High temperatures in Fresno and other Valley locations will remain in the upper 90s throughout the week, the National Weather Service says.

Meanwhile, overnight lows are expected in the mid-60s.

NWS Hanford says that an upper-level ridge over Canada is driving temperatures 15 degrees hotter than what is customary for the Valley this time of year.

High temperatures in the San Joaquin Valley will be near 100 degrees Sunday and Monday. Temperatures will remain well above seasonal values the remainder of next week. Water levels will rise on area rivers, streams, and creeks due to an increased rate of snowmelt. #CAwx pic.twitter.com/XX65MArslr

— NWS Hanford (@NWSHanford) May 12, 2023

Sierra Warmup Triggers Flooding Concerns

The hotter-than-normal temps also are accelerating a huge Sierra snowmelt and raising flooding alarms. NWS issued a Flood Watch for Yosemite National Park on Sunday that will be in place through at least Friday.

Three Yosemite campgrounds closed Monday because they are expected to flood: Lower and North Pines, and the Housekeeping Camp.

Releases from #Friant Dam are ramping up this morning for storage management in Millerton Reservoir… up to 9,000 cfs by 3pm.

Flows in the San Joaquin River are fast and cold; heed caution. pic.twitter.com/sfCHa7sDTr

— Bureau of Reclamation – California-Great Basin (@ReclamationCGB) May 15, 2023

“The combination of extended hot weather and abundant snow means the Merced River may remain above flood stage for some time,” the park said in a statement. An update on the closures is expected Monday evening.

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Late last month, eastern sections of Yosemite Valley closed for a few days over expected flooding that didn’t materialize.

The Sierra Nevada also may see a few daily thunderstorms in the afternoon beginning Friday, according to the NWS forecast.

(Associated Press contributed to this article.)