A disturbance rotating around an upper level low west of San Diego brought thunderstorm activity to the Bay Area on September 11. Widespread reports of lightning were received along with a few small hail reports and strong wind gusts.
Every event with a place to put it; 8 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MTR wrote it up.
A disturbance rotating around an upper level low west of San Diego brought thunderstorm activity to the Bay Area on September 11. Widespread reports of lightning were received along with a few small hail reports and strong wind gusts. It has been reported that there were over 40,000 lightning strikes across the Central Coast of California during this event. Several brush fires were also ignited due to lightning strikes.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Francisco Bay Area, CA wrote its last area forecast discussion before the first event at Sep 10, 9:39 PM PST, 3 hours 21 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
1039 PM PDT Sun Sep 10 2017
.SYNOPSIS...Shower chances will develop across the far southern
portion of the region late tonight and Monday, and then across
much of the forecast area Monday night and Tuesday. There is also
a slight chance of thunderstorms. Slightly cooler temperatures
are forecast for Monday, with much cooler weather expected by
midweek and beyond.
&&
.DISCUSSION...as of 9:20 PM PDT Sunday...After a pretty hot
afternoon, early evening temperatures are generally remaining
mild to warm, with an increasing cover of high cloudiness moving
in from the south. Downtown San Francisco reached a high of 89
deg, well below its extraordinary record highs of over 100 deg
Labor Day Weekend, but still well above its climatological norm
for Sept 10 of 70 deg. Oakland Airport did eclipse its previous
record for Sept 10, 91 deg in 1984, by 1 deg however while Santa
Rosa tied its record for the date of 100 (1944). Present forecasts
calling for afternoon temps Monday to be a bit below those today,
followed by more a more significant cool-down as the week
progresses, appear on-track.
Primary concern over the next few forecast periods is moisture
circulating northward into our CWA. Latest water vapor satellite
imagery indicates the primary upper low center is now about 350
miles WSW of San Diego and shifting slowly northward. A
disturbance rotating around it brought significant shower and
thunderstorm activity to southern California this afternoon, which
continues this evening. As it progresses to the WNW tonight and
Monday morning, latest model output remains basically consistent
with previous runs, and our present forecasts, in showing the
possibility of it clipping through the south end of our CWA.
Subsequent disturbances will spread shower and thunderstorm
chances farther northward and through the Bay Area Monday night
and Tuesday as the larger scale upper low progresses up the coast
offshore. With precipitable water values being unusually high for
this time of year, any thunderstorms that do occur would more
likely be accompanied by locally decent amounts of precipitation,
somewhat diminishing concern over "dry" lightning.
&&
.PREV DISCUSSION...as of 2:01 PM PDT Sunday...Hot temperatures
are developing as forecast this afternoon with lots of upper 80s
and 90s across the region that will continue to warm for a few
more hours. The marine layer has been washed out with only a
shallow fog deck around Monterey Bay while the Bay Area is sunny
and hot this afternoon with most of the RAWS network up on the
hills now bumping into the 90s.
We continue to watch a circulation sw of Point Conception that has
generated thunderstorms in the vicinity of the Channel Islands.
These storms and associated moisture will rotate northward
overnight but likely impact San Luis Obispo and Santa Barbara
Counties. However things get close enough to justify the slight
chance of showers and thunderstorms, mainly for southern Monterey
and San Benito counties late tonight into Monday morning.
Temperatures will cool a few degrees on Monday as the upper low
slowly drifts northward towards the Central Coast. The combination
of some increasing clouds, onshore flow and subtle airmass cooling
should be sufficient to send temps back in the other direction
but still expect 90s for the hotter inland valleys. In terms of
precip the overall trend of the gfs showing wetter solutions and
then nam/ecmwf drier solutions has continued. In general the
forecast updates have been to reduce pops as well as some of the
areal extent over the ocean waters and the far north bay. Precip
chances will continue later Monday into Tuesday due to the
proximity of the low but the reality is most places are not
expected to see organized precipitation.
Looking at the 18z nam the upper low is now progged to move
onshore around Morro Bay by 12z Weds morning. As this occurs any
precip chances should finally end for our district.
As this occurs we may fairly quickly return to a more typical
pattern of nw flow both at the surface and aloft. This would usher
in some cooler temperatures and turn on some ocean upwelling that
would ultimately see sst drop back into the 50s allowing for some
more seasonable overnight lows.
The long range ecmwf and gfs portend some cooler and wetter
troughs moving into the pacnw that may signal a start of autumn
with longer range trends on the cooler side of normal in terms of
temperatures but no significant precip for the Bay Area forecast
in the long range as would be expected.
&&
.AVIATION...As of 10:38 PM PDT Sunday...VFR conditions will
prevail through the forecast period. A low pressure system
rotating off the Southern California coast will bring increasing
high cloud rotating in from the south across area terminals. Light
to locally variable winds expected overnight.
Vicinity of KSFO...VFR condition will prevail through the
forecast period. Light winds. High confidence.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR condition will prevail through the
forecast period. Light winds. Moderate confidence.
&&
.MARINE...as of 9:00 PM PDT Sunday...Light northwesterly winds
will turn southerly overnight through Monday as a low pressure
system currently spinning out off the Southern California coast
slow moves northward. The low will bring an increased chance of
showers and thunderstorm beginning in the southern waters late
tonight. The chance of showers and thunderstorms will move north
Monday and impact some of our coastal waters into Tuesday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tngt...None.
PUBLIC FORECAST: Blier/RWW
AVIATION/MARINE: CW
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www.youtube.com/nwsbayareaThe San Francisco radar (KMUX), 115 km away, was scanning. The volume nearest the first event began at 14:50 UTC.
5 strong wind, 5 lightning, 3 wildfire, 2 hail, 1 flood. Local standard time, as filed.
Measured wind gust of 54 mph at Pebble Beach.
Reports of fallen trees and down power poles between 2:30 am PDT and 400 am PDT. Estimated wind gusts 30 to 40 mph.
Wind gust measured at 41 mph at San Jose International Airport.
Measured wind gust recorded at Spring Valley Raws of 54 mph.
Measured wind gust of 39 mph at the Los Gatos Raws.
Lightning strike caused damage to a residence on Tablot Ave in Pacifica.
Pea sized hail reported by spotter near Watsonville.
News report from KRON showed lightning struck a tree in San Mateo County on Pearl Ave causing a tree to crash into and damage a home.
Two fires broke out near Los Gatos due to lightning. One near Loma Prieta Ridge and the other near Twin Creeks Road |http://sanfrancisco.cbslocal.com/2017/09/11/lightning-strikes-recorded-bay-area/|http://sanfrancisco.cbslocal.com/2017/09/12/lightning-sparked-wildfire-burning-in-woodside/.
Trained spotter reported lightning.
Social media post on twitter from the public reports small hail as well as lightning near San Francisco.
Social media post on twitter from the public reports small hail as well as lightning near San Francisco.
Various lightning strikes sparked 15 fires in Monterey County that were contained on the same day http://www.mercurynews.com/2017/09/13/crews-contain-15-fires-started-by-lightning-in-monterey-county/. Timing has been estimated.
The Skeggs Fire was sparked by a lightning strike Monday night in the Santa Cruz Mountains. The fire peaked at 50 acres
http://www.fire.ca.gov/current_incidents/incidentdetails/Index/1826|http://www.mercurynews.com/2017/09/15/firefighters-fully-contain-skeggs-fire-near-woodside/.
Roadway flooding at 455 8th St.
News article from SF Gate reports that an airline worker was nearly struck by lightning at Ssan Francisco International Airport http://www.sfgate.com/news/bayarea/article/Airline-Worker-Nearly-Struck-By-Lightning-Monday-12191284.php.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2017Every state in 2017This day in weather historyWarnings in California, September 2017
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 718963, 718980, 718984, 718987, 718988, 718744, 718747, 718745, 719053, 718746, 718969, 718973, 719054, 719052, 718743, 719051.