Dry with near normal temperatures through mid-week. Mountain and foothill thunderstorms are possible today and Friday, otherwise dry. High pressure strengthens bringing a return of very hot weather over the weekend into next week, particularly for the northern and central Sacramento Valley.
Upper low near the Bay Area will slowly move east and weaken through Friday, as strong high pressure system develops over the Pacific Northwest. Increasing elevated instability ahead of the upper low will lead to mountain and foothill thunderstorms through Friday. Instability this afternoon will spread west from Nevada with widespread MUCape in the 200-600 J/KG range over the mountains and even spreading into the foothills.
Highest coverage and strongest storms look to be over eastern Shasta county and Plumas county, especially the area between Burney and Quincy. Main concern with the storms will be abundant lightning, which may start a few fires, especially over foothill locations where fuels are drier and little to no rain falls. A red flag warning has been issued for much of Shasta county, stretching southeast through eastern Tehama and Plumas county including Mineral, Chester and Quincy through 9pm tonight. There will also be a threat of hail and gusty winds in the stronger storms. Thunderstorms will also be possible farther south along the Sierra, but the storms are expected to have less coverage and remain at higher elevations.
Onshore flow will continue over the southern Sacramento Valley and Northern San Joaquin Valley with the upper low helping keep temperatures near normal, however much warmer conditions progressing northward. High temperatures will generally be in the upper 80’s and low 90’s today, although warming into the upper 90’s over the Northern Sacramento Valley, then low to mid 90’s, except 100 to 105 over the Northern Sacramento Valley Friday. Upper high will become firmly in place Saturday and Sunday with Valley high temperatures from 100 to 110 degrees. Excessive heat and high heat risk will be likely over the Central and Northern Sacramento Valley during the weekend starting Saturday. An excessive heat watch is in effect from Saturday through Wednesday for the Central and Northern Sacramento Valley. The heat will be moderated a bit over the Delta, Southern Sacramento and Northern Sacramento Valley due to weak onshore flow keeping temps in the upper 90’s to near 100 and these areas are forecast to remain below high risk levels at this time and are not included in the watch.
Extended discussion (Monday through Thursday)
Ensembles and cluster analyses continue to advertise a strong upper level ridge over the Pacific Northwest through the weekend and well into next week. The best potential for heat is over the northern and central Sacramento Valley, away from any Delta- influenced areas. Heat risk will fall into the high to very high category for the northern and central Sacramento Valley through at least next week Wednesday evening, so the Excessive Heat Watch remains in effect. High temperatures there are expected to remain around 104-108. Some records could be set. Heat risk near the Delta, southern Sacramento Valley, and northern San Joaquin Valley will remain in the moderate to locally high category into next week where temperatures are not forecast to be at hot.
The weather is expected to remain dry over much of the area through the extended. The exception is over the northern Sierra south of Highway 50, where some subtropical moisture advected northward could bring afternoon showers and thunderstorms.