Dr Terri Schmidt, an expert on hypothermia and mountain survival, said there was less than a 1 percent chance that Anthony Vietti and Katie Nolan had survived after going missing on Friday.
The body of a third member of their party, Luke Gullberg, 26, of Des Moines, Wash., was found on the mountain Saturday.
Schmidt spoke at a news conference called by rescue officials.
She talked with relatives of the missing climbers earlier in the day about the chances of survival in the extreme conditions on Mount Hood.
Schmidt said she gave them the same statistic and they asked that rescuers not be endangered by putting them at risk from an avalanche.
Steven Rollins, a rescue leader, said search teams would not be going back up the mountain any time soon because of avalanche dangers made worse by an ongoing storm that has created whiteout conditions.
Rollins, with Portland Mountain Rescue, said it would take four to five days of good weather to ease the avalanche risk, but such stretches were rare in the winter on Mount Hood.
"We can't get people off the ground ... our hands are really tied," Rollins said. "If there is anything we could do we would do it."
Even if the rescuers knew where the two climbers were, search teams would not be able to get to them because of the danger, Rollins said.
The Mount Hood ordeal began last Friday when the trio was reported missing. They had started up earlier in the day on what was expected to a one-day outing.
Ground teams and aircraft have been hindered in the search by snow, poor visibility and subfreezing temperatures.
Vietti, 24, is from Longview, Wash., and Nolan, 29, is from Portland.