June 21, 2020 – A prolonged period of dry weather and excessive heat is forecast this week. Moderate to very high heat risk is expected in the Valley and foothills, with little overnight relief.
Discussion
Temperatures early this morning around similar to this time yesterday except for in the northern Sacramento Valley where temperatures are around 5 to 10 degrees warmer. These warmer temperatures are likely due to northerly winds overnight whereas the wind was mostly calm in the area last night. Another factor is the Delta Breeze is not as strong as yesterday. Zonal flow currently over NorCal will transition into upper level ridging today as a trough begins to deepen over the Midwest. Temperatures today will be a few degrees warmer than yesterday, especially in Delta influenced areas which could see temperatures up to 10 degrees warmer.
An extended period of excessive heat truly begins Monday as the upper level ridge continues to build over California. Temperatures across interior NorCal will be 10 to 15 degrees above normal through the week. Widespread Valley temperatures in the triple digits are expected, hottest in the northern Sacramento Valley with forecast highs 105 to 110 degrees. The mountains will see temperatures in the upper 70s to 90s.
Not much relief from the heat is expected overnight either for much of the area. Low temperatures in the Valley and foothills will only fall to the 60s and 70s, with mid to upper 70s expected in the northern Sacramento Valley and thermal belts of the foothills. Marine influenced areas may experience slightly cooler overnight lows, but will remain hot during the day. Both the above normal daytime temperatures and warm overnight temperatures will cause moderate to high heat risk across the Valley and foothills with much of the mountains in moderate heat risk, as well.
The Excessive Heat Watch remains in effect for the Valley, surrounding foothills, and portions of the Delta from Monday through Friday, though ensembles indicate a chance of excessive heat remaining into the weekend. The general population should take precautions to prevent heat related illnesses given the potential for moderate to high heat risk. Also, area waterways are running fast and cold. Cold water safety should be practiced when on and near the water.
Aside from the heat, ensembles show the upper ridge axis shifting over Nevada Tuesday with a weak upper trough moving through the PacNW Tuesday into Wednesday. GEFS and NAM indicate the potential of enough moisture and instability for a slight chance of isolated mountain thunderstorms Tuesday afternoon over the high elevations of the Central Sierra.
Extended Discussion (Thursday through Sunday)
Little change to the extended forecast as the hot weather pattern persists later this week. There is still some uncertainty about the weekend (Fri-Sun) as models and ensembles are still in some disagreement, though the EC is trending more toward the GFS. GFS runs continue to point toward a weak trough over the region between two stronger highs, one over the Desert Southwest and the other to our northwest over the eastern Pacific which would support at least some weak Delta influence inland and possibly some diurnal deep convection over the Sierra.
Potential for a more significant cool down late next weekend and early the following week as a little more troughing develops. NBM viewer for Redding continues to shows increasing spread of high temperatures next weekend with the range on Saturday 99-111, Sunday 91-102, and Monday 88-97.