An unseasonably strong low pressure system will bring cooler temperatures, breezy onshore winds and shower and thunderstorm chances into the weekend. Warmer and drier next week with temperatures returning to near normal.
Short range ensembles indicate that the upper low currently over western Oregon will drop south over northwest California today and then eject east over the Great Basin Region Monday morning. Onshore flow will strengthen today and continue through the weekend with south to southwest wind gusts from 25 to 35 mph at times strongest near the Delta. This will drive the smoke and haze that moved into the region over the past few days north and east of the area.
A significant cool down will also beginning today with Valley high temperatures from the low 70’s to low 80’s today and Saturday, which is 15 to 20 degrees below normal for this time of year.
Significant cooling aloft spreading overhead with the upper low upper will increase instability and the chances for showers and thunderstorms today and Saturday with the best chances over the mountains and foothills. Vorticity center currently showing up nicely just northwest of Eureka will drop southeast towards the Coast Range and will act as a focus for thunderstorm activity today.
HRRR hourly rainfall amounts have consistently shown several pockets of greater than 1.00 inch an hour rainfall amounts over the August complex and McFarland fire burn scars from 11 am to 7 pm today.
The rainfall rates may be further enhanced by east and southeast low level jet moving up the Valley with strong onshore flow. A Flash Flood Watch for debris flow is in effect for these burn scars from 11 am to 7 pm today.
Abundant small hail will also be likely with storms given cold core under upper low and steep lapse rates. Main impacts elsewhere will be small hail, frequent lightning and brief heavy rain.
Lingering moisture and instability will shift south over the Sierra Saturday and Sunday with isolated thunderstorms possible. Diminishing instability and shear for storm organization will decrease the coverage and rainfall amounts of storms.
Valley temperatures will warm a bit back into the low 80’s Sunday. Dry weather and warmer temperatures will return Monday in the Northwest flow behind the trough, as it moves into the Great Basin Region. Temperatures will warm into the mid and upper 80’s.
Extended Discussion (Tuesday through Friday)
Ensembles and clusters show a flat ridge pattern over NorCal Tuesday shifting to more amplified ridging with height rises through the week. This will leave the area in a pattern of southwest flow due to a trough in the eastern Pacific and an upper high that will develop over the desert Southwest. Weather during this pattern will be mild and dry, with Valley high temperatures gradually rising to near normal levels in the low 90’s.