Above normal temperatures for the weekend, change to a wetter pattern possible by mid-week

May 10, 2019 – Chances of afternoon and evening showers and thunderstorms continue through the weekend, mainly south of I-80. A pattern change is currently projected for the middle of next week bringing cooler, potentially wetter weather across Northern California.

Discussion

Temperatures at 3 am were generally 2 to 4 degrees cooler than the same time yesterday. Marine stratus is remaining west of Cordelia this morning, with satellite showing some scattered low clouds over the northern San Joaquin Valley and over the Sierra. The lack of low marine clouds over the Sacramento metro area will allow temperatures to warm more quickly today, with highs in the mid to upper 80s rising about 10 degrees higher than seen on Wednesday. The northern Sacramento Valley is again the hot spot and is expected to reach the lower 90s this afternoon.

The upper level low pressure system which brought mountain thunderstorms yesterday afternoon and evening has shifted into southern California. Afternoon thunderstorm activity today should also shift southward and be limited to the mountains south of Highway 50, with just a few isolated storms expected. High Resolution Ensemble Forecast (HREF) projects the best time for thunderstorms around 3 to 5 pm, over Tuolumne County. Dry conditions are expected for most of the forecast area.

An upper ridge will continue to build to the north over the Pacific Northwest. The surface pressure gradient will bring increasing northerly winds this afternoon with gusts to 20 to 30 mph over the northern and central Sacramento Valley, weaker gusts of 15 to 25 mph from I80 south.

Easterly flow will maintain daily shower and thunderstorms chances for the Sierra this weekend, with dry weather elsewhere. Saturday chances of convection should remain south of Highway 50 but could spread further north along the crest on Sunday.

The Delta breeze should increase again over the weekend. Marine stratus is expected to move inland again over the Sacramento metro area on Saturday, but current HREF guidance suggests it will be less dense and not as long lasting as seen recently. The region can expect warm temperatures to continue through the weekend with Valley highs remaining in the low/mid 80s and low 90s for the northern Sacramento Valley.

Extended Discussion (Tuesday through Friday)

Another morning where ensemble and deterministic models show an up coming pattern change next week. The ridge/trough couplet will advance eastwards early next week while a broad trough develops over the Pacific. What this means for us is that we should see a dry and warm start to the work week before shifting to cooler, cloudier, and potentially wet weather. Have continued to use NBM guidance for PoP and temp for this period as deterministic models have been wavering from time to time.