Small Craft Advisory 90, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1043 PM PDT Thu May 23 2019

.SYNOPSIS...Mainly dry conditions are forecast through Saturday,
along with continued cooler than normal temperatures.
Unseasonably cool and unsettled conditions are forecast to develop
later in the weekend as a system drops in from the north, with
scattered showers and isolated thunderstorms possible on Sunday.
Dry weather should return by Memorial Day, but temperatures will
remain cool into early next week.

&&

.DISCUSSION...As of 9:05 PM PDT Thursday...An upper low is
centered over the Great Basin this evening while a vort lobe
rotates around the west side of the low and across Eastern
California. This vorticity lobe has been triggering isolated
shower activity over mainly the eastern edge of our forecast area
since mid afternoon. Showers recently developed over far eastern
Contra Costa County and are moving south into eastern Alameda
County. There have been reports of lightning and thunder, but
based on lightning detection networks, it appears that
thunderstorms are farther to the east over San Joaquin County.
Based on latest HRRR and the 00Z NAM, all shower activity should
end by late evening.

The upper low is forecast to move quickly off the the northeast
tonight and Friday. Thus, Friday is expected to be a dry day
under partly cloudy skies. Temperatures will remain cooler than
average.

A couple of weather systems lurking well to our north are forecast
to drop south late in the week and through the weekend. The first,
currently over British Columbia, is expected to move into
northeastern California by Friday night. Models keep precipitation
with this system to our north and east. A second, stronger,
system, currently located near the south coast of Alaska, is
forecast to follow closely behind the first. The models agree
that a cold upper low will dig south along or near the California
coast late Saturday night and Sunday, and perhaps directly over
the SF Bay Area on Sunday. This unseasonably cool and robust
system will likely generate scattered showers across our area on
Sunday and Sunday evening (showers could start as early as
Saturday night). In addition, there may be sufficient instability
for isolated thunderstorms on Sunday. Models indicate the best
potential for thunderstorms on Sunday will be across the southern
half of our forecast area, primarily from the South Bay southward.
Rainfall totals with this weekend's system will depend on the
exact track of the system. The 00Z NAM shows the upper low center
tracking just offshore, and thus gives the system move over-water
trajectory compared to the 12Z ECMWF. Thus, the NAM generates more
than twice the precipitation compared to the ECMWF. The 18Z GFS
QPF is similar to the wetter NAM. If the ECMWF were to verify,
precipitation totals would mostly be below a quarter inch, but the
00Z NAM shows precipitation totals close to an inch in some
sections of Monterey and San Benito Counties. All models agree
that the heavier rainfall totals with the Sunday system will be
over our the southern portion of our area - mainly Monterey and
San Benito Counties.

The Sunday system is forecast to be unseasonably cold, with the
NAM forecasting snow levels to drop as low as 4000 feet. The
National Blend of Models (NBM) is more conservative with snow
levels, maintaining levels above 5000 feet on Sunday. Once again,
much will depend on the exact track of the upper low. If the low's
cold core tracks over or very close to our area, we could see snow
accumulation on the higher peaks in the Santa Lucia Mountains of
Monterey County on Sunday. And it's not out of the question that
snow could fall as low as the highest peaks in the Bay Area (e.g.,
Mount Hamilton). Temperatures on Sunday are forecast to be as
much as 20 degrees below normal.

The upper low is forecast to move well off to the southeast of
our area by Monday. Thus, dry weather is expected to return by
Memorial Day. Temperatures will warm slightly on Monday, but still
remain well below normal.

Although temperatures are forecast to climb close to normal by the
middle of next week, there doesn't appear much evidence from the
longer range models of any significant warming trend. In fact,
some longer range model solutions show an upper low retrograding
from the Great Basin over California and bringing more shower
chances later next week.

&&

.AVIATION...as of 10:42 PM PDT Thursday...Southerly flow has
developed along the coast and is impacting SFO and SJC. This
favors stratus into OAK and the North Bay with stratus spreading
into SFO late.

Vicinity of KSFO...Becoming MVFR after 12Z. South to southewest
winds 5-10 kt. Clearing expected after 18Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR cigs through 18Z at SNS and 19Z at
MRY.

&&

.MARINE...as of 10:36 PM PDT Thursday...Gusty winds gradually
decrease late tonight and early Friday as low pressure over the
Nevada moves east. Gusty northwest winds  will redevelop over the
coastal waters Friday night through the  weekend as another low
moves inland over the Pacific Northwest into the Great Basin. A
large northwest swell will subside but  remain moderate through
the end of the week and into the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar until 3 AM

&&

PUBLIC FORECAST: Dykema
AVIATION: W Pi
MARINE: W Pi

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