The National Weather Service's Sacramento office has released a weather update indicating cooler temperatures expected through mid-week, accompanied by a chance for afternoon showers and thunderstorms over the mountains. The precipitation chances are set to increase Thursday into Friday as a weather system moves through, with the best chances of rain in the foothills and mountains.
According to the discussion from meteorologists, a robust Delta Breeze is currently underway due to the weakening and eastward shift of the strong upper ridge that prevailed over the past several days. Stratus clouds surged northward along the coast on Monday, and data from profilers indicate that the marine layer has deepened to around 3000 feet or more. The surface pressure gradient has transitioned from offshore to onshore, resulting in southwesterly gusts of 25-35 mph from the Sacramento area into the Delta region.
Mid-level moisture and instability are being drawn northward ahead of an offshore trough, leading to some elevated instability over the region on Tuesday morning. While the forecast soundings indicate potential instability, the question remains whether the lift will be sufficient to generate showers. Recent satellite imagery shows the development of mid-level clouds, with virga observed, potentially seeding drizzle or light rain as marine stratus spreads inland.
In addition to cooler temperatures, the forecast anticipates mainly diurnal mountain showers and thunderstorms later on Tuesday and Wednesday as an initial trough and closed low pass mainly south of the region into Southern California.
Looking ahead, a couple of stronger short-wave troughs are forecasted to drop into Northern California later in the week, reinforcing the cool and unsettled pattern. This will bring a better chance for showers later on Thursday into Friday. Quantitative Precipitation Forecast (QPF) suggests less than 0.10 inches of rainfall in the valley and around 0.50 inches over the mountains. Snow levels are expected to be around 7000 feet, with light snowfall accumulations possible over the higher peaks. Minimal impacts to travel are anticipated.