January 20, 2020 – Some light precipitation possible across the higher elevations of the northern Sierra today, otherwise mostly cloudy conditions. More widespread precipitation chances return tonight and Tuesday.
Discussion
Upper ridge has shifted east with with satellite imagery showing considerable mid and high clouds moving up from the southwest ahead of a weak weather system. Surface observations showing patchy light fog developing over portions of the Sacramento Valley while stratus is locked in again across the northern San Joaquin Valley. The extensive cloud cover is forecast to limit overall fog potential this morning.
The higher elevations of the northern Sierra may see a few sprinkles or light showers today as the weak system moves through. The remainder of the region is expected to see dry weather continue today. Forecast soundings indicate the passage of the weak system may be enough to disrupt the inversion over the Central Valley allowing for earlier mix-out of stratus and a little warmer temperatures today compared to Sunday.
Next stronger Pacific frontal system moves in tonight spreading light QPF across the northern half of the forecast area. Winds increase later tonight and Tuesday as the front approaches and moves inland. Bulk of precipitation expected during the day on Tuesday over the northern mountains and northern Sierra (0.50 to 1.25 inches) as the front moves through. The valley may see upwards of 0.25 to 0.40 inches, mainly to the north of Sacramento.
Forecast snowfall ranges from around 2-6 inches across the northern Sierra above 5000 feet with local amounts approaching a foot over the higher peaks in the northern mountains. Very little impact to travel possible until potentially Tuesday afternoon and evening across the northern Sierra passes.
Upper ridging returns mainly dry weather for the area on Wednesday and Thursday, except for some light warm-advection precip possible across the northern third of the forecast area as stronger systems pass to our north into the PacNW. Night and morning valley fog may return and become more extensive.
Extended Discussion (Friday through Monday)
A weak system will bring light showers on Friday, mainly over the northern mountains. Elsewhere, dry conditions are expected. Ensembles indicate a troughing pattern developing this weekend, with the potential for widespread precipitation across the region. Specific details will likely change given model disagreement in terms of timing and strength. At this point, the heaviest precipitation is expected to occur late Saturday into Sunday. Latest ECMWF ensemble is showing moderate to high probabilities of QPF exceeding 0.5 inches over higher terrain, and over 1 inch over the Sierra and Lassen National Park. This system could bring mountain travel impacts as snow levels lower below pass levels early Sunday morning.