Much cooler and breezier – Red Flag Warning today and tomorrow

September 11, 2018 – Below average temperatures expected for the remainder of the week. Locally gusty winds over higher elevations thru Wednesday. Areas of smoke and haze from wildfires will continue. Red Flag Warning from 1 PM this afternoon to 11 PM PDT Wednesday. A late summer season trough will bring gusty winds to northern California, especially over the higher terrain Tuesday and Wednesday. Relative humidity will remain low ahead of the trough, but increase late Wednesday with increased on-shore flow.

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Upper trough continues to deepen over the northeastern Pacific while the closed low drops south of the Queen Charlotte Islands. Skies are clear across interior NorCal with short-wave ridging presently moving across the area. Current temperatures range from the 30s in the mountain valleys to the upper 50s to around 70 across the Central Valley.

This week’s cooldown will commence today as the first shot of cooler air drops south into the region and onshore pressure gradients increase. More cool air arrives Wednesday and Thursday as the trough sags into NorCal. Forecast high temperatures are expected to be 5-15 degrees cooler than average Wednesday and Thursday with 70s to lower 80s across the Central Valley, and 60s to lower 70s over the mountains. Overnight temperatures will be the coolest so far this season with sub-freezing readings possible in the mountain valleys. The Central Valley will see chilly nights as well with some areas likely to dip into the 40s. Locally gusty west to southwest winds with gusts 25 to 40 mph will develop over the northern Sierra and the southern Cascade Range. This combined with very dry conditions will bring critical fire weather conditions this afternoon into Wednesday evening.

A Red Flag Warning is in effect for portions of Shasta County and the northern Sierra into Wednesday. Expect rapid fire spread with any new starts. Precipitation will likely remain outside of the forecast area, but stronger short-wave ejecting northeastward may be enough to pop some showers across far northwestern Shasta County late Wednesday afternoon into the evening. Wildfire smoke is expected to remain locally thick over the north end of the Sacramento Valley and surrounding foothills and mountains through at least Wednesday morning. Some improvement is expected each mid to late afternoon and early evening.

Extended Discussion (Saturday through Tuesday)

Longwave troughing will persist across the West Coast through at least this Saturday. The GFS and ECMWF have come into better agreement today, bringing a robust shortwave through northern California on Saturday. While dry weather is expected to continue, this pattern would likely bring enhanced southwest winds and cooler temperatures. Current blend of guidances suggests Valley high temperatures could be around or below 80 degrees over the weekend, or in the neighborhood of 10 degrees cooler than normal. The upper trough shifts eastward early next week, bringing a return to more seasonal temperatures.