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September 2, 2017 – Very hot weather with record highs likely today, with hot weather continuing through Labor Day weekend. Temperatures are expected to decrease to more seasonable levels by mid week. Increasing risk of showers and thunderstorms next week over the mountains. Excessive Heat Warning until 8 PM Monday.

Discussion

Hot weather peaks today with a strong high pressure ridge over the West. While the calendar shows that meteorological summer is over, this could be the hottest day of the year for some locations, with a forecast high at Redding of 113, 112 at Sacramento, 113 for Fairfield, and 110 for Stockton. An Excessive Heat Warning continues into Monday evening. Somewhat less hot weather is expected Sunday and Labor Day, especially for Delta breeze influenced areas.

Downtown Sacramento has never reached 110 in September, having reached 109 in the month only twice since records began in 1877, on September 1, 1950 and September 2, 1955 (the record for the date today). Only 2 days through the entire period of record have exceeded 112 degrees, 113 on July 14, 1972, and 114 on July 17, 1925.

The daily record for Redding Airport today is 112, set in 1998. For Stockton, it is 108 set in 1950.

One challenge with forecasting high temperatures this weekend is smoke and haze which have spread across from wildfires. Smoke could act to slightly reduce temperatures, if it is locally thick enough. Current visibility (at 4 am) ranged from 5 to 10 miles in the Valley. Another issue with the smoke is air quality, with forecasts posted at Airnow.gov.

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As the ridge shifts eastward, cooler onshore flow will slightly moderate temperatures on Sunday. The Delta breeze could bring more significant cooling for Labor Day for the Delta, the southern Sacramento Valley and northern San Joaquin Valley.

Elevated moisture and instability is forecast make it into the Sierra south of Tahoe later today for a slight chance of thunderstorms, then spread further north and into the Coastal Range early next week.

Extended discussion (Wednesday through Saturday)

Temperatures are expected to be closer to normal levels by the middle and latter half of next week. Monsoonal moisture will be in place over the region and that will bring the potential for thunderstorms over the mountains during the afternoons and into the evenings.

A trough of low pressure begins to develop and move towards California from the eastern Pacific late in the week. Currently, models suggest temperatures dropping to below normal levels by Friday. Chances for afternoon showers and thunderstorms for the mountains continue through the extended period.