Thunderstorm Wind — Mariposa, California

2012-01-21 · near Yosemite Vlg, Mariposa, California

Event narrative

A thunderstorm with heavy rain and lightning moved across Yosemite Valley on the morning of the 21st during a strong winter storm. The thunderstorm briefly generated gusty winds estimated at 40 to 45 mph which caused a tree to fall on a tent. Inside the tent was a park service employee who was killed by the tree fall.

Wider weather episode

The models began converging on a major change in the weather pattern beginning late in the day on January 19th. A Winter Storm Watch was issued for the Southern Sierra Nevada, and Tioga Pass was closed on the 17th; this was the latest date on record that the Pass had been open until.

A weak upper-level trough moved through the central California interior during the night of January 19th-20th. While the trough brought less rain than the computer models had indicated, it did knock down the ridge that had been in place over the state.

The second trough arrived during the evening of the 20th. From midnight to 4 AM on the 21st, 0.63 inch of rain fell on Fresno, a record-setting end to a 60-day stretch without measurable rain. The total rainfall for Fresno on the 21st was 0.79 inch; the old record rainfall had been 0.52 inch, set in 2010.

Lines of showers with a few embedded thunderstorms moved through the central California interior during the early morning hours of January 21st. These lines of convective activity brought locally heavy rain, gusty winds and a few lightning strikes. Scattered thunderstorms continued through the day, producing pea-size hail and a funnel cloud. Tree limbs were blown down by the winds, with downed limbs reported near Lake Isabella and in Yosemite National Park. One large tree limb in the Park fell on a tent cabin, resulting in the death of the occupant.

The third, and final, trough reached central California on the 23rd. This system also brought thunderstorms with small hail. A funnel cloud also was observed. In the Southern Sierra Nevada, snowfall totals from the three storms reached 1 feet in the high country.

An upper-level ridge built into California behind the departing storm. With abundant ground moisture, the stable airmass allowed the lower layers of the atmosphere to cool to saturation. Consequently, areas of dense fog developed the central and southern San Joaquin Valley. The next few nights saw less fog, mainly confined to west of Highway 99. Clouds over the region during the night of January 26th-27th kept the San Joaquin Valley mostly fog free. However, an upper-level short-wave passing through California did produce areas of drizzle on the Valley floor. The clouds also kept low temperatures in the central and southern San Joaquin Valley unseasonably warm. Both Bakersfield and Fresno bottomed out in the lower 50s and tied their record high minimum temperatures for the 27th (51 and 52 degrees, respectively).

Low clouds persisted over the San Joaquin Valley through much of the day on the 27th, and redeveloped over much of the Valley overnight. Areas of dense fog formed around sunrise, mainly in Kern and Merced Counties, and persisted through late morning. Dense fog redeveloped over San Joaquin Valley again during the early morning of January 29th, and persisted through late morning; a few fog patches lingered into the early afternoon hours.

Mid and high clouds moved into California on the 29th ahead of an upper-level trough that weakened the persistent ridge. Due to the clouds, fog development during the morning of January 30th was patchy, with some areas not seeing fog until around sunrise.

An upper-level disturbance approached the central California coast on January 30th, and moved inland during the afternoon. The disturbance had enough dynamics to trigger isolated light rain showers over the central California interior. The showers brought measurable rain to a few locations, including Fresno and NAS Lemoore, and lingered past midnight into the early morning hours of January 31st.

Bakersfield tied for its 10th warmest January on record, and Fresno had its 16th warmest January on record. For the rain season to date, the central and southern San Joaquin Valley had only received between a third and a half of normal.

View location on OpenStreetMap → (37.7500, -119.5800)

Source: NOAA Storm Events Database, event_id 367765. Narrative written by the NWS forecast office that issued the report.