Flat ridge will provide dry and warm weather Today and Friday with valley high temperatures in the low to mid 70’s. These readings are 5 to 10 degrees above normal for this time of year. Low pressure trough will swing across NorCal late Saturday into Sunday. This system appears on the weak side, as main energy remains north of the area over the Pacific Northwest. Some model disagreement exists in precip amounts with NAM on the extreme dry side, GFS much wetter and Euro in between. Current forecast is more in line with Euro compromise solution, valley amounts generally less than 0.10 and mountains 0.25-0.75. High snow levels above 7,500 feet will limit snowfall amounts.
Extended discussion (Monday through Thursday)
A larger scale disturbance will approach northern California for the start of next week. Height falls associated with the upper level trough will bring chances for rain showers and snow for the highest Sierra elevations Monday morning and afternoon. Towards late afternoon into the early evening hours Monday, the upper level trough is well forecasted to take a negative tilt into the region, which in combination with atmospheric instability could lead to the development of valley thunderstorms. Any storms that develop could produce gusty winds and small hail along with heavy downpours in heavy showers.
The upper level low rotates northward on Tuesday with another associated trough base swinging over the eastern Pacific into northern California for additional rain and thunderstorm chances for the valley and snow for elevations above 5000 feet. Moderate to heavy rainfall is possible for some Valley locations with heavier amounts confined to foothill and mountain locations. The mid to lower level jet will be somewhat strong according to model guidance with a south to southwest orientation that will lead to barrier jet development across the central valley. As a result, ground level winds and wind gusts could create some travel difficulties for smaller and higher profile vehicles across the forecast area Tuesday into Tuesday night.
Precipitation chances continue Wednesday as the upper level disturbance continues to create lift and instability. Snow for the Sierra above 4000 feet could result in travel delays for mountain passes.