Seasonable temperatures and generally dry weather are expected to persist this week, with just a slight chance of light showers over northern Shasta County today. Gusty winds tonight through Thursday will bring elevated fire weather conditions. A weather system early next week is expected to bring cooler temperatures, breezy winds, and the potential for precipitation.
Discussion
Temperatures this morning are quite cool, generally 3 to 10 degrees cooler than this time yesterday, with Valley lows dropping into the upper 40s for some locations. A fast-moving shortwave trough will skirt across the California-Oregon border today bringing scattered clouds and chances for scattered light showers over far northern Shasta County this afternoon. The NBM continues to have a 10-20% chance of 0.1″ of rain or more, so precipitation impacts will be fairly minimal if any. The majority of the area will see mostly sunny skies with temperatures a little warmer than yesterday.
Northerly winds will develop behind the exiting shortwave starting this evening and continuing through Thursday. NBM forecast winds have trended stronger, now with a 45% to 65% chance for gusts greater than 30 mph. Forecast gusts 20 to 30 mph are expected for for the Sacramento Valley, especially west of the I-5 corridor. The Delta and far western Sacramento Valley could see gusts up to 35-45 mph, peaking Thursday morning. Humidity levels will be quite low, with poor recovery (30-45%) Wednesday night for the western Sacramento Valley and adjacent foothills. Thursday afternoon humidity drops into the teens to 20%. The winds and low humidity will bring a short period with areas of elevated fire weather conditions, potentially locally near critical fire weather conditions.
Eastern Pacific upper level ridging builds back over interior Northern California late Thursday into Friday. This ridge axis shifts eastward with light onshore flow developing late Friday into Saturday as an upper level trough approaches. Temperatures trend a little warmer Saturday, with Valley and Delta highs near 80. Chances for light precipitation spread into the northern Coastal Range Saturday afternoon and evening, then into the northern mountains and northern Sacramento Valley overnight.
Extended Discussion (Sunday through Wednesday)
Ensemble guidance and cluster analysis continue to show the upper level trough moving onshore bringing an unsettled weather pattern for the weekend into early next week for interior Northern California. By Sunday afternoon there will be chances of rain showers spread across the mountains and Valley north of I-80. The National Blend of Models (NBM) currently shows a 40-80% probability of a tenth of an inch or more of precipitation over a 48-hour period (Sun AM-Tues AM). The highest precipitation chances will remain over the mountains north of I-80.
Across the northern Sierra, there is a 65-75% probability of a 0.25″ or more. Snow levels start above 8,000 feet Sunday, but are expected to drop below 5,000 feet Monday night. Snow totals of 3-6″ are currently forecast for above 6,500 feet Sunday night through the day Tuesday, so winter driving impacts are possible. Stay tuned for details on this as we get closer to the event.
In addition to precipitation, this system will bring cooler temperatures and increased onshore flow over the weekend with gusts 15 to 25 mph. Clusters are leaning towards this trough dropping into the Great Basin, with ridging building into the Pacific Northwest. This “Inside Slider” pattern will bring the potential for gusty northerly winds through the Valley with easterly/downslope winds across the Sierra and its foothills closer to the mid-week period. This pattern is weaker than the one that prompted last week’s wind event. The NBM is indicating a 40 to 70% chance of gusts greater than 30 mph Monday and Tuesday. This could potentially bring elevated fire weather concerns, mainly for Tuesday. Uncertainty remains how dry humidity will get, though, with cool temperatures peaking in the mid 60s to around 70 and lingering moisture over the area.