 50s to low 60s). High clouds ahead of the
next storm system will begin to encroach on the region from the
west through the day today then gradually thicken and lower through
tomorrow as the next storm system approaches the Golden State.
The weak ridge will strengthen inland through tomorrow,
bringing a warming and drying trend. Expect temperatures to climb
again for tomorrows daytime highs, with temperatures generally in
the mid to upper 60s.

The next storm system is evident about 1200 miles due west of
California/Oregon early this morning. This storm system is
currently breaking off from the mean westerly flow and digging
southward and is forecast to become a cutoff low over the next
several hours. As this system breaks away from the mean westerly
flow aloft, it will become coupled with a deep 1.4-1.6 TPW
subtropical moisture plume, advecting it towards California over
the next 48 to 96 hours. Forecast models have shown a clear trend
towards aiming this subtropical moisture plume at the central
Coast, ie Big Sur, southward towards Point Conception/Santa
Barbara county in southern California. As such, for now it
appears that most of the urban locations of the San Francisco Bay
area will dodge the heaviest rains. We will be closely monitoring
any northward trends of this moisture plume as it will greatly
affect the total forecast precipitation amounts. That said, the
San Francisco Bay area will not be excluded from some impacts from
this storm system. Instead, areas north of Big Sur should
anticipate more run of the mill storm system impacts associated
with the arrival of a moderate to strong cold frontal passage with
post frontal convective rain showers as the bulk of the enhanced
rainfall falls over Big Sur and SoCal. These impacts include gusty
southerly winds, with sustained speeds of 15 to 30 mph and gusts
of 35 to 55 mph possible ahead of and with the arrival of cold
front. The strongest winds will be over the usual locations, ie
the coast, higher elevation passes and peaks, and favorably
oriented valleys. Rainfall amounts will mainly range from 1-2
inches in most areas, while much more (3-5, locally up to 8
inches) are possible over the Big Sur/Santa Lucia mountains due to
enhanced orographic precip and positioning of the deepest
moisture plume. Our main forecast challenge from now through the
arrival of the storm system will be to fine tune the timing,
positioning, and amounts of the rainfall/winds associated with
this storm system. Due to the trajectory of this storm system
(west to east rather than our typical northwest to southeast),
expect rain to become widespread over the entire area around the
same time frame (lighter rain ahead of the deeper plume in the
warm sector Tuesday, then heavier rain in conjunction with the
arrival of the deeper moisture tap/cold frontal boundary by
Wednesday, then lingering post frontal convection/showers into

See previous forecast discussion for more details on the current
(overnight) forecast package.

.PREVIOUS of 04:01 AM PDT Sunday...Temperatures
are running a few to as much as 9 deg F cooler compared to 24
hours ago as mainly clear sky conditions prevail over the region.
Remnant low level moisture is helping to keep temperatures from
cooling too much and should prevent most locations from reaching
the freezing mark. With that said, patchy fog is already being
reported in the North Bay Valleys as well as the far southern
Salinas Valley as surface temperatures have cooled to dewpoint
values (generally in the middle 30s). In the Santa Clara and East
Bay Valleys, low clouds have developed early this morning. After a
cool start to the morning, temperatures will warm into the upper
50s to lower 60s as a short-wave ridge builds off of the coast.
Dry conditions along with additional warming is forecast for
Monday with most areas warming into the lower to upper 60s under a
mostly sunny sky.

A developing storm system well offshore is forecast to approach the
West Coast by the midweek with deeper moisture advecting into the
California coast. With this, cloud cover will be on the increase
late Monday into Tuesday morning with widespread rain likely to
impact the region around or slightly after sunrise Tuesday morning.
The latest WRF and NAM forecast output shows moderate rain spreading
across the region with PWAT values of 1.25" - 1.50" pointed toward
central and southern California through Tuesday afternoon. At this
time, the forecast models do point the deepest moisture into
Monterey County southward (southern California). Regardless,
additional rainfall will likely impact the entire region again on
Wednesday as a weak mid/upper level disturbance approaches northern
California. The core of the mid/upper level trough will then rotate
across the region Thursday with continued changes for rain turning
to showers Thursday afternoon/evening into Friday. Exact details on
the timing of each passing system and placement of the atmospheric
river remains difficult to pin-point at this time. Given the
uncertainly, folks should prepare for the potential for periods of
moderate to locally heavy rainfall and breezy winds during the
middle portion of the upcoming week.

While dry conditions will develop over the region late Friday
into Saturday, showers may linger across the northern portion of
the region during this time frame as one last mid/upper level
disturbance pushes inland across northern California. Be sure to
to check back for the latest forecast information in the coming
days we continue to sort out the details.

&& of 10:25 AM PDT Sunday...VFR today. A light
offshore wind this morning becomes onshore 5-10 knots this

Vicinity of KSFO...VFR. Light wind this morning
becoming onshore near 10 knots by early afternoon.

SFO Bridge Approach...VFR.

Monterey Bay Terminals...VFR. Light E-SE winds return this

&& of 8:23 AM PDT Sunday...High pressure over the
coastal waters will bring light northwest winds today and tonight.
Winds will shift to southerly on Monday and increase Tuesday as a
deep low tracks northeast across the offshore waters. Along with
gusty winds seas will become mixed Tuesday through Thursday.





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