Cooling trend will continue through Wednesday with below normal temperatures likely. Isolated showers and thunderstorms over the Shasta County Mountains mid-week, otherwise dry through Thursday.
Some high clouds are beginning to spread into the far northern portion of California as the trough over the northeast Pacific approaches. Elsewhere, skies are clear across the region early this morning. Surface pressure gradients continue to trend toward stronger onshore with southwesterly gusts around 30 mph in the west Delta, and southerly gusts in the teens to lower 20s mph common around the Sacramento area. The barrier jet along the western edge of the Sacramento Valley is weakening with a few locales reporting northerly gusts in the teens or lower 20s mph.
Trough will continue to deepen along 130W into mid-week with strong southwesterly flow remaining over NorCal the next few days. Breezy to windy conditions will be possible over the mountains, near the Delta, and locally in the Central Valley in the afternoons and evenings today into Wednesday. Along with the breezes, cooler temperatures are expected with highs today around 5-10 degrees cooler than Sunday’s. Tuesday and Wednesday are expected to be the coolest days of the week when highs will be around 10-15 degrees below average and top out only in the 70s across the Central Valley.
The upper trough is forecast to move across NorCal on Wednesday, and just enough moisture and instability may be present for a few showers or thunderstorms across the northern mountains during the afternoon into the evening. Little or no precipitation is expected with these showers, though some locally gusty outflow winds will be possible.
Temperatures may begin to inch up a bit beginning Thursday, but will still be well below average, as short-wave ridging moves overhead.
Extended discussion (Friday through Monday)
A cool closed upper low develops off the coast of British Columbia Friday. A short wave with a cold front moves into the northwest California coast Friday. Another shortwave approaches on Saturday. Below normal high temperatures are expected for the weekend, warming to around normal early next week.
Cluster analysis is showing a trend to a further westward location for the upper low, which could greatly limit how far inland the frontal systems and precipitation make it. The latest NBM runs are reflecting this lowering confidence in precipitation for the area. Much of the shower activity could end up limited to the northern Coastal Range. ECMWF and GFS ensemble members are now quite limited for showing significant QPF amounts for interior Northern California. Current forecast project a chance of rain showers over northwestern Shasta County, with a slight chance extending into the Sacramento Valley and western Tehama County Friday, spreading into western Plumas County on Saturday.
Sunday still has high uncertainty due to the potential track of the upper low. Overall trend is for precipitation to be further northward, with the NBM limiting precip chances to the northern portion of Shasta County. Most of the area is expected to be dry with temperatures near average. For the entire extended forecast, onshore flow remains with periods of breezy winds expected, especially near the Delta.