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November 23, 2017 – Mild temperatures this week with a weak weather system today. More widespread rain spreads south Saturday into Sunday followed by a colder system early next week.

Mostly cloudy across much of the area this morning, with some light, scattered precipitation having fallen from around Wheatland northward. Totals have ranged from a trace to 0.04″(Redding) in the Valley, to up to 0.14″ at Manzanita Lake and Mineral in the mountains of western Plumas County. The band of light precipitation is currently over the Paradise area, and is projected to slide southward through the day, reaching the I80 corridor and Sacramento metro area around 1-2 pm this afternoon.

Cloud cover limited morning Valley fog development, and the patchy fog that formed has quickly dispersed. Areas in the northern San Joaquin have been south of the main cloud deck and have warmed quickly. At 10 am, Stockton was already 65, Modesto 66. Increasing clouds should limit further increases, though highs should reach the low to mid 70s there. Most other Valley locations should remain in the upper 60s, which is still several degrees above the daily average highs of around 60. Forecast is on track, no updates made.

Discussion

Plenty of cloud cover continues to stream up from the southwest early this morning and some spotty light precipitation has been occurring over the northern half of the forecast area overnight along a weak frontal boundary. The cloud cover has inhibited dense fog development, but surface observations in the valley are saturated (or nearly so) so some patchy fog and stratus will be possible thru the morning. Current temperatures are milder compared to 24 hours ago and range from the mid 30s in the mountain valleys to the upper 40s and 50s elsewhere.

Deep moisture plume with TPW of 1.25 to 1.5 inches continues to feed into NorCal from the southwest as the upper ridge retreats slightly eastward. This moisture will interact with a weak front that is forecast to slide south to around I-80 by 00Z Friday resulting in a mostly cloudy Thanksgiving across the region. Some light rain may make it as far south as the Sacramento region by mid to late afternoon. Only light amounts of QPF are expected with less than a quarter inch in the mountains and only a few hundredths of an inch in the Sacramento Valley.

Precipitation is expected to retreat northward tonight and Friday as ridging reasserts itself across NorCal ahead of a stronger wave developing over the eastern Pacific. Most of the region will see dry weather for Friday into Saturday with mild temperatures.

A stronger frontal system is forecast to spread precipitation southward across interior NorCal Saturday night and Sunday along with stronger southerly winds. Snow levels will remain high ahead of the front limiting potential winter weather impacts to travel at the end of the holiday weekend.

Extended discussion (Monday through Thursday)

The 12z GFS is a bit quicker than the ECMWF with the easterly progression of the Sunday-Monday storm system, with precipitation ending by Monday evening. The ECMWF holds onto precipitation across the region into Monday night. By Tuesday into Wednesday, confidence is high that a strong ridge of high pressure will bring dry weather to Northern California. Depending on how much northerly wind develop behind the frontal system, we could also see fog development again in the valleys. Dry weather is likely to continue through the rest of the extended period.