Hot and dry weather returns today through Friday with Moderate to Major HeatRisk throughout the Valley, Delta, and Foothills. Next weather system arrives Friday into the weekend with chances of mountain showers and thunderstorms. Gradual cooling trend begins on Saturday through early next week.
Discussion
Morning GOES-18 Satellite reveals a closed low situated over central Oregon and ejecting eastward. Over interior NorCal, dry air and clear skies persist, and temperatures are falling into the upper 60s across the region. As the trough continues its eastward trek, a brief period of breezy to locally gusty northerly winds through the Valley is expected this morning into the afternoon hours. Current National Blend of Models (NBM) probabilities of gusts 20 mph or greater are around 30-50% along the I-5 corridor and areas westward during the aforementioned timeframe. The northerly winds and the resultant drying component will continue to bring elevated fire weather conditions across the Valley. As the trough departs, upper level ridging will develop and raise our heights and subsidence aloft, which will keep things dry today through Thursday.
As our heights rise due to the developing upper level ridging, high temperatures will begin to climb today through Friday. Rapid warming is anticipated which will bring Moderate to Major HeatRisk across the Valley, Delta, and Foothills. NBM probabilities of reaching 100 degrees or higher today are generally 20-40%, before rising to 60-100% tomorrow and Thursday, and 60-90% on Friday with the highest probabilities in the northern Sacramento Valley and thermal belts of the Foothills. Overnight low temperatures are expected to be in the mid 60s to mid 70s, and combined with the rapid warming and high temperatures in excess of 100 degrees, a Heat Advisory has been issued from 11 am PDT Wednesday through 8 PM PDT Friday for the Valley, Delta, and Foothills.
Friday, the upper level pattern begins to break down as the ridge axis moves eastward away from the Great Basin region. Off-shore troughing will develop off the coast western US coast will help “cool” temperatures slightly, but there will still be afternoon high temperatures in the triple digits for NorCal. Additionally, moisture will enter the northern Sierra/southern Cascades and the NBM continues to show slight (10-15%) chance of thunderstorms over the Sierra Crest, along with rain showers. Humidity values across the Valley will continue to be in the mid to upper teens and low twenties, leading to elevated fire weather conditions on Friday.
Extended Discussion (Saturday through Tuesday)
As we move into the weekend, the off-shore troughing will continue to build in and begin our gradual cooling trend. While the northern Sacramento Valley may still break 100 degrees Saturday, areas south of Red Bluff will be in the low to mid 90s for highs. Moisture will still advect into the into the mountains, so afternoon/evening rain showers will exist, mainly south of I-80 on Saturday. There is some disagreement amongst clusters regarding the influence of the off-shore troughing early next week, however clusters do relatively agree that temperatures will fall into the mid to upper 80s for the Delta and Valley. Onshore flow will be enhanced due to the off-shore troughing , so locally breezy to gusty conditions may develop, along with a gradual rise in humidity values.