September 16, 2020 – Smoke and haze will continue to impact temperatures and air quality. A weather system off the PacNW coast will keep onshore flow and periods of gusty winds through the Delta and over higher terrain this week. Stronger winds, cooler air, and a slight chance of showers in the far northern mountains possible late Thursday, with a better chance Friday. Dry weather returns for the weekend.
Other than areas of smoke and haze from the ongoing wildfires, skies are clear across the region early this morning. IR difference imagery shows stratus along the coast extending inland across the Bay Area, and may see a few bits of stratus around sunrise in the Sacramento region. Increased onshore flow is finally spreading better air inland into the Sacramento and Stockton region.
Onshore flow continues to bring moderating RH into the southern Sacramento Valley and northern San Joaquin Valley. However, RH recovery remains rather poor further to the north and across the foothills and mountains.
The gradual cooling trend is expected to continue across the region today as the large closed low off the PacNW slowly approaches. A little better mixing forecast today and Thursday may help to slowly improve air quality. Short-wave forecast to brush NW California on Thursday may bring a few late-day showers to the far north end of the forecast area, but will otherwise bring a little further cooling and help to induce stronger onshore flow in the afternoon. Better chance of seeing some morning stratus in the Central Valley as the marine layer deepens.
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Main low is expected to move onshore over the PacNW on Friday pushing a cold front southward into NorCal. A few showers will be possible across the northern mountains and northern Sacramento Valley, but the system will bring about 5-10 degrees of cooling compared to Thursday. Strong winds are expected to accompany the front over the mountains.
Light to moderate north winds, along with warmer and drier conditions, return Saturday as ridging builds into NorCal.
Extended Discussion (Sunday through Wednesday)
Zonal flow to slight troughing is expected through early next week. This will lead to warm and dry conditions across interior NorCal. Upper ridging may redevelop by mid-week, with a slight uptick in temperatures. High temperatures will range from the mid 80s to low 90s in the Valley through the period. Dry weather continues through the extended.