Pattern becomes drier and warmer into midweek. Unsettled weather returns late week into next weekend, bringing chances for widespread precipitation and gusty winds.
Discussion
Warm, calm weather dominates the area today and Tuesday, as a ridge continues to move over the area. Highs in the Valley will be around the low to mid 70s through Tuesday, with highs in the 60s in the foothills, and high 50s to low 60s in the mountains.
Winds today will be northerly and breezy into the afternoon, with occasional gusts between 15 to 30 mph, primarily along the I-5 corridor between Red Bluff and Sacramento and in the Sierra. As we move into the evening, winds will calm in the Valley, though the Sierra will continue to see breezy conditions into Tuesday.
Beginning Wednesday afternoon and evening, a new trough moves down along the coast from the Gulf of Alaska. This will keep highs closer to normal on Wednesday than on Monday and Tuesday.
The real cool-down occurs Thursday, with highs in the high 50s in the Valley, and 30s and 40s at higher elevations.
Additionally, on Wednesday, the area can expect breezy southerly winds in the evening, with gusts between 15 and 30 mph. Locally higher winds can be expected at higher elevations, especially over mountain peaks.
While this system will also bring some precipitation, between limited precipitable water availability and rain shadowing occurring in the majority of the Valley, precipitation totals will be limited (generally between a trace amount of rain and a tenth of an inch in the Valley). The exception to this will be any Valley locations impacted by thunderstorms (10 to 20% probability of occurrence on Thursday afternoon, with highest chances in the northern Sacramento Valley), which will cause some locally higher totals.
The foothills and mountains, along with Shasta County in the Valley, can expect slightly higher totals across Wednesday and Thursday, generally between 0.25 and 0.5 inches.
Snow levels will begin fairly high, around 5000 to 6000 ft. By Thursday afternoon, they will have lowered to around 2500 to 3500 ft; however, as mentioned before, limited availability of precipitable water means that snow totals currently look to remain fairly low, even at mountain peaks.
Currently, above 4500 ft, there is only a 5 to 25% probability of 8 inches of snow or more. Between 2500 and 4500 ft, there is a 20 to 65% probability of at least an inch of snow, with chances rapidly decreasing below 4000 ft. There is still some uncertainty associated with this system, so be sure to monitor the forecast closely for updates as we get closer to the event!
Extended Discussion (Friday through Monday)
Unsettled weather continues through the end of the week into the weekend as the closed low progresses eastward. Prior to this though, the closed low is expected to remain centered over central California early Friday. This will result in precipitation impacts likely lingering throughout the day, although primarily confined to foothills locations and higher along the Sierra, with only isolated showers at most for Valley locations.
Southerly winds will remain breezy on Friday as well, with strongest winds expected over the Sierra. Additional snowfall with this system looks to remain rather light through Friday and into early Saturday morning.
Rather low snow levels are expected with this cold system though, potentially falling as low as 1500′ to 2500′ by Friday morning. These levels should rise back to 2500′ to 3500′ through Friday afternoon and into Saturday morning.
Regardless, some light snowfall accumulations (up to an inch) may be possible as low as 3000′, with potential for flakes flying at 1500′-2500′ on Friday morning. The heaviest snow is expected to occur on Thursday, however, with primarily lingering snow showers throughout the day on Friday.
A potential secondary period of more widespread snowfall along the Sierra will then be possible Friday evening into early Saturday as the closed low begins to track eastward toward the Great Basin. Overall, probabilities of exceeding 6″ of additional snowfall from Friday through Saturday morning are only in the 10 to 25% range, primarily above 4500′.
Aside from a few lingering snow showers over the Sierra on Saturday, a brief period of relatively quiet weather is expected, with a slight 3 to 5 degree bump in high temperatures as a transient ridge moves through. Cluster analysis then indicates a 60% chance of another progressive shortwavetrough moving across interior NorCal on Sunday. This may bring additional precipitation impacts through the end of the weekend, primarily along the Sierra, but available moisture looks to be limited at this time.
Consensus then indicates a return of large scale ridging and attendant drier and warmer weather into early next week.