May 6, 2021 – Cooling trend today and Friday. Increased north winds and warmer into the weekend and early next week with elevated fire weather concerns.
Discussion
Satellite imagery reveals clear across interior NorCal this morning. The Delta Breeze has ushered in a cooler airmass near the Delta influenced areas early this morning with temperatures generally running 5 to 10 degrees cooler than yesterday. Not anticipating marine stratus intrusion this morning as the air mass is too dry, but conditions might be favorable Friday morning.
Ensembles in good agreement with a trough continuing to approach the West Coast today. This pattern will bring increased onshore flow, with southwest wind gusts up to 25-30 mph; locally up to 35 mph. Strongest wind gusts are expected through the Delta. This onshore flow will promote cooling across interior NorCal, though highs today will remain above average. Friday highs will be closer to seasonal averages.
Ensembles and cluster analysis show troughing developing over the northern Rockies and the Great Basin over the weekend. Tightening surface pressure gradients will lead to dry, gusty offshore winds Friday afternoon into early next week. Gusts in the 25 to 35 mph range, locally up to 40 mph, are forecast in the Sacramento Valley and surrounding foothills. Strongest wind forecast along the west side of the Sacramento Valley and adjacent Coastal Range foothills, especially on Saturday.
Daytime relative humidity values will drop into the single digits to mid teens in the Valley and foothills, while overnight recoveries will remain poor. These very dry conditions combined with gusty winds could bring elevated fire weather concerns this weekend into early next week. Avoid using outdoor equipment that could create a spark and obey local burn bans.
Warming trend is anticipated this weekend as an upper level ridge builds back over NorCal. High temperatures will warm back into the mid to upper 80s across the Valley Saturday and Sunday. Dry weather continues through the period.
Extended discussion (Monday through Thursday)
Dry and unseasonably warm pattern next week. Elevated fire weather concerns likely to persist into Monday as NorCal will remain on the backside of the western US trough with breezy north winds and low RH continuing.
Thereafter, lighter winds expected as broad upper ridging returns.
Highs expected to be around 10-15 degrees above average for most areas. Some weakening of the ridge may allow for an increased Delta Breeze after mid-week.