Dangerous heat through Saturday, cooling Sunday through early next week

June 24, 2020 – A prolonged period of hot weather continues through this week. Moderate to very high heat risk is expected in the Valley and foothills, with little overnight relief. Slight chance of afternoon and evening thunderstorms over the mountains today. Significant cooling trend Sunday and into early next week, with a slight chance of some mountain showers.

Discussion

Convection which brought large hail to the Sierra Crest exited eastward in the early evening, with clear skies across the area. The Delta Breeze has increased overnight, bringing cooling in the marine/Delta influenced areas. Wind at Suisun was gusting to 35 mph at 3 am. The Fort Ord Profiler shows the marine layer is fairly shallow at around 1200 feet, so not expecting low clouds into the Sacramento area this morning. Areas over the northern Sacramento Valley were actually a little warmer than this time yesterday, with Redding at 80 degrees.

An upper level trough will move across the Pacific Northwest today, brushing Northern California. This will bring a chance of isolated thunderstorms this afternoon to the Sierra Crest south of I-80. HREF probabilities continue to show that most convection will be east of the Crest.

High temperatures this afternoon will be similar to those on Tuesday, with widespread triple digit heat. Minor synoptic cooling from the trough is forecast to bring temperatures a couple degrees less hot over the mountains and the northern Sacramento Valley, though.

NBM has high confidence in max temperatures increasing as eastern Pacific ridging rebuilds Thursday and Friday in the wake of the departing trough. These will be the hottest days of the week, particularly Friday, when Redding is forecast to reach 111. While very hot, the daily record of 113 set in 2006 only has a projected 13% chance of being tied or broken.

For overnight lows, Delta influenced areas will continue to see a some cooling. The northern Sacramento Valley is expected to only drop into the low to mid 70s. Foothill locations within the temperature inversion and strong thermal belts will experience some very warm nights, with some spots remaining in the upper 70s to near 80. Saturday highs will see several degrees of cooling as a trough from British Columbia approaches, but temperatures will still be fairly hot. A Heat Advisory remains in effect through 8 pm Saturday evening.

Extended Discussion (Sunday through Wednesday)

Large upper low forecast over the PacNW Sunday, edges into the Intermountain West Monday. This will result in continued cooling over interior NorCal. Cyclonic upslope flow will lead to a threat of showers over higher terrain. Deterministic models differ significantly by next Tuesday with GFS more progressive with wave, while EC maintains more troughing over the area. Ensemble solutions support the EC, and thus forecast continues below normal high temperatures with a threat of mountain showers day 7.