Wildfire in California, July 20, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
Issued by National Weather Service Los Angeles/Oxnard CA
200 PM PDT Mon Jul 20 2020

.SYNOPSIS...Expect cooler-than-average conditions to continue
through the middle of the week with even a little drizzle near
the ocean at times. Warmer conditions will slowly return later in
the week.

&&

.DISCUSSION...as of 1:50 PM PDT Monday...Its July, inland
sunshine and coastal clouds. Despite the inland sunshine this
afternoon the latest 24 hour trends show a downward trend in
temperatures. A few things are at play for the slightly cooler
temperatures. The persistent southerly flow along the coast
finally relented over the last 24 hours and has now switched to
northwesterly. The more summerlike northwesterly flow over the
waters will keep more coastal clouds piled up at the coast.
Additionally, the ridge of high pressure over the region has
shifted eastward. In its wake, an upper low offshore is deepening
and slowly tracking eastward. Northwest winds, onshore flow and
approaching upper low led to the slight cool down today.

No major changes from previous forecast. The upstream upper low
will continue to move over the region through midweek. As a
result, night and morning low clouds will continue. Given the
lowering 500mb heights would not be surprised to see the marine
layer depth actually deepen over the next few days. Some of the
models develop some precipitation along the coast, which is most
likely drizzle. Do expect a daily rollback to the coast of the
clouds, but despite afternoon inland sunshine temperatures will
remain on the cool side and below seasonal averages for late
July. The other weather item of note through midweek will be the
monsoon. As the upper low moves eastward flow aloft will become
more southerly. The southerly flow will tap into some upper level
moisture and pull it northward up the Sierra Nevada and into
NorCal. Immediate impacts to the Bay Area are non-existent, but
locations N and E have a shot for afternoon thunderstorms. The
concern would be fire weather with lightning and new starts.

By Thursday the upper low weakens and becomes more of an open
wave as it slides eastward. Even though the low weakens a broad
upper trough will linger over the region keeping temperatures
below seasonal averages Thursday and into next weekend.

&&

.UPDATED AVIATION...as of 11:45 AM PDT Monday...for 18Z TAFs. Low
cloud cover has been eroding over most areas. Stratus with
mostly MVFR ceilings has been slower to clear out over OAK and
around the Golden Gate and Bay Bridges. Patches of stratus
may linger along the immediate coastline for much of the day.
Breezy onshore winds are expected this afternoon and early
evening. With the large scale pattern remaining in place, a
return of similar cloud cover is this evening as winds subside.

Vicinity of KSFO...Cloud cover has cleared out over most areas,
however MVFR ceilings lingered over OAK and around the Golden Gate
and Bay Bridge of 19Z. The stratus is expected to clear out in these
areas by 21Z. Onshore winds will prevail and become breezy this
afternoon. A return of stratus and lower cigs is forecast this
evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Conditions have been improving away
from the coastline and Monterey Bay where MVFR ceilings persisted.
Conditions are forecast to improve after 21Z in these locations
where the stratus has been more stubborn to clear out. Some patches
of low clouds may linger near the coastline in the afternoon. Winds
will remain onshore and breezy, especially funneling through the
Salinas Valley. Stratus will make another early return to the
peninsula as winds ease overnight and return to cigs IFR.

&&

.MARINE...as of 10:02 AM PDT Monday...Light westerly winds will
prevail near the California coastline, with breezier northwest
winds farther offshore. Locally gusty onshore winds are forecast
this afternoon and evening across the northern San Francisco Bay
from the Golden Gate out into the west Delta as onshore breezes
continue.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...SF Bay until 9 PM

&&

PUBLIC FORECAST: MM/EB
AVIATION: DB@EKA
MARINE: Sweet

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