Isolated to scattered shower and thunderstorm chances continue for the mountains, portions of the foothills, and northern half of the Sacramento Valley through the holiday weekend and next week. Temperatures remain near or slightly below normal. Periods of locally breezy onshore winds. Slight warming for the end of the week into next weekend possible.
Discussion
Latest GOES-West nighttime microphysics RGB satellite imagery depicts some mid to high clouds moving over NorCal as well as some marine stratus along western portions of Solano County. A modest Delta breeze is creating locally breezy conditions this morning, with southwest wind gusts to 32 mph currently at Fairfield/Travis Air Force Base, as of 2 AM PDT. Morning low temperatures today are forecast to be in the low 50s to low 60s for the Valley and foothills with 30s to 50s in the mountains. An upper level low looks to drop down over the California Coast over the next 24 hours or so, and will continue to progress southward through midweek and then shift inland.
Seasonably cool temperatures will continue in the short-term, with near to below daytime high temperatures. For today, the Valley and lower foothills can expect daytime highs to approach the mid 70s to mid 80s, and the 50s to 70s for the upper foothills and mountains.
Another round of locally breezy onshore winds will allow for those cooler temperatures in the Delta area and adjacent Valley locations where highs will be slightly cooler, in the low to mid 70s. Similar conditions are expected over the next few days.
Isolated to scattered shower and thunderstorm chances return today, with continued chances over the next several afternoons and evenings. The best chances today (15 to 35 percent probability) will be in the Coastal Range, southern Cascades, northern Sierra Nevada and portions of the northern/central Sacramento Valley over the next few days.
For today and Monday, the HREF suggest CAPE values in those areas up to 500 to 1000 J/kg. The main concerns with any thunderstorms that develop would be dangerous lightning, hail, brief heavy rain, and gusty winds, which could impact outdoor recreationists over Memorial Day Weekend. Motorists should slow down and use wipers/headlights when encountering slick roads.
Extended Discussion (Thursday through Sunday)
There is some divergence in the models over the extended forecast period in the ensemble guidance and cluster analysis. However, mountain shower and thunderstorm chances look to return each afternoon and evening later this week and potentially into next weekend.
The National Blend of Models (NBM) advertises generally a 10 to 25 percent probability of thunderstorm development over the northern Sierra and southern Cascades during this timeframe.
Additionally, model guidance is indicating a potential warming trend, bringing temperatures back up closer to normal, or even slightly above normal for the end of the week into next weekend.