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August 28, 2020 – Smoke and haze from wildfires will continue to impact air quality and temperatures over interior Northern California for the next several days. Overall dry weather this week but a few late day storms possible over the high Sierra today. Hot temperatures continue through next week.

Discussion

Observed temperatures across much of interior Northern California early this morning are around 3 to 8 degrees warmer than this time yesterday. The Delta Breeze overnight has not been as strong as last night and therefore has not cooled temperatures as much. This is because the upper level shortwave has begun to move to the east and heights are slowly rising over the area. Heights will continue to rise slightly today, allowing daytime temperatures to briefly rise to around 3 to 12 degrees above normal for the day. There is another weak chance of thunderstorms over the high Sierra as the upper trough moves through, though moisture looks to be less than yesterday with PWATs generally around and less than 0.5 inch.

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In terms of air quality and smoke, the diminished Delta Breeze could allow smoke from the active wildfires to funnel back into portions of the Valley that have not seen as much smoke in the past couple days, which the HRRR smoke forecast model agrees with. Wildfire smoke will continue to impact air quality and visibility across interior NorCal for the next several days.

Ensembles continue to indicate upper level heights lowering slight over NorCal this weekend as a weak trough moves through the PacNW. Temperatures will fall a few degrees with high temperatures in the mid to upper 90s across the Valley and 70s to 90s in the mountains and foothills. This will also bring back onshore flow and a moderate Delta Breeze, which will keep overnight temperatures mild and possibly clear some smoke out of the Valley again.

On Monday, ensembles have shifted the timing of the shortwave that is expected to move through the Rockies. This shortwave continues to remain too far to the east to produce any impact to the forecast area except for possibly slightly stronger northerly winds to the Shasta County area, though winds are still expected to be below 10 to 15 kts. A strong Pacific ridge begins to build, and temperatures will begin rising again. Currently forecasting Valley temperatures in the upper 90s to around 102, but depending on the location of the shortwave and strength of the ridge, temperatures could be a few degrees warmer.

Extended Discussion (Tuesday through Friday)

Ensembles support an extended period of dry and anomalously hot weather over NorCal during the middle and latter portion of next week as strong high pressure settles overhead. Temperatures will likely be well above average for early September with highs potentially exceeding the upper bounds of the NBM forecast.