Severe Thunderstorm Warning 4, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
453 AM PDT Sun Aug 16 2020

.SYNOPSIS...Thunderstorm chances continue through Sunday and
into Monday morning. Temperatures will be slightly cooler today
however excessive heat will continue again today and through at
least mid week.

&&

.DISCUSSION...as of 3:30 AM PDT Sunday...Extremely busy night at
the office between extreme heat and a red flag warning for
thunderstorm risk, but the main focus has been a powerful, fast
moving severe thunderstorm that developed off the Central Coast
late last evening and has been producing frequent lightning and
erratic gusty outflow winds of 45 to 60 mph (update peak gusts 60-
75mph since the AFD was written). Due to the very frequent
lightning, expect wildfire starts to start where lightning occurs,
especially given the particularly dry vegetation and hot and dry air
mass near the surface below the elevated moisture aloft.

Latest WRF/NAM output show a much more aggressive push of MUCAPE
(a measure of potential energy at a specific level in the
atmosphere), PWATs (total moisture available in the atmosphere),
and modified total totals >34 across the region through Sunday and
into early Monday. Thunderstorms will develop primarily in the
core of a pair of elevated moisture surges rotating into the
region around the oppressive high pressure driving the excessive
heatwave and a 578dm 500mb trough centered just off the NW
California coast. This trough will further enhance elevated
convection as it transitions into the area of best upper level
dynamics on the eastern flank of the trough, which is what we have
seen occurring over the last day, and particularly tonight. Expect
new thunderstorms to rapidly enhance as they moved into this
preferred region throughout the next 24-36 hours.  Due to these
conditions, the Red Flag Warning for dry thunderstorms has been
extended until Monday morning.

In addition, the excessive heat warning remains in effect through
at least Wednesday. Today's forecast high temperatures were
increased on the overnight shift as previous days have
outperformed even the hottest model data and it is expected that
both day and night time temperatures will run warmer than
modeled, even after bias corrections. In addition, the heat
advisories for the nearshore zones will not longer just be issued
through the day but also through the night as overnight lows
struggle to drop below 70 degrees even at the coast. Without
significant overnight cooling, the days will start warmer and stay
hotter midday even longer through the day, both of which will add
to accumulating heat stress across the major urban areas. Many
have asked if these thunderstorms will mean it will be much cooler
today, and the answer unfortunately is no. This is because the
hottest, driest air is hovering just above the surface and up a
few hundred to thousand feet. Meanwhile, these thunderstorms are
sitting at about 10000 feet or higher, much too high for the
moisture to make any significant difference for us at the ground.
The increased albedo from these clouds in place will bring some
very minor cooling related to additional insolation heating from
the sun, however, the main reason we are so hot is because of the
hot dry air near the surface, not so much the insolation from the
sun.

As it is a very busy shift, will leave it here, so please check
the previous discussion for additional details.

.PREVIOUS DISCUSSION...as of 9:15 PM PDT Saturday...Active
weather continues at this hour with most of the activity remaining
just east of our area. With that being said models continue to
suggest the possibility of thunderstorm development overnight with
moisture streaming in from the south and moderate instability
over the region. We are currently monitoring a patch of clouds
located just off the Southern California coast that is gradually
making its way northward toward Santa Barbara. Models suggest this
area of moisture will continue northward across the Monterey and
San Francisco Bay Area overnight into Sunday morning. The
trajectory of the moisture coupled with the instability over the
region has the potential to trigger thunderstorms and dry
lightning over our area. This may result in additional fire starts
overnight. As a result the Red Flag Warning remains in effect.

The only significant change that was made to the forecast this
evening was the extension of the Heat Advisories through tonight
and Sunday. Overnight temperatures will provide limited and
short-lived relief with lows only expected to drop into the 60s
in the valleys and 70s and 80s in the hills and mountains.
Temperatures are forecast to ramp up rapidly once again on Sunday.
While temperatures will be slightly cooler than what we saw
today, the prolonged duration of this record breaking event will
increase the potential for heat related illnesses, particularly
among vulnerable populations.

A list of todays records is located in the Climate section below.

From previous discussion...Two main items of focus for the
current forecast:

1) Red Flag Warning and thunderstorms: Fire weather watches have
been upgraded to Red Flag Warnings. About 36 hours ago the models
started picking up on a monsoon push for today into Sunday. This
morning after 7 am the first lightning strikes occurred over Santa
Barbara county. This initial shortwave has kept most of the
convection over San Luis Obispo county but several strikes and
t-storms have made it into extreme southern Monterey county with
some action now shifting into Kings county. There are numerous
confirmed small fires in southern Monterey county and into San
Luis Obispo county and now one near Avenal. Even though the
storms are creating some rain drops the fuels are so dry there
is a high ignition efficiency. As always t-storm forecasts remain
difficult around the Bay Area for summertime events. In the short
term the Red Flag Warning is focused on Monterey and San Benito
counties. Expect more activity across this region through this
evening.

Earlier model runs showed good agreement that a second round of
activity would fire somewhere between the Santa Cruz mtns and Mt
Hamilton after dark tonight and then shift northward overnight.
The more recent runs of the hrrr/rap/nam have changed slightly.
However rather than chasing models decided to keep the forecast
idea the same. The 12z gfs seems to capture a shortwave currently
near the CA/Mexico border and rotate it northward overnight. The
18z gfs and short terms mesoscale models now show the better
chances for convection by morning somewhere around Pt Reyes and up
into Sonoma/Napa. Will continue to monitor short term convective
trends closely this evening and overnight. Main concern will be
any new ignitions. Any storms should end by midday Sunday across
the Bay Area and shift northward by Sunday afternoon. Must be
stated that most areas will not seeing thunder and lightning but
any new ignitions will be problematic.

2) Heat: Hot temps continue this afternoon with record high temps
falling once again. No changes to inland excessive heat warning
which goes through Weds evening. Heat Advisories remain in effect
around the Bay through 9 pm this evening. Idea remains that we
will do targeted advisories for those locations closer to the
water based upon what is reported for highs today. But either way
it will be another hot day on Sunday with record or near record
warmth. Not much time spent analyzing the long range but ecmwf
still showing a strong ridge at least through midweek.

&&

.AVIATION....as of 04:50 AM PDT Sunday...for 12Z TAFs. A large
line of thunderstorms moved through the Bay Area between 10 and
12Z with frequent lightning strikes and strong winds reaching 40
to 50 kts. New storms have already begun to form over the ocean
west of Monterey Bay and moving northward for the next round of
lightning and stronger wind gusts. Clouds will remain elevated
through out the day.

Vicinity of KSFO...The next round of storms approaches the
terminal that will bring frequent lightning and strong wind gusts
over 35 knots as the storms move through the terminal. These
storms are high based and cigs should remain elevated through the
day. Winds will remain breezy and onshore in breaks between
storms.

SFO Bridge Approach...Potential for clouds that produce lightning
and reduced vis on approach as storms move through the area.

Monterey Bay Terminals...Storms are currently out over the ocean,
but the threat for frequent lightning from high based clouds still
exists for area terminals. Winds are breezy and from the south,
but will increase locally as storms push through and develop
through the day.

&&

.CLIMATE...

RECORD HIGH TEMPERATURES SATURDAY, AUGUST 15, 2020

LOCATION                    HIGH           PREVIOUS RECORD
-----------------------------------------------------------------
SANTA ROSA                   103    TIED    103 IN 2019
NAPA                         103            102 IN 1906
LIVERMORE                    108    TIED    108 IN 1967
SFO AIRPORT                   94    TIED     94 IN 2019
REDWOOD CITY                 100    TIED    100 IN 2019
OAKLAND DOWNTOWN              93    TIED     93 IN 2015
GILROY                       108            104 IN 2019
MONTEREY                      91             90 IN 2015

Record High Temperatures for Sunday.
			    August 16
------------------------------------------------------
Santa Rosa		100 in 2019
Kentfield		101 in 2015
Napa			104 in 2015
Richmond		 94 in 2015
Livermore		105 in 1951
San Francisco	 90 in 2015
SFO				 90 in 2015
Redwood City	101 in 2015
Half Moon Bay	 72 in 2015
Oakland Dtown	 92 in 2015
San Jose		 97 in 2015
Gilroy			107 in 2019
Santa Cruz		 94 in 1983
Salinas			 94 in 1933
Monterey		 83 in 2015
King City		103 in 2015

&&

.MARINE...as of 02:18 AM PDT Sunday...Scattered thunderstorms are
moving over the southern waters this evening and will continue to
move northward Sunday morning. These storms are producing
frequent lightning and may produce rough seas and locally gusty
winds. Away from he storms are light west to northwest winds will
continue over the coastal waters through the weekend. Mixed seas
will persist with a short period northwest swell and a couple of
longer period southerly swells.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Excessive Heat Warning...CAZ507-510-511-513-516>518
             Red Flag Warning...CAZ516>518-528-530
             Red Flag Warning...CAZ006-505>513-529
             Heat Advisory...CAZ006-506-508-512-528>530
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm from 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 5 PM

&&

PUBLIC FORECAST: DRP/CW
AVIATION: DK
MARINE: DK

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Special Weather Statement
National Weather Service San Francisco CA
142 AM PDT Sun Aug 16 2020

CAZ509-512-516>518-528>530-161015-
Northern Salinas Valley/Hollister Valley and Carmel Valley CA-
Mountains Of San Benito County And Interior Monterey County Including
Pinnacles National Park CA-San Francisco Peninsula Coast CA-
Southern Monterey Bay and Big Sur Coast CA-Santa Cruz Mountains CA-
Santa Lucia Mountains and Los Padres National Forest CA-
Southern Salinas Valley/Arroyo Seco and Lake San Antonio CA-
Northern Monterey Bay CA-
142 AM PDT Sun Aug 16 2020

...A LINE OF THUNDERSTORMS WILL LEAD TO FREQUENT LIGHTNING WHICH MAY
LEAD TO WILDFIRES...

At 140 AM PDT, doppler radar and trained weather spotters reported
strong thunderstorms along a line extending from 46 miles west of
Del Monte Forest to 6 miles south of Gorda. Movement was north at 40
mph.  Expect these thunderstorms to move across the areas of Big
Sur, Monterey Peninsula, Santa Cruz, and Santa Cruz mountains over
the next two hours.

The main threat is frequent lightning from this line of
thunderstorms. This lightning may ignite wildfires where it strikes.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3579 12146 3583 12152 3611 12174 3629 12199
      3660 12207 3670 12199 3671 12190 3681 12188
      3689 12195 3688 12208 3697 12228 3716 12248
      3729 12249 3728 12248 3696 12163 3689 12158
      3692 12154 3681 12126 3579 12135
TIME...MOT...LOC 0840Z 180DEG 37KT 3634 12273 3584 12144

DRP