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June 22, 2017 – Extended heat wave with widespread highs over 100 degrees and warm nights through the end of the week. Isolated afternoon thunderstorms possible over the mountains. Not as hot next week. Red Flag Warning until 11 PM  this evening for the lower elevations, Heat Advisory until 11 PM  Saturday.

Discussion

Clear skies across interior NorCal early this morning. Strengthening high pressure and northerly surface flow has weakened the Delta Breeze and is resulting in warmer temperatures through much of the Sacramento Valley. Current temperatures range from the upper 40s and 50s in the mountain valleys with 70s to lower 80s elsewhere, except current temperatures remain around 90 across the northern Sacramento Valley.

Breezy northerly flow increases across the area today shutting off the flow from the Delta by mid to late morning. The strongest winds will be over the north and west side of the valley. Hotter temperatures will be the result with record heat again and highs most likely the hottest of the year for a number of locations. The high heat, low relative humidity and increased winds will continue high fire danger for a number of areas.

Thunderstorm chances likely to be limited to the Sierra south of Lake Tahoe over the next few days.

Hot weather continues across the northern half of the Sacramento Valley and in the mountains late this week while cooler weather gradually returns to the Delta and southern Sacramento Valley as the Delta Breeze returns.

Extended discussion (Monday through Thursday)

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Cooling trend continues Monday as the upper level ridge axis shifts eastward into the Great Basin and an upper trough moves into the eastern Pacific. Even with significant cooling, daytime max temperatures are still expected to come in a few to several degrees above normal.

Stable southwest flow aloft will keep skies fair over the CWA while a moderate onshore surface gradient brings an increasing delta breeze. Weak upper trough moves over the west coast on Tuesday bringing more cooling.

Although models differ a bit, all models keep NorCal under stable dry conditions. Extended models show another weak upper low dropping into the eastern Pacific around the middle of next week. Under this scenario, there should be some amplification of high pressure over the western U.S. and a weakening delta flow. This would bring a slight warming trend towards the end of the extended period.