Residents of mountain towns like Arnold, Twain Harte and Groveland, and the rest of the Stanislaus National Forest are being advised of red flag warnings for very dry, gusty winds with low relative humidity levels from Tuesday afternoon to late Wednesday and early Thursday.

The red flag warnings for critical fire weather conditions include the 57-square-mile Donnell Fire burn in the Middle Fork Stanislaus River watershed, and they are coming from forecasters in Sacramento and Reno. The warnings cover the north half of the Sierra Nevada range and most of the state of Nevada.

These fire weather warnings are for dry, windy conditions with slightly cooler temperatures, not hotter. At high elevations, winds could gust 25 to 35 miles per hour, and up to 70 mph on the most exposed, wind-vulnerable peaks and ridges. The warnings mean fires may start easier and spread faster, and in areas where fires are already burning, there could be locally poor air quality for sensitive groups due to increased wildfire smoke.

Forecasters in Reno are warning, “This is a particularly dangerous situation with extremely low
humidity and high winds. New fires will grow rapidly out of control, in some cases people may not be able to evacuate safely in time should a fire approach. Avoid outdoor activities that can cause a spark near dry vegetation, such as yard work, target shooting, or campfires.”

The strongest winds are expected Tuesday and Wednesday afternoons and each night.

Mother Lode foothill towns along Highway 49, including San Andreas, Angels Camp, Murphys, Columbia, Sonora and Jamestown are just outside the red flag warnings area. Yosemite National Park is also just outside the warnings area, on its south edge.

As of Monday, the Donnell Fire was estimated to be 90 percent contained at 36,450 acres, or 56.9 square miles. There were still more than 125 personnel assigned to the Donnell Fire as of Monday morning.

For Arnold, Twain Harte and Groveland, Tuesday is expected to be sunny with daytime highs from 77 to 82, with winds to 10 miles per hour. Tuesday night lows will dip into the highs 40s to mid 50s, with winds to 8 mph. Wednesday continues mostly sunny with daytime highs up to 72 with winds to 13 mph. Wednesday night is expected to be partly cloudy with lows from 44 to 50 degrees Fahrenheit.

Forecasters say the cause of dry, windy, fire weather conditions this week is a deepening atmospheric trough along the West Coast, resulting in cooler temperatures and enhanced atmosphere pressure gradients.

Craig Shoemaker, a forecaster with the National Weather Service in Sacramento, said the atmospheric trough is happening as cooler air moves in, collides with and pushes warmer air east and northeast, increasing pressure and creating winds.

Relative humidity levels could fall as low as 10 to 20 percent in the Stanislaus National Forest and west of the Sierra Nevada crest. That means summer-dried fuels, including live trees, dead trees, and dead fuels on the ground, are going to be even drier. Forecasters in Sacramento say the highest threats for critical fire weather during the red flag warning include the Donnell Fire burn.

West of Sonora Pass, the red flag warnings are expected to expire at 11 p.m. Wednesday. East of Sonora Pass, red flag warnings expire at 5 a.m. Thursday.

Over the past weekend, firefighters and pilots with Cal Fire’s Tuolumne-Calaveras Unit battled a 500- to 600-acre grass fire near Tulloch Road in Tuolumne County, east of Knights Ferry. The fire broke out Saturday afternoon. It was declared 100 percent contained Monday. No information about the cause of the fire was available Monday.

The cool spell this week is bringing temperatures 5 to 15 degrees cooler than normal, Tuesday to Friday, followed by slightly warmer temperatures this weekend.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or 588-4585. Follow him on Twitter at @GuyMcCarthy.

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