April 13, 2021 – Dry, warm, and breezy conditions through early Wednesday with strongest winds expected today. Chances for mountain showers over the Sierra Tuesday and Wednesday. Warmer temperatures and dry weather late week into the weekend.
The forecasted upper level trough is beginning to move south into northern California & Nevada early this morning. Ensemble guidance remains in good agreement that this trough will form a cutoff low as it moves south along the CA/NV border today. Winds are forecast to increase this morning as the trough first moves over the area and surface pressure gradients tighten.
Strongest winds are expected over the northern Sacramento Valley this afternoon and early evening with northerly gusts up to 25 to 35 mph, locally up to 40 mph. A Wind Advisory remains in effect today for the northern Sacramento Valley. Locally breezy winds are also possible over the Sierra, strongest over the Central Sierra crest with westerly gusts up to 35 to 45 mph.
Locally gusty winds will continue through Wednesday. Dry conditions with minimum daytime relative humidity values ranging from 5 to 15 percent in the Valley in combination with these winds will bring elevated fire weather conditions. Any outdoor burning is highly discouraged the next couple days.
Precipitation chances continue to increase over the mountains and upper foothills today through Wednesday as ensemble guidance shows the upper trough lingering over Nevada with multiple impulses moving along it through the forecast area.
QPF amounts for the two day period range from a few hundredths in the foothills up to 0.25 to 0.5 inches above about 5000 feet. Snow amounts up to 3 to 6 inches are possible for higher elevations above 6500 feet with locally lighter amounts possible down to 5500 feet. Best chances of showers are for locations in southern Plumas county and southward.
Isolated thunderstorms also remain possible in the afternoons as models suggest enough instability (CAPE values 100-300 J/kg) and moisture over the mountains and foothills. The main impact with stronger cells will be small hail and locally heavy but brief downpours.
Ensembles agree the upper low will move east into the Great Basin on Thursday bringing drier weather and less breezy conditions. Temperatures are forecast to cool around 5 to 10 degrees today compared to yesterday with Valley temperatures expected to remain in the mid to upper 70s through Thursday.
High pressure will then build back over the West Coast on Friday allowing temperatures to raise again.
Extended discussion (Saturday through Tuesday)
Good agreement that high pressure will result in warm and dry weather across the region this weekend and early next week. Highs will be around 15-20 degrees above average through Monday, reaching the upper 80s to lower 90s throughout the Central Valley.
Slight cooling will be possible beginning Tuesday as the ridge is weakened by a weather system passing to the north.