Rip Current in California, August 18 to 19, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
548 PM PDT Wed Aug 18 2021

.SYNOPSIS...Dry conditions remain in place on Wednesday with
smoky and hazy conditions lasting through Thursday afternoon.
Offshore winds are forecast to briefly increase over the North
Bay Mountains on Wednesday night keeping fire concerns high.
Seasonable temperatures continue into the weekend.

&&

.DISCUSSION...as of 01:42 PM PDT Wednesday...An upper level low
centered over southwestern Idaho continues its gradual movement
east over the Rocky Mountains. This low was the driving force
behind the critical fire conditions that were present last evening
which brought dry conditions and gusty offshore winds. Winds have
generally subsided, but are still out of the north-northeast with
gusts between 15 to 20 mph, with highest elevations occasionally
gusting up to 30 mph. With winds that have weakened and humidity
levels from 20 to 40 percent, it was determined that the Red Flag
Warning over the North Bay Mountains and the East Bay Hills could
be cancelled early. However, dry conditions still prevail. Even
though winds have weakened, a new fire, the Cache Fire, near
Clearlake in Lake County outside has started recently. Even though
a Red Flag is not valid in our county warning area, caution is
 urged so that no one becomes the spark that the critically dry
fuels could use to start another fire.

Higher resolution HRRR, NAM and WRF models are all depicting an
increase in winds later tonight, mainly over Napa County. Winds
will likely increase between 20 to 25 mph, with isolated gusts
around 30 to 35 mph along the highest peaks and ridgetops with
relative humidity levels down in the 20 percent range with brief
periods of time in the upper teens. While the increase in
offshore winds and dry conditions make for elevated fire concerns,
the short temporal range and minimal land area covered is the
main reason there is no valid red flag warning specifically issued
for this event. But this event will be monitored through the
night.

The upper level trough has also provided an influx in smoke to the
region. The northerly winds are pushing smoke from the River Fire,
the Dixie Fire, and other fires in Northern California and Oregon
southward. Many ASOS sites around the Bay Area are recording a
ceiling between 4000 and 5000 feet. But that ceiling is not from
clouds, it is from the smoke.

Additionally to the fire concerns and increase in smoke, the
northerly winds are pulling a cooler air mass southward toward the
Central Coast. Many stations are reporting temperatures in the 70s
with isolated low 80s for far interior areas expected. While much
of the cooler temperature is brought to the area from the cooler
air mass, the previously mentioned smoke will also act to prevent
more daytime heating to occur Wednesday afternoon.

By Thursday, the trough starts to move far enough away eastward to
providing more zonal westerly flow in its wake. The westerly winds
aloft will help to push some of the smoke toward the Central
Valley on Thursday afternoon. With onshore winds, expect
temperatures to be fairly similar tomorrow.

Another upper level trough looks to develop over southwestern
British Columbia Friday afternoon, sinking down over the Pacific
Northwest on Saturday. It looks to drop southward, tightening the
pressure gradient and bringing offshore winds back to the North
Bay. Exact timing and magnitude of the winds are still to be
determined. For now it will be closely monitored. If the track of
the trough remains where models currently project it to go, it
should remain far enough away to have conditions be similar to
tonight, where an increase in an occasional higher gust at higher
elevations may be possible keeping fire concerns high.

The long range forecast shows a 594 dm high at 500 mb develop over
Texas early next week, and slowly retrograde back towards the
state of California. A stationary short wave trough looks to
develop along the coast extending down past the Mexican border. If
it remains in place, it will keep southwesterly onshore winds in
place, but how it interacts with the high will be watched closely.
Temperatures will likely increase next week as a result, but not
resulting in a heat wave at this time.

&&

.AVIATION...as of 05:35 PM PDT Wednesday...For the 00z TAFs.
Skies over the district are free of clouds but filled with
widespread smoke and haze. A weak marine layer lid is evident at
about 1200 feet in the new 00Z KOAK sounding, and at around 2000
feet in the latest Bodega Bay and Fort Ord profilers.

Near surface smoke remains the primary aviation concern tonight,
though marine stratus ceiling onset is expected later this
evening at both KMRY and KSNS. Bay Area terminal visibilities
will continue to vary between MVFR and VFR, and slant range
visibilities will be reduced.

Vicinity of KSFO...VFR first part of the evening, then low
confidence forecast of visibility reduction into the MVFR range
beginning around mid-evening due to wildfire smoke. Marine
stratus ceiling development is not expected either overnight or
Thursday morning. Onshore winds to around 15 kt, diminishing later
in the evening.

KSFO Bridge Approach...similar to KSFO.

Monterey Bay...VFR through mid-evening, then low-to-moderate
confidence in forecast of MVFR stratus ceiling development late
evening. Hazy aloft due to smoke, with some reduction in both
slant range and surface visibilities possible at times. Onshore
winds 10 to 15 kt into the first part of the evening then
diminishing.

&&

.MARINE...as of 02:42 PM PDT Wednesday...Strong northwest winds
prevail over the northernmost outer waters that can generate
locally steep waves and result in hazardous seas conditions. Mixed
seas continue with a longer period southerly swell and short
period northwest waves at 8 to 10 seconds.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...SF Bay until 9 PM

&&

PUBLIC FORECAST: DK
AVIATION: Blier
MARINE: Bingaman

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