Increasing chances for more widespread precipitation arrive with a midweek pattern change, persisting into the weekend. Lower snow levels (5000 – 6500 feet) and less precipitation expected compared to last week’s storm.
Discussion
As of early this afternoon, morning fog has mostly dissipated and partly to mostly cloudy skies are evident across interior NorCal. Despite this, temperatures have already made a run at the mid 50s in the Valley and foothills and 40s at higher elevations.
High temperatures by the end of the day look to rise a few more degrees, topping out in the upper 50s to low 60s in the Valley and upper 40s to low 50s in the mountains.
Although ridging aloft remains in place over the region, a weak shortwave looks to pass through the Pacific Northwest later today. This will result in a glancing blow to the far northern forecast area, with a slight (10 to 20%) chance for an isolated shower or two over the coastal range into this evening. This activity will remain light, with only a 5 to 15% chance of exceeding 0.1″ of QPF.
Otherwise, another round of fog, possibly dense at times, is expected for the northern San Joaquin Valley and potentially into the southern Sacramento Valley by Tuesday morning. At this time, there is a 40 to 60% chance of seeing visibilities drop below 1/2 mile in these areas. After the potential morning fog, quiet, dry, and seasonable weather then looks to prevail through the remainder of Tuesday.
Moving toward mid to late week, a pattern change is expected as troughing from the Gulf of Alaska drops southward toward the Pacific Northwest. Despite the anticipated further northward trajectory, a modest southwesterly fetch around the trough into interior NorCal will introduce some precipitation to the region Wednesday, tapering off by early Thursday.
This trajectory will, however, keep the bulk of the precipitation generally from the I-80 corridor northward. Latest probabilities of exceeding 0.25″ of QPF are in the 75 to 90% range from the I-80 corridor northward, with 40 to 60% probabilities further southward.
Heaviest precipitation is expected to remain confined to the mountains. Furthermore, snow levels drop into the 5000 to 6000 foot range on Wednesday, bringing the possibility of some light accumulations to pass level.
Given the overall lighter QPF anticipated with this system, probabilities of snowfall exceeding 2″ are only in the 20 to 40% range along the Sierra and southern Cascades.
Along with this system, breezy southerly winds look to increase throughout the day on Wednesday as well. Gusts of 15 to 25 mph will be possible in the Valley, with a 70 to 90% chance of gusts exceeding 35 mph along the Sierra.
Extended Discussion (Friday through Monday)
A secondary trough will move over the area starting Friday, bringing even more chances for widespread rain and better chances for mountain snow than earlier in the week. Most of the rain from this weekend system will occur on Friday, with showers tapering off through Saturday and Sunday.
Between Friday and Saturday, there is a 50-80% chance of at least half an inch accumulating over the 24 hour period. The new year looks to be starting off cloudy with a slight chance of scattered showers due to a third low pressure system off the coast of California.
Temperatures will be slightly above or near seasonable levels through the extended forecast period, with highs in the Valley around the high 50s to low 60s, and at higher elevations near the mid 30s to high 50s.