Tropical moisture streamed northward into central California around a strong area of upper high pressure which brought record heat to much of the area. The moisture was sufficient enough for scattered thunderstorms to break out over the Sierra Nevada during the afternoon of August 17.
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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 HNX wrote it up.
Tropical moisture streamed northward into central California around a strong area of upper high pressure which brought record heat to much of the area. The moisture was sufficient enough for scattered thunderstorms to break out over the Sierra Nevada during the afternoon of August 17. One slow moving thunderstorm produced flash flooding along SR 120 in Yosemite National Park near Tuolumne Meadows.
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 Aug 17, 1:29 PM PST, 1 hour 31 minutes earlier.
Area Forecast Discussion National Weather Service Hanford CA 229 PM PDT Mon Aug 17 2020 .SYNOPSIS...Dangerous heat will continue across the district through at least Thursday with a daily threat of afternoon and early evening thunderstorms over the higher elevations of the Sierra. There is also a slight chance of thunderstorms for portions of the desert and Fort Tejon area Tuesday. && .DISCUSSION... Central California continues to bake under the influence of strong high pressure currently centered near southwest Utah. Record high temperatures were set across much of our area Sunday. Current 24 hour temperature trends are mostly running down a few degrees and we may not see as many record highs broken today but conditions are still much hotter than normal. Moisture drawn up around the edge of the high pressure center has been fueling thunderstorms across our mountain and desert areas during the afternoons and we are seeing similar convective development again this afternoon. The high pressure ridge is progged to sink slowly south and gradually weaken over our area after Tuesday as shortwave energy pushes into the northwest US coast. Tuesday highs will be similar to today and then just a degree or so lower Wednesday as the ridge begins to retreat. Moisture will also continue to affect our area, continuing the chances for afternoon mountain convection and keeping our overnight lows elevated. The excessive heat during the afternoon, with little overnight relief creates a high risk of heat related illness for much of the population, and an Excessive Heat Warning remains in effect for central California outside of the mountain areas. Thursday will see additional reduction in high temperatures as the ridge continues to retreat and temperatures will run around 4-8 degrees above normal into the end of the week. Still hot, but not quite as hot as we are currently experiencing. The drier, more westerly flow over the flattened high pressure ridge will also inhibit our thunderstorm activity. By the start of next week, models show the ridge expanding into our area again, with a slight uptick in our temperatures but not looking like the heat we are currently experiencing. && .AVIATION... Isolated thunderstorms can be expected over the mountains and desert until 04z Tuesday and again after 18Z Wednesday. Otherwise, VFR conditions will prevail over most of the central California interior during the next 24 hours. && .AIR QUALITY ISSUES... On Monday August 17 2020... Unhealthy for sensitive groups in Fresno... Kern and Tulare Counties and Sequoia National Park and Forest. Further information is available at Valleyair.org && .CERTAINTY... The level of certainty for days 1 and 2 is high. The level of certainty for days 3 through 7 is medium. Certainty levels include low...medium...and high. Please visit www.weather.gov/hnx/certainty.html for additional information an/or to provide feedback. && .HNX WATCHES/WARNINGS/ADVISORIES... Excessive Heat Warning until 9 PM PDT Thursday for CAZ179>191- 198-199. && public...JEB aviation....MV weather.gov/hanford
The San Joaquin Valley radar (KHNX), 176 km away, was scanning. The volume nearest the first event began at 22:58 UTC.
1 flash flood. Local standard time, as filed.
Park Service Rangers reported flash flooding on SR 120. A nearby weather station at Tuolumne Meadows reported 0.42 inches of rain and hail between 1500 and 1600 PST.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2020Every state in 2020This day in weather historyWarnings in California, August 2020
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: 903137.