Warming and drying trend with triple digit heat today through Friday for the Central Valley. The hottest day will be Friday, with little overnight relief from the heat. There’s a chance for showers and thunderstorms for the northern mountains and northern Sacramento Valley later this weekend. Excessive Heat Warning from 11 AM to 10 PM PDT Friday.
Upper level ridge will begin to build into the four corners region today and will amplify into the end of the week. This will bring hot temperatures to NorCal as we will be on the western edge of the ridge.
The hottest day is still looking to be Friday but highs will push to near to just above the 100 degree mark today in the lower elevations with widespread 100s for Friday. This will bring widespread moderate to high heat risk for the Valley and foothills and heat products have been issued for these areas.
We are seeing a Delta Breeze tonight although it is weaker then yesterday at this time. As the ridge builds in the Delta Breeze will get cut off and we can expect hot temperatures even in the Delta region the next few days.
Overnight lows are also going to be mild starting tomorrow night and will continue into Saturday morning. Lows in the Valley will be in the upper 60s to lower 70s with low to mid 70s throughout the thermal belts. Short waves spinning into the PacNW off of a low in the Gulf of Alaska will bring some high clouds to the region for today with less cloud cover expected on Friday.
We will also see some increased northerly flow on Friday with lowering humidity. This could bring some increased fire weather concerns to the Valley, with the heat providing additional drying of fuels.
This upper level low will start to push to the east on Saturday and the back side of the trough will dig over the Pacific reaching CA on Sunday. This has slowed some which has resulted in slightly warmer highs on Saturday and the heat products may need to be extended an additional day. We will see the return of onshore winds later in the day along with the Delta Breeze developing. This will bring some relief to the heat but will not come until the evening.
A cold front associated with the digging trough will push into the northern part of our area late in the day. This will bring light shower chances to the Coastal Range with shower chances continuing to overspread northern areas overnight as the front pushes south and east. Shower activity will be pretty limited in the northern Sacramento Valley initially due to dry air below 700 mb but profiles should become a bit more saturated overnight with some light showers possible. Moisture with this trough and front are quite impressive for this time of year with PWATs pushing above 1.0″ but the drier low levels will limit some of the QPF. The majority of the rain accumulation will be over the northern mountains with some light accumulation possible in the northern Sacramento Valley.
The trough axis will be pushing through the northern part of the area during the day Sunday. We do see instability build over the northern mountains and northern Sacramento Valley by the afternoon. This will bring the chance for some afternoon thunderstorms. The latest GFS is quite high with CAPE values but when you dive into some of the ensembles things becoming a bit more mixed. This makes sense as front timing and cloud cover will be a big factor for how much instability we see in the afternoon along with what the environment will look like. Right now it looks like we’ll see some thunderstorms with brief heavy rain along with some hail.
Temperatures will be much cooler on Sunday running as much as 20 degrees cooler than Friday. Shower and thunderstorm chances diminish Sunday evening as the trough pushes east.
Extended Discussion (Monday through Thursday)
High pressure builds inland through the first half of the new week with warming temperatures and some gusty north to east wind. This could lead to a potential for increased fire weather conditions.
Heating peaks Wednesday as associated upper ridge shifts through interior NorCal. Increasing onshore flow Thursday with some cooling as troughing in the Pacific approaches.