On September 5th, copious low-level moisture over the deserts combined with extensive daytime heating and an upper-level trough triggered a massive severe thunderstorm over the San Diego County deserts just south of Ocotillo Wells. Surface dew point temperatures in the area were as high as 75 degrees, with air temperatures reaching 105 degrees.
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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 SGX wrote it up.
On September 5th, copious low-level moisture over the deserts combined with extensive daytime heating and an upper-level trough triggered a massive severe thunderstorm over the San Diego County deserts just south of Ocotillo Wells. Surface dew point temperatures in the area were as high as 75 degrees, with air temperatures reaching 105 degrees. According to radar data, storm tops were at least 60 kft. The storm, which was nearly stationary and lasted for over 90 minutes, produced flash flooding, damaging wind, and large hail. No hail size reports were received, but based on radar data it is likely that at least golf ball sized hail was being produced by this storm. Strong downdrafts knocked over several power poles along Split Mountain Road. Radar data also indicated a rather impressive anti-cyclonic mesocyclone and strong low-level rotation, with up to 106 kt gate-to-gate shear around 4800 ft AGL.
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 Diego, CA wrote its last area forecast discussion before the first event at Sep 5, 12:25 PM PST, 3 hours 30 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN DIEGO CA 125 PM PDT SAT SEP 5 2009 .SYNOPSIS... A TROUGH OF LOW PRESSURE WILL BRING DRYING AND SOME COOLING THROUGH THE HOLIDAY WEEKEND...AND A SLOW DEEPENING OF THE MARINE LAYER FOR MORE EXTENSIVE NIGHT AND MORNING COASTAL LOW CLOUDS AND FOG. ISOLATED THUNDERSTORMS ARE POSSIBLE AGAIN THIS AFTERNOON AND EVENING IN THE MOUNTAINS AND DESERTS. OTHERWISE...A RETURN OF A MORE GENERAL MONSOONAL MOISTURE IS POSSIBLE TOWARD THE MIDDLE AND LATTER PART OF NEXT WEEK FOR A SLIGHT CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS IN THE MOUNTAINS...DESERTS...AND INLAND VALLEYS. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... AREAS OF MID-LEVEL CLOUDS ASSOCIATED WITH A WEAK UPPER-LEVEL TROUGH WERE DISSIPATING OVER EASTERN SECTIONS OF THE FORECAST AREA EARLY THIS AFTERNOON BASED ON SATELLITE IMAGERY. HEATING WAS DRIVING ISOLATED DEEP CUMULUS DEVELOPMENT BY 20Z. DEW POINTS REMAINED ELEVATED...WHILE TEMPS WERE A FEW DEGREES LOWER THAN YESTERDAY AFTERNOON AT 20Z. SURFACE PRESSURE GRADIENTS WERE WEAK ONSHORE. DEWPOINTS REMAINED QUITE HIGH (MID 60S TO MID 70S F) OVER THE LOWER DESERTS. BASED ON MOUNTAIN OBSERVATIONS...THE MOIST LAYER WAS ABOUT 4K FT DEEP. MODEL SOUNDINGS IN THE LOWER DESERT INDICATED A WEAK MID-LEVEL CAP WILL BE PRESENT...BUT INSTABILITY WAS SUBSTANTIAL WITH CAPES ABOVE 1500-2000 J/K AND A K INDEX NORTH OF 35. BASED ON THE DEEP MOIST LAYER AND FORECAST MAX TEMPS ABOVE 100...EXPECT A FEW PARCELS WILL GAIN ENOUGH SIZE AND VELOCITY TO BUILD INTO THUNDERSTORMS THIS AFTERNOON OVER THE MOUNTAINS AND SHOULD TRACK NE INTO THE DESERTS. OTHERWISE...DRY WEST OF THE MOUNTAINS AND WARM. ADDITIONAL MID-LEVEL DRYING IS LIKELY BY SUN...SO THE CHANCE OF SHOWERS AND THUNDERSTORMS WILL DIMINISH FOR THE REMAINDER OF THE HOLIDAY WEEKEND. DRY...WESTERLY FLOW ALOFT AND INCREASING ONSHORE SURFACE PRESSURE GRADIENTS WILL TREND US MORE TOWARD SEASONAL CONDITIONS. WHILE NOT A FORECAST...A DROP IN SEA SURFACE TEMPS WOULD GO A LONG WAY TOWARD THAT END AS WELL. LATER IN THE WEEK...EASTERLY FLOW IS FORECAST TO REDEVELOP AS LOW PRESSURE DEVELOPS OFF THE COAST AND HIGH PRESSURE STRENGTHENS INLAND. WHILE NOT PARTICULARLY HOT FOR SOCAL...THIS FLOW COULD BRING MORE MID-LEVEL MOISTURE AND CLOUDS BACK TO THE REGION AND A RENEWED THREAT OF MOUNTAIN AND DESERT CONVECTION. REGARDING SEA SURFACE TEMPS...NEAR SHORE TEMPS ARE ELEVATED NOW (MID 70S). THIS IS ABOUT 6 TO 10 DEGREES ABOVE AVERAGE...AND WHILE IT IS GREAT FOR BEACH GOERS...IT LIMITS THE COOLING POWER OF THE SEA BREEZE INLAND AND ELEVATES HUMIDITY...MAKING IT FEEL MORE MUGGY. PERSISTENT NW FLOW ALONG THE CA COAST USUALLY RESULTS IN OVERTURNING AND COOLING OF THE WATER. WHILE SOME DECREASE IN HUMIDITY IS LIKELY OVER THE NEXT COUPLE OF DAYS...A DEFINITIVE DROP PROBABLY WILL NOT OCCUR FOR COASTAL AND VALLEY COMMUNITIES UNTIL THE SEA SURFACE TEMPS FALL BACK INTO THE 60S. && .AVIATION... 051900Z...MARINE LAYER DEPTH WAS AROUND 1000 FEET THIS AFTERNOON AND LITTLE CHANGE IS EXPECTED THROUGH SUNDAY. AREAS OF STRATUS SHOULD RETURN TO THE COAST THIS EVENING WITH IFR CEILINGS AT COASTAL AIRPORTS SUCH AS KSAN AND POSSIBLY KSNA AND LIFR CONDITIONS POSSIBLE AT SOME MESA AIRPORTS AS WELL AS KCRQ BETWEEN MIDNIGHT AND 16Z ON SUNDAY. ABOVE THE MARINE LAYER...SCATTERED CU/TCU THIS AFTERNOON. THE AIR MASS REMAINS UNSTABLE ENOUGH THAT ISOLATED CBS/TSTMS CAN NOT BE RULED OUT BUT MOST TSTM ACTIVITY SHOULD BE SOUTH OVER NORTHERN BAJA AND OVER THE EASTERN DESERT AREAS. ANY ACTIVITY SHOULD END BY SUNSET. OTHERWISE MOSTLY CLEAR THROUGH SUNDAY. VISIBILITIES MOSTLY UNRESTRICTED EXCEPT VICINITY OF WILDFIRES. HORTON && .SGX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...NONE. && PUBLIC...JAD AVIATION/MARINE...HORTON NWS ON THE WEB AT WEATHER.GOV/SANDIEGO
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE SAN DIEGO CA 430 PM PDT SAT SEP 5 2009 CAZ042-043-048-050-055>058-060>062-061200- ORANGE COUNTY COASTAL AREAS-SAN DIEGO COUNTY COASTAL AREAS- SAN BERNARDINO AND RIVERSIDE COUNTY VALLEYS-THE INLAND EMPIRE- SAN DIEGO COUNTY VALLEYS-SAN BERNARDINO COUNTY MOUNTAINS- RIVERSIDE COUNTY MOUNTAINS-SANTA ANA MOUNTAINS AND FOOTHILLS- SAN DIEGO COUNTY MOUNTAINS-APPLE AND LUCERNE VALLEYS- COACHELLA VALLEY-SAN DIEGO COUNTY DESERTS- 430 PM PDT SAT SEP 5 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR ALL OF EXTREME SOUTHWESTERN CALIFORNIA. .DAY ONE...THIS AFTERNOON AND TONIGHT THERE IS A SLIGHT CHANCE OF THUNDERSTORMS FOR SOUTHERN MOUNTAIN AND DESERT AREAS THROUGH EARLY EVENING. THE FLASH FLOOD POTENTIAL INDEX IS LOW...THE RISK OF FLASH FLOODING IS MINIMAL. .DAYS TWO THROUGH SEVEN...SUNDAY THROUGH FRIDAY NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .SPOTTER INFORMATION STATEMENT... SKYWARN WEATHER ACTIVATION IS NOT REQUESTED FOR THIS AREA...HOWEVER WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER CONDITIONS. MOEDE
The San Diego radar (KNKX), 87 km away, was scanning. The volume nearest the first event began at 23:56 UTC.
1 thunderstorm wind, 1 flash flood. Local standard time, as filed.
Imperial Irrigation District Energy reported 16 broken power poles and associated equipment along Split Mountain Road in San Diego County due to strong winds from the thunderstorm near Ocotillo Wells.
A nearly stationary thunderstorm with heavy rain near the town of Ocotillo Wells caused flash flooding along the Fish Creek Wash and Split Mountain Road. Several power poles were down in the area, but this may have been attributed to high winds as well as the flooding.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2009Every state in 2009This day in weather historyWarnings in California, September 2009
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: 196859, 196858.