It's starting to feel like summer in Portland, and the 80-plus degree days beckon a dip in the river. But local agencies warn that while outside temperatures are creeping towards sweltering, river temperatures are still dangerously low.

The National Weather Service of Portland issued a warning Tuesday that unprepared swimmers could experience cold water shock by jumping into water that is still 40-50 degrees Fahrenheit.

Last Saturday, The Oregonian reported, an early-season float down the Washougal River was likely fatal for a Portland man, who is still missing after being swept off his inner tube by quick-moving water.

NWS cautions people not to be "fooled by the warm air temperatures this week. "

"Most river temperatures remain in the 40s and 50s," the agency tweeted, "which can lead to cold water shock and eventually death for the unprepared."

In an infographic, NWS explains that less than a half a cup of water in your lungs can result in drowning and that it takes just an hour to two hours to lose consciousness in extremely cold water.

NWS advises that people wear life vests, keep a close eye on children, avoid swimming in areas where water is flowing quickly and refrain from drinking alcohol before hitting the river—as booze can enhance hypothermia.