Near to slightly below normal temperatures are expected to persist through the week. Even cooler temperatures, breezy/gusty winds with showers and mountain thunderstorms possible late this week.
Late-week weather system will bring the potential for showers and thunderstorms north of I-80. Any thunderstorm will bring the potential for heavy rainfall, gusty winds, lightning and small hail. Highest chances on Friday.
Discussion
The area remains between an upper level trough centered off the Pacific Northwest and upper level ridging centered over the Rocky Mountains, with dry weather through mid week. The trough drifts a little northward today, with the upper level ridging building in a little further over the area. While relatively cool onshore flow continues, high temperatures will edge slightly higher, about 2-4 degrees warmer than on Sunday. These temperatures will still be below normal.
Temperatures will continue to trend slightly higher on Tuesday, which will be the warmest day of the week. Highs will be generally a little below normal to near normal, ranging from the upper 80s to lower 90s in the Valley and Delta, with higher elevations in the 70s to the 80s.
A Delta breeze in the late afternoon and evening will continue to bring some breezy winds and cooling to influenced areas. Humidity levels over the area will be a little lower today, with afternoon levels dropping into the upper teens for the foothills and northern Sacramento Valley, then trending higher into mid week.
Extended Discussion (Friday through Monday)
Ensembles and clusters show an anomalously cold low pushing through northern California late Thursday and Friday, potentially into Saturday. Current National Blend of Model probability of precipitation is 20 to 50 percent over the northern mountains and northern Sacramento Valley Thursday night. This extends southward into the central Sacramento Valley and Sierra on Friday, with the potential for thunderstorms over the southern Cascades, Burney Basin, far northern Sierra and the northern Sacramento Valley as cold air aloft moves through and destabilizes the atmosphere.
The probability for thunderstorms is currently projected 20-30% over those areas, with a lesser potential further southward. This is worth keeping an eye on. Some showers may linger into Saturday, though uncertainty remains on the speed and exact track of the low. Can’t rule out that some high peaks in the Sierra and Lassen National Park could potentially see a mix of light snow and rain showers late Friday night into early Saturday, though no accumulation is expected. Some northerly winds may develop on Sunday as the low exits.
Temperatures are expected to cool down quite a bit late in the week. Valley highs are forecast to remain in the 80s on Thursday, and only reach the mid 70s to around 80 on Friday. Temperatures are forecast to warm back to near normal (low to mid 90s in the Valley) Sunday as the low fully moves out of the area and ridging begins to replace it. Above normal temperatures are expected to return Monday as ridging continues to build.