Triple digit heat continues across the Central Valley through the weekend with gradual cooling next week. Monsoon activity will lead to some afternoon showers and isolated thunderstorms over the northern Sierra today, and again Friday and Saturday, south of Highway 50. Dry weather is expected through the extended.
The upper level low pressure system that has been pulling up monsoonal moisture is shifting eastward and is now centered over the Great Basin in Nevada. Instability over the northern Sierra is increasing, with showers and isolated thunderstorms developing over the Sierra crest. Precipitable water is around 0.7″, so expect any storms to develop to be wet, limiting fire weather concerns to an extent.
Storms have developed as far north as Plumas County, though most convective activity on the west slope of the Sierra is from around I-80 southward. The main threat with the storms is small hail and a brief period of heavy rain. Thunderstorms will diminish quickly in the early evening as solar heating ends, with dry weather overnight.
Some lingering showers and isolated thunderstorms are possible south of Highway 50 on Friday afternoon. A weak shortwave will again bring the potential to the same area Saturday afternoon. Temperatures in the Valley started the day very warm with cloud cover keeping temperatures up.
Downtown Sacramento had only dropped to 75 by daybreak, 15 degrees above normal. This is very warm but fell short of the record high minimum of 78, set in 1909. The clouds cleared quickly, allowing solar heating in the Valley and Delta. Temperatures this afternoon are already in the upper 90s to low 100s, with widespread triple digits here again.
More heat is expected through the weekend with little change in highs. The good news is that onshore winds with a Delta Breeze should bring some relief from the heat during the evenings and overnights in Delta influenced areas.
Extended Discussion (Monday through Thursday)
Upper level ridging will continue to build over the west for the start of the extended period. This will keep warm temperatures in place with highs near 100. The ridge will start to push east mid week as a trough begins to dig into the PacNW. This will bring some cooling with highs moderating closer to average for this time of year. There is some uncertainty with how quick this trough will dig and we could see the cooling delayed into the later part of the week. Onshore flow will continue and a Delta breeze should bring some relief from the heat during the evenings and overnights in the Delta influenced areas.