Seasonable temperatures and generally dry weather are expected to persist this week, with unsettled weather returning over the weekend into early next week. Weak weather system with light rain and high elevation snow, mainly north of US 50 later this weekend into early next week. Limited impacts expected. Potential for increased northerly winds and lowering relative humidity leading to elevated fire weather concerns behind the passage of the system mid next week.
Discussion
A quiet pattern is expected today with light onshore flow, dry weather and slightly cooler high temperatures than Monday. These highs are about normal for this time of year, in the 70s to near 80 in the Valley and lower foothills, and in the 50s to 70s in the upper foothills and lower mountains. Little change is expected through Friday.
A shortwave trough brushes by to the north over southern Oregon and far Northern California, with weak eastern Pacific ridging building in behind it starting Wednesday afternoon. The day will start out cool, with Valley and Delta lows in the mid to upper 40, but afternoon temperatures will trend a few degrees warmer than today. Precipitation is expected to remain mainly north of the area, though completely can’t rule out some isolated light mountain shower chances over far northeastern Shasta County.
Much of interior Northern California will be mostly clear. Behind this trough, some increased northerly flow is expected Wednesday night into Thursday, along with a warming and drying trend as the eastern Pacific ridging builds back into the area through the rest of the week.
Forecast highs are in the upper 70s to low 80s. Daytime relative humidity around 15-30%, lowest on Thursday on the west side of the Valley, where some northerly winds gusting to 20-25 mph are possible.
The flow pattern shifts to more onshore Friday afternoon as a Pacific trough approaches and the ridge axis shifts eastward, with increasing humidity levels and cloud cover over the area.
Extended Discussion (Saturday through Tuesday)
Ensemble guidance and cluster analysis has an upper level trough off the coast by early Saturday. The trough is then projected to move onshore sometime later in the weekend into early next week bringing an unsettled weather pattern back to interior northern California. The NBM currently shows the potential for light precipitation beginning over the northern Coastal Range Saturday afternoon/evening, with chances becoming widespread north of Interstate 80 on Sunday.
The National Blend of Models (NBM) continues to show a 40-80% probability of a tenth of an inch or more of precipitation over 48 hours through Monday night, with the best chances over higher terrain and north of Interstate 80. There is a 25-65% probability of a quarter of an inch or more.
Cooling temperatures and increased onshore flow are also anticipated with the troughing over this period. Some clusters are also suggesting the trough digging southward over the Great Basin for the middle of next week with ridging building in over the eastern Pacific, leading to the potential for some increased northerly flow developing behind the trough.
Significant uncertainty remains, though, so we will be closely monitoring the situation.