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May 5, 2020 – Dry and warm through this week. High temperatures warm to around 15 to 20 degrees above average by Thursday into the weekend with some locally gusty north to east wind late in the week. Cooler weather returns next week.
Upper level ridging builds a little today, with high temperatures this afternoon similar to slightly warmer than yesterday. Most locations in the Valley and Delta will be in the mid 80s. Onshore flow will pick up tonight, with a Delta breeze increasing and keeping Wednesday morning temperatures cool. A passing shortwave is forecast to push through the Pacific Northwest early Wednesday. Some morning stratus clouds are possible for the Delta and portions of the Valley south of Marysville. Sunny skies are expected Wednesday afternoon. As this trough exits eastward, tightening surface pressure gradients will increase northerly winds over the Valley with expected gusts now looking stronger, up to 20 to 30 mph late afternoon into early evening.
Thursday will mark a shift toward much hotter weather as an upper level low over the Gulf of Alaska pumps up eastern Pacific ridging along the West Coast. The ridge builds northward into southeastern Alaska. Morning lows will be mild, dropping only into the upper 50s to lower 60s in the Valley and foothills. Breezy northerly winds continue through the morning, gradually decreasing in the afternoon. High temperatures jump up by around 6 to 9 degrees from Wednesday. Widespread low 90s are likely but will be well below triple digit daily records set back in 1987. Overnight lows will tend to be warm given the continued north to easterly breezes downslope winds, with low 60s into the foothills.
Temperatures peak on Friday as the ridge builds further and the axis moves over the forecast area. Widespread mid to upper 90s are expected. The current forecast has high temperatures exceeding records for Redding and Red Bluff at 97 degrees (Records 96 in 2001). NBM probabilities suggest a much lower chance (less than 1 out of 3) for records being broken over the southern Sacramento and northern San Joaquin valleys, though forecast highs are 15 degrees above normal. Heat risk is in the moderate category for those sensitive to heat. Outdoor workers should prepare for hot conditions and should have access to sufficient shade and water.
Extended Discussion (Saturday through Tuesday)
Upper ridge axis shifts into the Great Basin Saturday while high temperatures remain upwards of 20 degrees above normal. Some locations in the Northern Sacramento Valley could approach or surpass record levels.
Pacific frontal system progresses towards the West Coast Sunday bringing cooling and increasing onshore flow. Models push frontal system precip inland Monday afternoon, then move baroclinic zone across the CWA Monday night into Tuesday morning. Unsettled weather continues Tuesday as upper low remains off the West Coast and additional waves rotate inland. High temperatures return to near normal by day 7.