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The National Weather Service office in Medford has issued a heat advisory for Sunday and Monday for a large swath of southwestern Oregon and parts of Northern California.

The weather service said in a post Saturday that hot temperatures could cause heat illnesses, plus area rivers remain cold and dangerous due to ongoing mountain snowmelt. The advisory is in effect from 11 a.m. Sunday to 8 p.m. Monday.

Forecasters said temperatures could reach 103 degrees in some locations. Medford’s forecast high for Sunday is 101, with 100 on Monday. Ashland’s highs are predicted to hit 96 on Sunday and 95 on Monday.

People are urged to stay hydrated, seek air conditioning and refrain from exertion outdoors.

Areas covered by the advisory include the Rogue, Applegate and Illinois valleys in Jackson and Josephine counties, and the Shasta, Scott, Klamath and Salmon valleys in Siskiyou County, California. Cities include Medford, Ashland, Grants Pass, Provolt, Cave Junction, and Yreka, Happy Camp, Fort Jones, Somes Bar and Sawyers Bar in Northern California.