May 12, 2020 – Precipitation chances and cooler temperatures continue through Thursday. High mountain snow is possible with light accumulations at pass levels into this evening, bringing some slippery road conditions at times. Isolated thunderstorms are possible this afternoon and evening, mainly for Shasta and Tehama counties. Some breezy southwest winds are expected today. Warmer and drier Friday, followed by more wet and cooler weather into next week.
Upper level low is now currently west of the Oregon-Washington border. Some light showers continue over the foothills this morning, mainly over the Motherlode.
A shortwave rotating around the southern portion of the low will bring the next wave of precipitation for interior Northern California today. Radar shows a band of showers just off the coast. This will move inland, bringing the best potential for rain showers for the Valley this morning.
Snow levels at 3 am over the I-80 corridor were still quite high, around 8000 feet, but will be dropping to around pass levels (7200 feet) by daybreak as colder air advects in. Oroville profiler was showing snow levels down to around 7000 feet. Some wet snow is possible for Sierra passes into the early evening, maybe a couple inches of accumulation. This could bring some slippery conditions at times, most likely in the afternoon. Drivers should be prepared for winter driving conditions and slowdowns.
Models build in some weak instability late morning into the evening over the northern Sacramento Valley and adjacent mountains. Given the timing of the energy there wont be much clearing and that will likely limit instability and thunderstorm chances but a few isolated storms are possible over Butte, Tehama, and Shasta Counties. Precipitable Water will be 0.50-0.75″ in that area and brief heavy downpours will be possible with any thunderstorms. Some small hail is also possible. No issues are expected for burn scars. Showers will become less numerous tonight.
Well below average highs are expected today and Wednesday with them running as much 10-15 below average in the Valley, up to 20 degrees below average in the mountains.
Some breezy south to south west winds will pick up this morning with Valley gusts up to 20-25 mph possible once again mid morning into the afternoon.
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The general pattern lingers on Wednesday, with a weaker shortwave around the base of the upper low tracking into Northern California. This will bring another period of showers, especially north of I-80. wettest locations should be the northern mountains. On Thursday the low will be pushing to the east but some energy moving over the northwest part of the area will bring some showers early in the day.
Upper level ridging begins to build in late in Thursday and continues Friday bringing a return to dry conditions. Valley highs in the 70s are expected Thursday afternoon, reaching the low 80s on Friday, which is normal for this time of year.
Extended Discussion (Saturday through Tuesday)
Pacific frontal system approaches early Saturday with precipitation spreading into northwestern portions of the CWA by the afternoon, and across the remainder of interior NorCal Saturday night into Sunday morning. Snow levels look to be at or above pass levels through much of the weekend. Models differ with handling of associated upper low, but it appears unsettled weather will continue Monday into Tuesday. Near normal highs Saturday cool to upwards of 10 degrees below normal Monday with continued below normal Tuesday.