SACRAMENTO (CBS13) — The unofficial start of summer is going to feel a whole lot like mid-summer, weather forecasters say.

An Excessive Heat Watch went into effect Memorial Day weekend for the Sacramento Valley – with the region expected to see 100-degree days earlier than usual.

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Valley highs already crossed the century mark on Sunday, with Downtown Sacramento reaching a high of 100. While very hot for this time of year, NWS says the record was 103 set back in 1910.

Monday and Tuesday are expected to be the hottest days, though, with valley highs expected between 100 to 108. Overnight lows are expected to be relatively mild, contributing to the daily high.

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Forecasters say it will be Northern California’s first widespread triple-digit heatwave, with the heat expected to continue through the middle of next week.

The California Department of Public Health is warning that the high heat will bring a greater risk for heat stress or illness in people and animals. It is advised to drink a lot of water, stay indoors between 10 a.m. to 8 p.m. and in air-conditioned buildings, if possible.

Two cooling centers will be open in Sacramento on Monday: The Hagginwood Community Center at 3271 Marysville Blvd. and Hart Senior Center at 915 27th St. Both will be open from 10 a.m. to 6 p.m.

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According to NWS, the valley typically doesn’t reach 100-degree highs until early to mid-June. Sacramento usually sees its first 100-degree day on June 18.