May 8, 2020 – Dry weather with above average temperatures continues into the weekend. Cooler weather along with widespread precipitation returns early next week.
Clear skies cover the region early this morning. The dry north to northeast flow of the past couple of days has subsided across most of the area, though lingering east gradient (RNO-SAC gradient is still around 6 mbs) is still resulting in local gusts of 15-30 mph across the foothills and west slopes of the northern Sierra Nevada. Dew points and humidities are considerably higher across much of the area this morning compared to 24 hours ago, and the humidity at Redding and Vacaville are both up 50% compared to this time yesterday.
Unseasonably warm temperatures will continue through Saturday as a ridge of high pressure amplifies/strengthens along the West Coast. Readings will peak today with highs running about 10-20 degrees above climatology. Portions of the northern Sacramento Valley may see their first triple digit heat of the season.
Long outdoor exposures could increase chances for heat-related illnesses among sensitive groups given the potential for moderate heat risk. Outdoor workers should prepare for hot conditions and stay hydrated.
Ensembles in good agreement indicating the upper ridge axis will begin to shift east into the Great Basin by Saturday afternoon. This should allow a return of the Delta Breeze resulting in slight cooling across the southern half of the Sacramento Valley while the remainder of the region stays hot for another day.
More substantial cooling forecast to begin on Sunday as stronger southwest flow spreads across the area ahead of the approaching trough over the eastern Pacific. Valley highs will be in the 80s, generally 5-10 degrees cooler than Saturday.
The eastern Pacific trough is forecast to pivot toward the West Coast on Monday sending a late-season frontal system into NorCal. At this time, light precipitation is forecast to spread into areas north of I-80 during the afternoon before spreading south Monday night. Some accumulating snowfall will be possible across the higher northern Sierra passes by sunrise on Tuesday.
Extended Discussion (Tuesday through Friday)
The forecast remains on track for the pattern change as low pressure digs towards the West Coast. Cooler and weather wetter is in store for interior northern California Tuesday and Wednesday as the low digs towards CA and then lifts back towards the PacNW. Showery precipitation is expected, with thunderstorms being possible as well. Tend to agree with the dayshifts thinking that the best chances for thunder activity will be from BAB/OVE to CIC/RBL/RDD. Model guidance depicts a reasonable swatch of instability over these areas, which combined with with the axis of instability due to the mid level CAA, steepening lapse rates, and topographic influences. The NBM point viewer also falls in line with the chance for thunderstorms so confidence has increased in this portion of the forecast.
On top of the thunder forecast, there is still respectful QPF amounts forecast for this system. Looking at the NBM 1D Viewer for our respective sites during this multi-day wet pattern, Mon- Wed of next week. For example, 1 to 2+ inches is forecast at some points in the Shasta Mtns (down a little from yesterday), 1 to 2 inches in the Sierra N of I-80, 1.5 inches Nrn Sac Vly, tapering to around a quarter inch towards the I-80 corridor, and then next to nothing in the Nrn SJV. There`s also a chance for some high elevation snow, with possibly one-half inch to 3 inches over most of the Sierra, with Lassen NP possibly getting up to a foot.
After the low lifts back to the north on Wednesday, zonal flow sets up over the region. Some embedded shortwaves could bring mountain showers to the region on Thursday and Friday, with dry weather expected elsewhere. Temperatures will rise a touch, putting them closer to their seasonal averages.