A gradual warming trend will lead to moderate heat risk through much of this week. Mostly dry weather forecast except for a chance of isolated mountain thunderstorms Wednesday into the weekend.
Ensemble models are persistent in forecasting a large ridge of high pressure building over the West Coast this week. This will continue to bring warming temperatures to interior Northern California through the end of the week. Temperatures warm a few degrees each day today and tomorrow and peaking midweek. Overall, can expect to see Valley temperatures in the upper 90s to near 108 degrees in the northern Sacramento Valley with mountain temperatures in the 80s to upper 90s. Overnight temperatures will remain fairly warm in some areas, including the northern Sacramento Valley and thermal belts in the foothills, only falling to the 70s to around 80 degrees. These temperatures will produce widespread moderate heat risk for those who are sensitive to the heat and those who spend time outdoors.
With this pattern, onshore flow is forecast to continue with generally west to southwest winds, though northerly in the northern San Joaquin Valley, in the daytime hours. A decent Delta flow is expected each night as surface pressure gradients tighten along the coast. This will provide better overnight cooling for Delta influenced areas. This will also keep the majority of wildfire smoke north of area wildfires, though light diurnal winds in the early morning hours could allow smoke to drain into the northern Sacramento Valley.
Monsoonal moisture is slated to move back into Central and Northern California beginning Wednesday which will bring a renewed chance of isolated mountain thunderstorms. At this time, best chances for instability remain near and east of the Sierra crest south of Highway 50, potentially spreading north into the Southern Cascades and Shasta County mountains late in the week. PWAT values range from 0.5 inch to just under 1 inch, so thunderstorms will likely contain some precipitation. However, with current dry conditions, any lightning strikes could cause fire starts. Those who have outdoor mountain plans should be weather aware.
Extended Discussion (Saturday through Tuesday)
Pretty good agreement between the GEFS and EC ensemble indicating strong ridging continuing over the weekend maintaining the hot and dry weather, then modest cooling possible early next week as a weak trough develops. However, cluster analysis indicates there is some uncertainty on the depth of the trough.
It’s looking like some moisture from Kevin will works its way across NorCal on Saturday bringing an increase in clouds and perhaps some sprinkles as deeper moisture moves in (TPW anomaly around 3 standard deviations), but shower and thunderstorm chances likely to be limited to the Sierra south of Tahoe as organized lift appears lacking for more widespread showers or thunderstorms. However, bears watching.
Drier mid and upper flow associated with the trough shifts moisture well east of the region early next week.