Lightning Strike and Strong Wind in California, August 23, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
532 PM PDT Sun Aug 23 2020

.SYNOPSIS...Scattered thunderstorms are expected Sunday night
through Monday morning as elevated moisture from Baja California
moves northward. The main threat will be lightning and gusty,
erratic winds as the storms develop. A Red Flag Warning remains in
effect through 5 PM PDT. Seasonably warm and dry conditions will
continue to the remainder of the week.

&&

.DISCUSSION...as of 02:11 PM PDT Sunday...Conditions around the Bay
Area and the Central Coast are warm and dry. Stratus clouds have
developed over the Pacific, but a virtually non-existent marine layer
and light winds have allowed relative humidities to be dry inland
once again. Lower level winds over the ocean are from the northwest,
and much more variable over the rugged terrain inland. Aloft
however, winds are out of the west southwest ahead of an approaching
system.

High clouds have been moving over the Bay Area all day ahead of a
low pressure system that was remnant tropical Genevieve. The 18z
sounding shows temperatures have cooled at mid-levels as the surge
of moisture moves northward as values of precipitable water as
nudged upward. Along the northern flank of the circulation, elevated
thunderstorms continue to move northward from the coastal waters off
southern California and up the Big Sur coast. This cluster of storms
has already created lightning strikes just west of Morro Bay, as
detected by GLM on GOES-17 satellite. As we approach the time of day
for peak heating (and added instability), scattered storms are
expected to continue to push northward, over the Central Coast and
Monterey Bay later this afternoon and over the Bay Area this
evening. Lightning and gusty erratic winds are the main concern with
these storms. With conditions as dry as they are presently around
the region, a Red Flag Warning was issued and remains in effect
through 5 PM PDT on Monday. We have been broadcasting that the "main
event" of this system will be Sunday evening through Monday morning,
and based on recent model runs bringing the ingredients together,
that looks to remain the same.

As this low moves northward, the moisture will wrap around its
center point and provide a chance for thunderstorms to continue on
Monday. While models are showing decreased instability present for
the system to work with, scattered storms are still possible through
Monday evening, with decreasing chances as a weak upper level short
wave is expected to move toward Northern California. However, a lot
of it will depend on how the current event transpires and the
progress of the longer wave trough over the Pacific moving eastward

Beyond that remains a lot of model uncertainty. It appears that the
upper level ridge over the Four Corners will remain in place, which
will keep temperatures warm and dry conditions to be prevalent. The
good news is that once this moisture passes, it does not appear that
there will be another large influx of moisture that would trigger
additional storms Wednesday onward. As the long wave trough moves
over California, it will set up consistent southwesterly flow aloft
(a new direction for the wildfire fight).

&&

.AVIATION....As of 5:32 PM PDT Sunday...for 00z TAFs. It's a mix
of LIFR-IFR in wildfire smoke and poor slant range visibilities
and MVFR-VFR coastside and bayside. The marine layer is near
1,000 feet deep per profiler data. Patchy coastal stratus and fog
are also reducing visibilities and ceilings resulting in LIFR-IFR.
Scattered high based showers are moving up from the south with a
swath of warm air advection, echo tops 27 thousand to isolated 30
thousand feet. Lightning is 300 miles west of Point Conception
near 700 mb to 500 mb troughing.

Vicinity of KSFO...VFR, vicinity showers this evening, northwest
to west wind near 15 knots decreasing to near 5 knots overnight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR, a few evening showers are possible
otherwise wildfire smoke and haze likely mixing with patchy fog
and low stratus ceilings /VLIFR-IFR/ tonight and Monday morning.
A return to VFR is likely Monday afternoon with onshore breezes.

&&

.MARINE...as of 02:05 PM PDT Sunday...Remnants of Hurricane
Genevieve will bring a moist and unstable air mass across the
coastal waters resulting in a chance of thunderstorms through late
tonight. Northwest winds will continue to decrease tonight
becoming light Monday through Wednesday across the inner coastal
waters.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ006-505>513-516>518-528>530
             SCA...Pt Arena to Pt Reyes 0-10 nm from 7 PM until 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 7 PM until 3 AM

&&

PUBLIC FORECAST: DK
AVIATION: Canepa
MARINE: CW

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