Dry and seasonably warm through Saturday. Breezy north to east winds Saturday but little to no impacts expected. Pattern change with cooling trend returns Sunday. Chances for precipitation Monday and again mid to late week, with periods of locally breezy winds.
Discussion (Today through Monday)
Latest GOES-West satellite imagery shows a fairly dense band of high clouds moving into CA along with some patchy fog in the Modesto area. Localized areas of patchy fog will be possible in the northern San Joaquin Valley through this morning, briefly producing visbilities as low as 3/4 to 1 mile at times. The ridge axis has shifted inland, currently over NV, allowing weak southwest flow over much of NorCal.
Slightly cooler high temperatures expected this afternoon compared to yesterday’s.
The next weak weather system can also been seen on satellite this morning, currently moving into southwestern British Columbia. As this system continues to shift southeast through today, it will arrive in NorCal overnight tonight with an associated a weak front, mainly providing increased cloud cover overnight. Increased cloud cover overnight will mainly limit maximum cooling of overnight temperatures.
Increased cloud cover will also help limit radiative cooling fog development. Though, a brief period of patchy fog will be possible in the Stockton to Modesto area before the more dense cloud cover arrives by early morning Saturday in the northern San Joaquin Valley.
Following the weak front Saturday morning, expect mostly clear skies, slightly cooler temperatures and some northerly breezes develop in the afternoon across the north and western Sacramento Valley. Expect strongest northerly winds 10-15 mph with gusts 20-25 mph primarily along and west of the I-5 corridor.
By Sunday, northwesterly flow overhead will continue, continuing to filter in cooler air and allowing a weak piece of energy to move through NorCal, just bringing a period of passing clouds. Temperatures on Sunday will be closer to seasonal normals following the weak front Saturday with lighter winds.
The ‘better’ weather system previously advertised as coming on Sunday continues to trend slightly slower and drier, with slight to chance (15-35%) PoPs of seeing light precipitation limited to northern Shasta County overnight Sunday/early Monday. Slight to chance (20-40%) PoPs will spread southeast Monday morning, with the ‘best’ chances of seeing measurable precipitation mainly confined to the Sierra and upper foothills late morning-afternoon.
Snow levels with this system will be between 3,500-4,000 feet. Since the best chances of seeing light precipitation are mainly limited to the mountains, with snow levels around 4000 feet, precip. will primarily fall as snow. Considering the continued drying trend with this system, snow amounts have once again decreased with the latest forecast, with 24 hour snow totals of 0.5″-2″.
Even looking at the 75th and 90th percentile snow totals, which only amount to 2″-4″, so even the upper-end percentiles are below Winter Weather Advisory criteria. Even so, light snow along mountain passes may create some minor impacts along the major thoroughfares in the Sierra for travelers ill-prepared for lightly snow-covered roadways.
Extended Discussion (Tuesday through Friday)
Dry weather Tuesday as short wave upper ridging moves through. Some morning frost possible in the Central Valley. Cluster analysis/ensembles showing a deeper next system, farther west. EC has picked up on this better than GFS at this time and NBM POPs look similar to this solution. This would suggest some precip chances over northern portions of the forecast area Wednesday afternoon into evening, then spreading south over the remainder of interior NorCal Thursday.
Decreasing showers expected Friday from north to south as the system moves south and east of the CWA.