Grass Valley, CA 95945 Weather

Key Messages

- Widespread precipitation late today through Thursday, with below normal temperatures, thunderstorm chances, and high elevation snow showers.

- Ridging builds into the region end of the week and into the weekend, with a slight chance for precipitation Sunday into early next week.

Discussion

...Today through Thursday...

Satellite has a broad area of broken clouds across CA this afternoon, with a few isolated showers in parts of the Sierra. Current HIRES guidance has showers and isolated thunderstorm potential along the Sierra as the main upper level trough remains offshore pushing moisture and instability into CA. Best chances for any shower or isolated storm for today will be this evening and into the overnight hours for the Sierra before the main upper level system starts to shift onshore. HRRR and NAM guidance has the most robust chances, while others favor more isolated shower potential. None the less, chances are 10-20 percent for areas in the Sierra this evening and overnight for thunderstorm potential. A few storms may be on the stronger side, with small hail, brief heavy rain, gusty winds, and frequently lightning being the main hazards.

Further instability and moisture push into the Valley, Foothills, and Mountains Wednesday. Best chances for widespread precipitation will be during the afternoon through the evening hours. This timeframe is when the highest PWATS, Shear, Instability, and Lift will be within the region. Shear is roughly around 20kts through the first 6km and instability is sitting around 500-750J/kg. This combined with the strength of the forcing moving through the low to mid levels will bring the thunderstorm chances through the mountains and into the foothills/valley. A few of the storms could be on the stronger side once again, with the potential for a low top supercell. The chance of a low top supercell is low for the region, but the area of interest for this development is within the southern Sacramento Valley, northern San Joaquin, and into the adjacent terrain. Main hazards in the stronger storms would be small hail, gusty winds, lightning, and brief heavy rain. We will continue to monitor the latest CAMS and observations as the system moves into the region bringing the thunderstorm chance.

Current NBM guidance increased snow levels to 8500 feet and above Wednesday through Thursday. Snow totals at the highest elevation (areas south of Highway 50) are roughly 3-6 inches at this time. Higher chances and totals are toward the Yosemite region. Conditions slowly dry out on Thursday with lingering thunderstorm potential in the mountains Thursday afternoon and evening. By the overnight conditions dry and skies start to clear. Besides the thunderstorms, rain, and snow chances, temperatures will continue to be below normal to near normal across the region through the end of the week.

...Friday and beyond...

As the system tracks east of the region late Thursday into Friday, conditions dry and slight ridging builds into the area. Winds will turn toward the north to northeast on Friday, with a few gusts up to 30mph. Minimum RH values will be in the 20s to 30s for portions of the northern Sacramento Valley and adjacent Terrain Friday, but at this time chances for fire weather concerns are low. Dry and below normal temperatures continue through Saturday and into Sunday, with slight chances for precipitation in the higher terrain late Sunday and into early next week. Guidance continues to struggle with the ridge evolution and if any trough can break through and bring further unsettled weather.

Aviation

VFR conditions prevail across the region through the TAF period. Winds will remain light and less than 12kts. A few isolated showers will be possible in the Valley after 05Z Wednesday, before more widespread showers move into the area after 18Z Wednesday.

Sto Watches

None.