An unseasonably strong low pressure system will bring cooler temperatures, breezy onshore winds and increased shower and thunderstorm chances into the weekend.

Rain rates are expected to remain below debris flow thresholds at this time


Trough developing along the West Coast today will deepen tonight into Friday as energy dropping south from Western Canada initiates development of unseasonably strong upper low progged to be over southwestern Oregon by early Friday morning. Short range ensembles indicate this low will drop south over Northern California Saturday and then eject east over the Great Basin Region Sunday.

Onshore flow will develop today strengthen this afternoon continuing through the weekend with south to southwest 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 day north and east of the area, although some haze may linger into midday.

Wind gusts on Friday

A significant cool down will also occur beginning Friday after one last warm day today with Valley highs in the low and mid 90’s. Expect Valley high temperatures from the low 70’s to low 80’s Friday and Saturday, which is 15 to 20 degrees below normal for this time of year.

Showers and isolated thunderstorms (20-40% probability) are possible Friday-Sunday, mainly over the mountains and the northern Sacramento Valley. Main impacts are lightning, small hail, gusty winds, and brief heavy rain. Remember, if thunder roars, go indoors.

Significant cooling aloft spreading overhead with the upper low upper will increase instability and the chances for showers and thunderstorms Friday and Saturday for areas north of interstate 80 with the best chances over the mountains.

Initial guidance keeps rainfall rates below 0.50 inches per hour well below debris flow flood criteria.

Best shear and potential for storm organization looks to be over the southern Cascades and northern Sierra from Butte and Plumas counties north into Eastern Shasta county, so this would be the area to keep an eye on for heavier rainfall in future model runs.

Main impacts from storms should be small hail, frequent lightning and brief heavy rain. In general the current expectation is that the inland trajectory of the system will limit moisture and precipitation amounts.

Lingering moisture and instability will shift south over the Sierra Sunday with a thunderstorms possible, especially south of Interstate 80. Valley temperatures will warm a bit back into the low to mid 80’s.

Extended Discussion (Monday through Thursday)

Ensembles and clusters indicate a southwest flow pattern developing over NorCal early next week, as the region becomes positioned between upper high developing over the desert Southwest and persistent eastern Pacific trough.

This will bring drier and warmer weather with temperatures warming to near normal levels in the low 90’s by mid-week.