Widespread sprinkles, or light showers, possible through Tuesday. Mainly late day thunderstorm chances expected over the Sierra Nevada through the week. Cooler temperatures early this week before warming to above average again by mid-week.


Satellite imagery shows that considerable mid and high cloudiness has moved northward into NorCal as the monsoon push continues (satellite derived TPW now around an inch or above across most of the forecast area). The large MCS that developed over southern Nevada Sunday is now decaying as it moves southwest into SoCal. Radar has been picking up some light returns over NorCal since Sunday evening, but only sprinkles or a brief light shower are expected given the dry lower levels (below about 600 mb).

Surface pressure gradients have trended a bit stronger onshore compared to 24 hours ago, but profiler data indicate the depth of the marine layer remains around 1.5k ft. The increase in cloud cover is also affecting cooling rates as current temperatures are running around 4-8 degrees milder compared to 24 hours ago across the mountain areas and northern Sacramento Valley. The Delta Breeze is keeping temperatures pretty similar through the Delta into the southern Sacramento Valley.

The increase in cloud cover will result in some cooling across the area today as the Delta Breeze is not expected to provide much additional cooling given the depth of the marine layer. We’ll continue to see isolated sprinkles and very light showers across much of the area today, but lack of strong elevated instability and lift is expected to limit more extensive activity. We may see an increase (compared to the past several days) in surface-based late day thunderstorms over the northern Sierra, but extensive cloud cover could pose a limiting factor.

Still appears the best chance for widespread light showers will be tonight and early Tuesday as the remnants of the inverted trough that’s been affecting the Southwest over the weekend move up from the south. Overall elevated instability does not look too impressive, but increased lift should be enough for more extensive light showers (and perhaps some isolated thunder). The wave lifts north on Tuesday with decreasing clouds and showers expected from the south.

By mid-week it appears we’ll be back into our hot and dry weather pattern with limited Delta Breeze and isolated diurnal deep convection over the northern Sierra.

Extended Discussion (Friday through Monday)

Interior NorCal remains situated between ridging over the Rockies and troughing over the Eastern Pacific through the extended. This should bring above normal temperatures that gradually cool towards the end of the period. Monsoonal moisture surge could cause a few thunderstorms to develop along the Sierra crest, mainly south of I-80.