June 20, 2020 – Hot and dry weather is forecast this weekend, followed by a prolonged period of excessive heat next week. Moderate to high heat risk is expected in the Valley and foothills, with little overnight relief.
Discussion
Persistent upper level ridging will result in hot and dry weather for interior Northern California through next week. A weak weather system moving through the PacNW this weekend will cause the ridge to flatten over NorCal. This is allow for continued onshore flow through the weekend which will keep Valley temperatures in the upper 90s to around 101.
As the upper trough moves east into the Northern Plains, the upper ridge will build significantly over California Monday through midweek. This will bring even hotter temperatures to NorCal with daytime temperatures around 10 to 15 degrees above normal. Widespread triple digits are expected in the Valley and lower foothills, hottest in the northern Sacramento Valley with forecast highs around 105 to 110 degrees. This will cause moderate to high heat risk across the Valley and foothills. The mountains will be particularly hot as well with daytime temperatures in the upper 70s to 90s, causing moderate heat risk in much of the mountains, as well.
Little overnight relief is expected for much of the area early next week as low temperatures are only expected to fall to the 60s to low 70s, though some foothill and northern Valley locations may see low temperatures as high as the mid 70s. Marine influenced areas may experience slightly cooler overnight lows, but will remain hot during the day.
An 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 next 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.
Extended Discussion (Wednesday through Saturday)
Hot weather pattern expected to continue during the second half of next week, though the magnitude of the heat is uncertain by late in the week due to persistent differences in the EC and GFS ensembles. 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. The EC on the other hand points toward a strong broad ridge extending from the eastern Pacific down into Arizona which would cap any deep convection and squash the marine layer. NBM viewer for Redding shows this increasing spread of high temperatures later next week with the range on Friday of 103-111, Saturday 98-112, and Sunday 95-106.