Hot again today with a cool down Sunday. Good chance of showers and thunderstorms Sunday over northern areas, followed by breezy north winds early next week. Excessive Heat Warning from 11 AM this morning to 11 PM Saturday for Carquinez Strait and Delta-Central Sacramento Valley-Mountains Southwestern Shasta County to Western Colusa County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Southern Sacramento Valley. Heat Advisory from 11 AM this morning to 11 PM Saturday for Motherlode.
Discussion
Upper level ridge will continue to amplify over the four corners region today and this will bring another hot day to NorCal. Yesterday many Valley locations saw highs right around 100, today we will add another 2 to 7 degrees to that with widespread 100s for the Valley, 90s in the foothills, and 70s and 80s in the mountains.
Moderate to high heat risk is expected today and you should take heat precautions if you have outdoor activities planned. We will continue to see passing high clouds throughout the day as short waves pass to our north over the PacNW.
Today will be the peak of the heat event as the ridge will start to push east Saturday as a trough in the Gulf of Alaska starts to push east. This will bring some cooling on Saturday but it is still looking to be another hot day with highs near 100 in the lower elevations.
Overnight lows tonight are mild generally running 4 to 12 degrees warmer than this time last night. Lows Friday night will be mild once again helping to add to the elevated heat risk. Northerly flow develops today and this will help to bring a period of elevated fire weather conditions during the afternoon for the Valley.
Northerly flow will shift to onshore Saturday afternoon and we will see the return of the Delta Breeze, this will bring some relief to the heat for Delta regions but not until the evening.
The back side of the trough in the Gulf of Alaska will dig over the Pacific and reach CA on Sunday. This will bring much cooler conditions with highs as much as 20 degrees cooler than today.
It will also bring shower and thunderstorm chances to the northern mountains and the northern Sacramento Valley. The cold front will approach the region late in the day Saturday and a few light showers will be possible over the Shasta County mountains by the end of the day. Shower chances will continue to overspread northern areas overnight as the front pushes south and east.
In the lower elevations it will take some time for profiles to saturate and this will limit rainfall there but some light showers will be possible by daybreak. The trough axis will be pushing through the northern part of the area during the day Sunday and this will keep showers in the forecast.
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. Soundings show quite a bit of shear and some more organized thunderstorms aren’t out of the question.
Breezy winds, small hail, and brief heavy downpours are going to be the main threat with any thunderstorms. Timing of the front and cloud cover will be a factor for how much instability we see in the afternoon along with what the environment will look like. 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. Upper level trough will push east by Sunday night with any shower or thunderstorm activity wrapping up shortly after sunset.
Dry conditions return Monday as ridging begins to build in. With the trough off to our east we will see some northerly winds develop by the afternoon. They don’t look overly strong but gusts 20-30 mph look possible over mainly the Sacramento Valley. This should be enough to bring a period of elevated fire weather conditions.
Extended Discussion (Tuesday through Friday)
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 concerns, dependent upon amount of wetting precip Sunday. Heating peaks Wednesday as associated upper ridge shifts through interior NorCal. Increasing onshore flow Thursday into Friday with some cooling as unseasonably deep troughing in the Pacific approaches.