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

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FXUS66 KMTR 182342

Area Forecast Discussion
National Weather Service San Francisco Bay Area
442 PM PDT Sun Mar 18 2018

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

&& of 01:40 PM PDT Sunday... The broad upper low
that brought several days of unsettled weather to the region has
now exited California. In the wake of this storm system, high
pressure is building into California from the eastern Pacific --
resulting in light winds and slightly warmer temperatures today,
with a gradual warming trend into tomorrow. Tomorrow will be the
warmest day of the last several, with afternoon highs in the mid
to upper 60s.

That said, high level moisture out ahead of the next storm system
has already begun to advect from the west. As a result of this
moist air mass, the sky and horizon are more of a gray- white hue
rather than a blue one early this afternoon. This initial moisture
surge is the spear head of 1.4 - 1.7 TPW subtropical moisture
plume that will be advected towards California through the next
several days.

Visible satellite imagery shows a 534dm 500mb cut off low in the
making approximately 1300 miles west of the California/Oregon
border. This storm system is expected to completely separate from
the mean westerly jetstream aloft through the next few to several
hours. Next, the trajectory of this storm system will shift
to the east, towards California. As this low moves towards
California, the cyclonic flow will allow the storm system to tap
into the aforementioned subtropical moisture plume and couple its
trajectory to that of the southern periphery of the storm system.

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 is looking increasingly
likely that the greater San Francisco Bay area will dodge the
heaviest rains. 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 as the bulk of the enhanced rainfall falls
farther to the south over Big Sur and SoCal.

Rainfall amounts will mainly range from .75-1.5 inches in most
urban 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.
The latest forecast QPF values have been ramped up significantly
over the Big Sur peaks and slightly elsewhere around the Monterey
Bay (ie Santa Cruz mountains, Salinas valley), while slightly
reduced farther northward, particularly in the North Bay. The main
impact could be some nuisance flooding for most urban locations,
however, rock/mud slides could occur in susceptible areas of Big
Sur during the highest rain rates.

Snow levels are forecast to be well above our highest elevations
as the initial moisture surge comes ashore Tue-Wed due to the
tropical nature of the air mass. Snow levels will eventually drop
down to the 3500-4000 ft range in the post frontal environment.

In addition to the rain, gusty southerly winds, with sustained
speeds of 15 to 30 mph and gusts of 35 to 55 mph are expected
ahead of and with the arrival of cold front, particularly on
Wednesday. The strongest winds will be over the usual locations,
ie the coast, higher elevation passes and peaks, and favorably
oriented valleys. Downed trees and power lines will be the main
risk here.

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 Thursday into Friday).


.AVIATION...As of 4:45 PM PDT Sunday...Lower humidities will make
for VFR conditions through Monday.

Vicinity of KSFO...VFR.

SFO Bridge Approach...VFR.

Monterey Bay Terminals...VFR.

&& of 4:45 AM PDT Sunday...High pressure over the
coastal waters will bring light northwest winds 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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