SNOQUALMIE PASS - Interstate 90 across Snoqualmie Pass will remain closed until at least 9 a.m. Friday morning, DOT officials said, and some estimates say the shutdown could last longer.

I-90, the main arterial connecting the eastern and western halves of Washington state, has been open for only about six hours since Tuesday morning, due to crushing amounts of heavy snow that continue to dump relentlessly in the mountains.

State transportation officials were unable to estimate when I-90 might reopen, but said it will remain closed at least until Friday morning, when DOT crews will reassess the situation. Earlier plans to open the freeway Thursday morning were cancelled when twice as much snow fell as was predicted Wednesday night and Thursday morning.

State troopers said the arterial could be closed until Saturday. The economic impact of the closure is estimated to be $700,000 for each hour it remains in effect.

Up to two or three feet more snow could fall in the passes by Friday, forecasters said.

Gov. Chris Gregoire declared a state of emergency in several counties, including King County, to help local agencies hire more help and equipment to clear snow. The emergency declaration also covers Adams, Clark, Columbia, Kittitas, Klickitat, Lincoln, Okanogan, Pend Oreille, Skamania, Spokane, Stevens, Walla Walla, Whitman and Yakima counties.

Gregoire said I-90 would not be reopened until it is safe to do so.

The snow was creating havoc in Eastern Washington as well. Washington State University in Pullman cancelled all classes today, and parts of Spokane were paralyzed by snowfall. Public schools remained closed in Spokane as well.

The state Transportation Department advised motorists to take alternate routes if they need to cross the Cascades, but those options were dwindling.

Highway 12 at White Pass and some routes through Southwest Washington and Northern Oregon, including Highway 14 are open. Highway 2 at Stevens Pass was closed through much of Thursday afternoon and finally reopened in both directions shortly after 5 p.m.

Another major route, I-84 through northern Oregon, is closed between Pendleton and Ontario due to blowing and drifting snow, but you can still cut over to the Tri-Cities via I-82 at Hermiston and and connect to I-90 via U.S. 395 before the closure. But winter driving conditions persist across Eastern Washington.

Motorists crossing the mountains also were advised to get up-to-date information about road closures before leaving and to keep a winter car kit in their vehicle. You can also call 511 from your cell phone for the latest conditions.

An avalanche on I-90 temporarily trapped two cars Wednesday afternoon near Snoqualmie Pass after two lanes of the freeway had been open a few hours during a lull in the snow, but their occupants were rescued unharmed, a state Department of Transportation spokesman said.

The pass, which had reopened late Wednesday morning after a 28-hour closure due to heavy snowfall and earlier avalanches, was closed in both directions after the Wednesday afternoon slide, which was the worst avalanche the DOT has ever faced.

There were two people in each of the cars hit by the avalanche, he said. One of those cars is Alex Walker's.

"Wasn't anything on my mind other than just stay alive," said Walker.

Walker said seeing the cloud of snow storming down in front of him was the scariest thing he has ever seen.

"There was no control; it was blind. When it went blind I felt some spinning going on, but I had no reference so I just held on and let it ride," he said.

Workers probed the snowslide, which measured 400 feet long by 30 feet deep in spots, as a precaution but no other vehicles were caught.

DOT worker Al Mminerich said he's never seen anything like it.

"No, not at all. Nothing close to this," he said.

The slide occurred in the westbound lanes about 1½ miles west of the summit of Snoqualmie Pass just before 3 p.m., Westbay said. The pass is about 50 miles east of Seattle.

Motorists were being turned around at North Bend on the west side of the pass and at Ellensburg on the east side, Westbay said.

Interstate 90 across Snoqualmie Pass, elevation 3,011 feet, usually carries about 27,000 vehicles per day. Traffic was backed up for miles on Wednesday afternoon.

Forecasters predicted that another incoming storm could drop 2 more feet of snow in the Cascades. Snowfall at Snoqualmie Pass since Monday has totaled 3 feet.

A snowslide that blocked the eastbound lanes prompted closure of the pass about 7:30 a.m. Tuesday. It remained closed for 28 hours until one lane in each direction was reopened at 11 a.m. Wednesday.

The Transportation Department said the pass was closed for 22 hours back in 2002.

Before dawn Wednesday, workers detonated about 500 pounds of explosives to bring down more loose snow and plowed snow and debris equal to about 130,000 dump truck loads, the Transportation Department said.

Why So Much Snow At Snoqualmie Pass?

By the time this week is over, as much as 8 feet of new snow may have fallen at Snoqualmie Pass. How can that one area get so much snow when it's not raining nearly that much in the lowlands?

It's all about the upper-level winds, which are in the perfect alignment for a heavy, consistent snow in the Cascades.

We have a very strong jet stream pointed right at Western Washington, and the general air flow in the wake of Monday night's storm has been almost straight from the west. That air is also still very moist as it comes off the ocean.

As that air slams into the western side of the Cascades, the air is lifted up the mountains' slopes. When air lifts, it cools and condenses -- in essence, it's like squeezing a soaked sponge, only in this case, it's snow that comes pouring out.

(That's the physics behind why there's a big rain forest on the southwestern slopes of the Olympic Mountains as our flow is predominantly out of the southwest. That air hits the southwestern slopes and squeezes out the moisture there on a near-daily basis. The rain forests there get around 200 inches of rain a year.)

Back to the Cascades, getting 1-2 feet of snow with a passing storm isn't too unusual for the mountains in winter, but usually the westerly flow abates somewhat after the storm and the snow lets up. This week, the jet stream has just been relentless in keeping that west flow going, and in turn, the snow falling.

The snow isn't expected to stop anytime soon. Forecasters expect 6-10" of new snow during the day Thursday, followed by 6-12" Thursday night and another 6-10" on Friday. By late Friday afternoon, new snow totals will range from 18-36" (That's 1.5-3 feet for those who have given up on doing math today) -- that's on top of the 6-12" that fell Tuesday night.

We are working to get some official snow totals from the pass, but the ski resort at Snoqualmie Summit reports 26" of snow in the past 48 hours.