June 17, 2019 – A heating trend is expected early this week, peaking on Tuesday, followed by less hot weather. Isolated mountain thunderstorms are possible this afternoon and evening, mainly over the higher elevations of the Sierra Nevada.
The upper level trough which has brought afternoon mountain thunderstorms in recent days shifts south and east of the area later today. This should limit the bulk of the thunderstorm activity this afternoon and evening to the northern Sierra south of I80, though a slight potential continues over the Lassen National Park. Easterly flow may move some showers and isolated thunderstorms towards the upper elevations of the Motherlode and locally down to near Sonora late in the afternoon and early evening. Eastern Pacific ridging will then extend over the area, bringing dry weather and increasing temperatures across the area on Tuesday.
High temperatures this afternoon are forecast to be 5 to 8 degrees warmer than yesterday. The Delta breeze continues to blow but is expected to be a little weaker today. Breezy northerly winds over the northern Sacramento Valley will develop with the upper level ridge building to the north, with gusts up to 20 to 25 mph this afternoon on the west side. This will lower minimum humidity levels into the teens in that area. The northerly winds continue on Tuesday with additional drying, which will bring some increased local fire weather concerns in the afternoon and early evening.
Temperatures are expected to peak on Tuesday, with a trough then approaching the Pacific Northwest. This will flatten the ridge on Wednesday and then extend into NorCal on Thursday. Valley temperatures on Tuesday will reach the upper 90s over the northern San Joaquin Valley and southern Sacramento Valley with a weakened Delta breeze. Higher temperatures are forecast over the northern Sacramento Valley with northerly flow keeping morning temperatures in the 70s and afternoon highs peaking around 105. This could bring some heat impacts, especially to sensitive groups. The record for the 18th (Tuesday) at the Redding Airport is 110, set in 2017. This was followed by daily records of 113 on the 19th, 111 on the 20th, 110 on the 21st and 109 on the 22nd, all set in 2017. So this will be a minor heat event in comparison.
The Delta breeze strengthens again Tuesday into early Wednesday, bringing some rapid relief to influenced area. Some heat persists longer over the northern Sacramento Valley, with the Redding high on Wednesday projected at 103. More widespread cooling arrives Thursday, with Valley highs in the more seasonable upper 80s to upper 90s. This means the heat event will be a relatively short one.
Extended Discussion (Friday through Monday)
Current ensemble guidance is in better agreement than yesterday on the pattern for the extended period. Both the GEFS and European ensemble show a low pressure system digging into the intermountain west Friday and Saturday. This will allow temperatures to remain near normal through the extended period, with Valley highs in the 90s and mountain highs in the 60s to 80s.
However, this pattern will bring gusty north winds to the Sacramento valley Friday. Minimum relative humidities will remain in the teens and 20s for the Valley. Although nighttime RH recoveries remain decent, the gusty winds along with dry daytime RH could lead to increased fire weather concerns on Friday.