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June 15, 2021 at 9:40 AM Update: the Excessive Heat Warning now starts at 2PM Wed Jun 16. Highs will range from 100-113° with warm overnight lows.

Warming trend this week with widespread triple digit heat expected across the lower elevations by mid-week continuing into the weekend. Very high heat risk is expected with well above normal temperatures. Excessive Heat Warning now in effect from 2 PAM Wednesday to 9 PM Saturday.

Discussion

A few clouds linger across the northern mountains early this morning, otherwise skies are clear over interior NorCal. Action in the form of showers and thunderstorms remains well to the north over northeast Oregon and eastern Washington in advance of the trough now approaching the PacNW coast. Onshore surface pressure gradient persists, but it’s weaker than 24 hours ago with only a few locales reporting gusts in the teens mph early this morning.

High pressure will begin to rebuild across NorCal today as the upper trough pushes east toward the far northern Rockies. Highs today will be a little warmer than Monday’s and will be close to average for mid-June.

Much warmer weather returns beginning Wednesday as the Four Corners high extends westward cutting off any remaining onshore flow. Highs will be around 10-15 degrees hotter than today as low-level flow turns northerly, and will likely exceed triple digits throughout the Central Valley.

Heat is expected to peak on Thursday and Friday with widespread high to very high heat risk expected. Hottest portions of the Central Valley will see highs around 110 degrees. Overnight lows will also be very mild beginning Wednesday night with readings remaining in the upper 60s to around 80 in the Central Valley and foothill/mountain thermal belts.

Extended discussion (Saturday through Tuesday)

Strong upper ridge responsible for the heat wave looks to weaken over the weekend and shift east early next week as low pressure develops offshore. This will result in a cooling trend through the extended forecast period. 90s are forecast for much of the Central Valley for highs on Monday with triple digit heat lingering in the Northern Sacramento Valley. Valley highs Tuesday expected in the upper 80s to upper 90s.