The search for two pairs of hikers missing for nearly a week in Washington's Mount Rainier National Park resumed today after freezing rain and heavy snow kept crews inside Friday.

National Parks spokeswoman Patti Wold says conditions are rough, but searchers are optimistic that the four will be found alive.

"Visibility is poor. There are winds up to 50 miles an hour, but the folks we have searching are some of the best of the best, and the most physically able to do this search under those conditions," said Wold.

"We know that they are still very likely alive, as long as they have been cautious in how they've conducted themselves up there," she said. "We're still optimistic. We are trying to make sure that we keep our searchers safe out there and that's our number one priority."

Mark Vucich, 37, of San Diego, and Michelle Trojanowski, 30, of Atlanta, were supposed to return Sunday from a camping trip on the Muir Snowfield, about 10,000 feet up, but have not been heard from in days.

The pair's car was found in a parking lot about halfway up the mountain, according to Wold.

Mount Rainier is about 70 miles southeast of Seattle, which was battered by snow and ice on Wednesday.  Mount Rainier was hit with snow over the weekend and temperatures continued to hover well below freezing.

A team of 10 Park Service staff set out Thursday to travel the same route Vucich and Trojanowski would have taken, Wold said.

On Tuesday, the team was only able to conduct a limited search after conditions of zero visibility and 100 mph winds hampered their efforts.

"The weather still isn't good up there," Wold said. "We are doing what we can."

Meanwhile, two other climbers, an unidentified couple from Springfield, Ore., are also missing after failing to return Monday from a summit attempt on the Disappointment Cleaver route.

Park officials said they believe both pairs of climbers chose to ride out the storm and wait for conditions to clear before returning.  All four are thought to be well-equipped with tents, sleeping bags and other cold weather gear.

Wold says park officials are not sure of the experience level of the two campers and two climbers who are missing, but remain optimistic that they will be found alive.

"The more experienced they are, the better, the more likely they would actually be to understand how to ration their food and supplies," said Wold.

Vucich's uncle told the U-T San Diego his nephew is an experienced hiker, but said the family is worried.

"We hope he is hunkered down," Jack Anthony said. "It's not a good situation."

The missing hikers' families and park officials are hoping their story ends in a fashion similar to that of Yong Chun Kim, the 66-year-old snowshoer who was found alive on Mount Rainier after a two-day search.

Rescuers found Kim, of Tacoma, Wash., alive and well Monday afternoon after he fell down a steep slope Saturday while snowshoeing and became separated from his group.  Kim told park officials he burned dollar bills from his wallet to create warmth and stay alive.

National Park Service officials had to rescue Kim using a special Sno-Cat vehicle because the snowy and windy conditions prevented a helicopter rescue.

ABC News' Katie Kindelan, Maria Nikias and the Associated Press contributed to this report.