A moist southeast flow rotating around a large area of high pressure over the central plains region brought sub-tropical air into central California on July 27 and 28. This provided for enough moisture for afternoon and evening thunderstorms over the Sierra Nevada and adjacent foothills on both days.
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.
A moist southeast flow rotating around a large area of high pressure over the central plains region brought sub-tropical air into central California on July 27 and 28. This provided for enough moisture for afternoon and evening thunderstorms over the Sierra Nevada and adjacent foothills on both days. A few of the thunderstorms produced locally heavy rainfall over the Sierra Nevada, while a thunderstorm on the 28th produced strong outflow winds in Yosemite Valley blew down several drought killed or weakened trees.
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 Jul 28, 10:45 AM PST, 5 hours 15 minutes earlier.
Area Forecast Discussion National Weather Service Hanford CA 1145 AM PDT Wed Jul 28 2021 .SYNOPSIS... Slightly warmer than average high temperatures look likely into the foreseeable future over the forecast area. Monsoonal moisture will provide the Sierra Nevada with a chance of afternoon thunderstorms each afternoon through this weekend. && .DISCUSSION... Latest satellite imagery from GOES-17 is showing developing showers and thunderstorms over the Sierra Nevada this afternoon. This is due to moist southeasterly flow that is typical for this time of the year due to ridging over the central/western US. This is fueling the Sierra Nevada with instability for afternoon convection. Wouldn't be surprised to see some of these storms to have periods of heavy rainfall due to elevated PWATs over the area. High resolution model guidance is showing afternoon thunderstorms going through the evening afternoon hours before waning by sunset as instability drops. This pattern of elevated moisture and instability will stay in place through the weekend as easterly waves rotating around the ridge of high pressure enter the region. Blended model guidance is showing Friday afternoon and Saturday afternoon as easterly waves coupled with a closed low off the CA coast funnel even more moisture into the area. This may lead to convection occuring in the desert areas of Kern County as well as the Sierra Nevada Friday and Saturday, with periods of heavy rain possible in these storms. Afternoon high temperatures across the San Joaquin Valley are only expected to oscillate a couple of degrees through next Wednesday, with highs ranging around 100-104 degrees. Cloud cover associated with the monsoonal moisture will likely contribute to some of the fluctuations in afternoon high temperatures as well. After Sunday, ensemble guidance is trying to show the return of drier, southwest flow over the area, but knowing exactly how long the ridge of high pressure giving us the monsoonal moisture is unclear this far out. && .AVIATION... Showers and isolated thunderstorms likely through 03Z Thur in the Sierra, primarily north of Sequoia National Park. Otherwise, VFR conditions will prevail through the next 24 hours. && .AIR QUALITY ISSUES... On Wednesday July 28 2021... 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... None. && public...Bollenbacher aviation....Bollenbacher weather.gov/hanford
Special Weather Statement
National Weather Service San Joaquin Valley CA
259 PM PDT Wed Jul 28 2021
CAZ192-193-282245-
Central Sierra CA-North Kings River CA-
259 PM PDT Wed Jul 28 2021
...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN FRESNO COUNTY UNTIL
345 PM PDT...
At 258 PM PDT, Doppler radar was tracking strong thunderstorms along
a line extending from 17 miles south of Mammoth Lakes to 12 miles
southwest of North Lake Campground. These storms were nearly
stationary.
Pea size hail and winds in excess of 30 mph will be possible with
these storms.
Locations impacted include...
Blackcap Basin, Upper Burnt Corral, Volcanic Knob, Florence Lake and
McClure Meadow Ranger Station.
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.
&&
LAT...LON 3736 11878 3734 11878 3732 11873 3726 11866
3724 11868 3720 11868 3719 11866 3716 11867
3696 11886 3735 11905 3745 11889
TIME...MOT...LOC 2158Z 172DEG 2KT 3739 11898 3708 11876
AndersenThe San Joaquin Valley radar (KHNX), 158 km away, was scanning. The volume nearest the first event began at 00:00 UTC.
1 thunderstorm wind. Local standard time, as filed.
A tree was blown down in gusty thunderstorm winds and crushed 3 unoccupied tent cabins at Curry Village in Yosemite National Park.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2021Every state in 2021This day in weather historyWarnings in California, July 2021
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: 963977.