While high pressure was strengthening over the West Coast, an upper-level low off Baja California began spinning subtropical moisture into California from the southeast. A few thunderstorms developed over the Southern Sierra Nevada on the 17th, but convection was widespread across the region the next day.
Every event with a place to put it. 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 HNX wrote it up.
While high pressure was strengthening over the West Coast, an upper-level low off Baja California began spinning subtropical moisture into California from the southeast. A few thunderstorms developed over the Southern Sierra Nevada on the 17th, but convection was widespread across the region the next day. By the afternoon of May 18th, thunderstorms had developed over the Kern County desert near Edwards Air Force Base, and over extreme southeastern San Luis Obispo County. The latter storm drifted north into the far western portion of Kern County. Four Severe Thunderstorm Warnings were issued during the afternoon; two verified, including the storm near Edwards Air Force Base. There was a lull in the activity around sunset, then an upper-level disturbance moved northwest along the Southern Sierra Nevada, triggering another round of thunderstorms. The strongest storms were over the mountains of Madera and Mariposa Counties, and as these thunderstorms collapsed, outflow winds caused more storms to develop. Ultimately, the thunderstorms propagated westward into Merced County before finally ending late in the evening. Rainfall from these storms mostly was under a tenth of an inch, although Yosemite Valley recorded 0.33 inch, with a quarter inch of rain falling in one hour.
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 Hanford, CA wrote its last area forecast discussion before the first event at May 18, 8:05 AM PST, 4 hours 59 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN JOAQUIN VALLEY - HANFORD CA 905 AM PDT MON MAY 18 2009 .DISCUSSION...ALL PARAMETERS INDICATE AN ISOLATED TO SCATTERED THUNDERSTORM DAY OVER THE CENTRAL CALIFORNIA INTERIOR AS THE MOIST SOUTHEAST FLOW IS PUSHING WELL INTO THE AREA. THE EDWARDS SOUNDING FROM 12Z SHOWS LIFTED INDEX AT -5 AND CAPE TO 1600 J/KG AND WITH RADAR ALREADY SHOWING SOME LIGHT SHOWERS MOVING INTO KERN COUNTY FROM THE SOUTHEAST AND THUNDERSTORMS IN PROGRESS MOVING NORTHWEST FROM SOUTHEAST CALIFORNIA, ISOLATED TO SCATTERED THUNDER IS A SLAM DUNK. AS FOR TEMPERATURES, ANOTHER HOT DAY WILL OCCUR AND ALTHOUGH CURRENT GRIDDED FORECAST MAX TEMPS MAY BE A LITTLE COOL AT SOME LOCATIONS, TRIPLE DIGITS WILL BE WIDESPREAD AGAIN TODAY. ALL IN ALL, THE PATTERN IS MUCH MORE AUGUST LIKE THAN MAY AND A VERY EARLY TASTE OF THE SUMMER MONSOON WILL OCCUR. NO UPDATES THIS MORNING. && .AVIATION...AN INFLOW OF MONSOON MOISTURE WILL PROVIDE MID TO HIGH LEVEL CIGS OVER MUCH OF THE REGION. DAY TIME HEATING MAY TRIGGER THUNDERSTORMS OVER THE MOUNTAINS DURING THE AFTERNOON. THE STORMS MAY DEVELOP AROUND MID-DAY AND LAST INTO THE EARLY EVENING. THE STORM MOVEMENT TOWARDS THE NW MAY BRING A THUNDER THREAT INTO THE EAST SIDE AND SOUTH END OF THE SAN JOAQUIN VALLEY. ANY THUNDER ACTIVITY WILL RESULT IN LOCAL MVFR CONDITIONS IN HEAVY SHOWERS. OTHERWISE VFR CONDITIONS ARE EXPECTED ELSEWHERE THROUGH 18Z TUESDAY. && .AIR QUALITY ISSUES... ON MONDAY MAY 18 2009... UNHEALTHY FOR SENSITIVE GROUPS IN FRESNO... KERN... KINGS AND TULARE COUNTIES. FURTHER INFORMATION IS AVAILABLE AT VALLEYAIR.ORG && .HNX WATCHES/WARNINGS/ADVISORIES... NONE. && PUBLIC...DUDLEY AVN/FW...MOLINA WEATHER.GOV/HANFORD
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE SAN JOAQUIN VALLEY - HANFORD CA 500 AM PDT MON MAY 18 2009 CAZ090-092>094-191200- EAST CENTRAL SAN JOAQUIN VALLEY-SOUTHEASTERN SAN JOAQUIN VALLEY- MARIPOSA MADERA AND FRESNO COUNTY FOOTHILLS- TULARE COUNTY FOOTHILLS- 500 AM PDT MON MAY 18 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE EAST SIDE OF THE SAN JOAQUIN VALLEY AND THE ADJACENT FOOTHILLS. .DAY ONE...TODAY AND TONIGHT THERE IS A SLIGHT CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS FOR THE EAST SIDE OF THE SAN JOAQUIN VALLEY AND THE ADJACENT FOOTHILLS. SOME OF THESE THUNDERSTORMS MAY PRODUCE LOCALLY HEAVY RAIN...GUSTY WINDS...SMALL HAIL AND LIGHTNING. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY HAZARDOUS WEATHER IS NOT EXPECTED. .SPOTTER INFORMATION STATEMENT... SKYWARN SPOTTER ACTIVATION MAY BE NECESSARY FOR THE EAST SIDE OF THE SAN JOAQUIN VALLEY AND THE ADJACENT FOOTHILLS AFTER NOON PDT TODAY. WEATHER SPOTTERS ARE ENCOURAGED TO REPORT ANY SIGNIFICANT OR UNUSUAL CONDITIONS TO THE NATIONAL WEATHER SERVICE. WEATHER.GOV/HANFORD CAZ098-099-191200- INDIAN WELLS VALLEY-SOUTHEASTERN KERN COUNTY DESERT- 500 AM PDT MON MAY 18 2009 THIS HAZARDOUS WEATHER OUTLOOK IS FOR THE KERN COUNTY DESERTS. .DAY ONE...TODAY AND TONIGHT THERE IS A SLIGHT CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS FOR THE KERN COUNTY DESERTS. SOME OF THESE THUNDERSTORMS MAY PRODUCE LOCALLY HEAVY RAIN...GUSTY WINDS...SMALL HAIL AND LIGHTNING. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY HAZARDOUS WEATHER IS NOT EXPECTED. .SPOTTER INFORMATION STATEMENT... SKYWARN SPOTTER ACTIVATION MAY BE NECESSARY FOR THE KERN COUNTY DESERTS AFTER 11 AM PDT TODAY. WEATHER SPOTTERS ARE ENCOURAGED TO REPORT ANY SIGNIFICANT OR UNUSUAL CONDITIONS TO THE NATIONAL WEATHER SERVICE. SANGER WEATHER.GOV/HANFORD
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE SAN JOAQUIN VALLEY - HANFORD CA 1030 AM PDT MON MAY 18 2009 CAZ095-097>099-182030- KERN COUNTY MOUNTAINS-TULARE COUNTY MOUNTAINS-INDIAN WELLS VALLEY- SOUTHEASTERN KERN COUNTY DESERT- 1030 AM PDT MON MAY 18 2009 ...THUNDERSTORMS EXPECTED TO FORM IN THE NEXT ONE TO TWO HOURS... CONDITIONS ARE BECOMING CONDUCIVE FOR SHOWER AND THUNDERSTORM FORMATION OVER THE FAR SOUTHERN SIERRA NEVADA...AND KERN COUNTY MOUNTAINS AND DESERTS...INCLUDING THE COMMUNITIES OF RIDGECREST...MOJAVE...TEHACHAPI...LAKE ISABELLA AND ROSAMOND. A FEW STRONG STORMS WILL BE POSSIBLE WITH SMALL HAIL...HEAVY DOWNPOURS...FREQUENT LIGHTNING AND GUSTY WINDS UP TO 40 MPH. HOWEVER...THE RISK FOR SEVERE WEATHER APPEARS LOW AT THIS TIME. CAZ089>094-096-182030- WEST CENTRAL SAN JOAQUIN VALLEY-EAST CENTRAL SAN JOAQUIN VALLEY- SOUTHWESTERN SAN JOAQUIN VALLEY-SOUTHEASTERN SAN JOAQUIN VALLEY- MARIPOSA MADERA AND FRESNO COUNTY FOOTHILLS- TULARE COUNTY FOOTHILLS- SIERRA NEVADA FROM YOSEMITE TO KINGS CANYON- 1030 AM PDT MON MAY 18 2009 ...THUNDERSTORMS POSSIBLE THIS AFTERNOON... WITH MOISTURE STREAMING NORTH ACROSS CALIFORNIA THIS MORNING...CONDITIONS MAY BECOME CONDUCIVE FOR THUNDERSTORM FORMATION THIS AFTERNOON ACROSS THE SAN JOAQUIN VALLEY...AND THE ADJACENT SIERRA FOOTHILLS AND YOSEMITE PARK AREA. HEAVY DOWNPOURS WILL BE POSSIBLE WITH ANY THUNDERSTORMS THAT FORM. HOWEVER...THE RISK FOR SEVERE WEATHER APPEARS LOW AT THIS TIME. JMB WEATHER.GOV/HANFORD
The national radar mosaic for the moment the first event began.
5 thunderstorm wind, 1 funnel cloud. Local standard time, as filed.
Thunderstorm outflow caused winds to gust up to 74 mph near Edwards Air Force Base. Winds were measured on a NASA wind tower at 33 feet. Winds lasted for 30 minutes or more.
A trained spotter reported a funnel cloud.
Thunderstorm outflow generated severe wind gusts.
California Highway Patrol reported 3 trees downed across SR 140 near Briceburg.
Severe criteria wind gusts from a severe thunderstorm.
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, May 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: 176487, 176482, 176484, 176489, 176485, 176491.