One of the most severe forest fires in the Yukon’s recent history is raging out of control on the doorstep of Whitehorse.

About 150 residents of Echo Valley -- just west of the Mayo road cutoff -- were forced to flee their homes overnight. Hundreds more people in the subdivision of Crestview were put on an evacuation alert.

An Emergency Measures Organization meeting was underway at press time today. Officials were to decide upon evacuation procedures should the fire threaten downtown Whitehorse this afternoon.

The fire covers 1,500 hectares at Haeckel Hill, fewer than three kilometres from Crestview, and fewer than seven kilometres from main street.

"This is a high-intensity fire in a high-priority area," Al Beaver of the Forest Fire Control Centre in Whitehorse warned today.

It’s so intense that forestry officials were forced to call off water bombers and fire crews Thursday evening.

"I’m not putting anyone at the head of a fire like that," he said. "It’s a half-mile wall of flames."

Fire retardant released by the bombers evaporates immediately in the heat of the flames, he said. "It’s having absolutely no effect."

Attempts overnight to build a fireguard failed.

Beaver said crews are working frantically to get a guard built on the north side of the fire before temperatures rose and humidity dropped this afternoon.

He hoped it would be established by 1:30 p.m. today. "The Cat bosses have told me it’ll be in, and I trust them."

Then it becomes a matter of containing the fire inside the lines, he said.

"It’s a matter of when it starts to stress these lines… It’s going to be the same day today as yesterday, the same high-intensity type fire."

The fire began at 1:24 p.m. Thursday at the old Gun Club. It doubled in size within 15 minutes to five hectares, and by 6 p.m. covered 800 hectares.

By midnight, 1,500 hectares of 150-year-old boreal forest were aflame.

The critical factor in this fire is wind, said Beaver. Because winds were light, the fire did not spread as rapidly as it could have.

However, it was incredibly intense, exceeding the maximum forestry level for intensity and strength, called Rank Five Plus.

"The energy released in that fire today was equivalent to a small nuclear explosion," he said. "Fortunately, we had light winds. If we’d had stronger wind, heaven only knows where we’d be."

The fire is definitely human-caused, said Beaver, but that’s as much as fire officials may ever know.

"It’s a human-caused fire of some nature. But whether we ever will be able to tell how it started is hard to say. Often, suppression efforts obliterate a fire’s origin."

Things settled overnight with the cooler temperatures and higher humidity. But forecasters at the Yukon Weather Centre are warning of "another day like yesterday," with temperatures of 28 degrees C, and thunderstroms.

Winds are predicted to remain light northerly, which will push the fire into the mountainous region away from the city.

But the thunderstorms are a concern.

"They can get unpredictable," said Ken Clark, a Yukon Weather Centre forecaster. "We’re expecting brief, fairly strong wind gusts this afternoon and evening."

Firefighter reinforcements have been called in from British Columbia to help keep the fire within the lines. Three divisions of 20 people have been called in, and now, 150 firefighters are attacking the blaze.

Eight air tankers and two helicopters are standing by. Three "bird-dogs" are monitoring the fire and potting course for the air tankers.

RCMP closed off the Alaska Highway overnight for three hours. We could be closing things down again," said Sgt. Brian Gudmundson of the Whitehorse detachment. All road access to the fire has been blocked off.

The fire started at the base of Haeckel Hill and spread uphill. Following a westerly wind direction, it seemed to run parallel to the Alaska Highway.

The roadside was crowded with people watching in amazement as the fire roared uphill. They remained, snapping pictures and recording video footage, despite entreaties from the RCMP to move on.

There were a number of near-misses between vehicles as drivers gaped out their windows at the scene.

At 9 p.m. the wind shifted to a north, northwesterly direction, and pushed the fire around the hill into the Echo Valley subdivision.

Residents were warned to leave at 8 p.m. Trucks carrying freezers, televisions, and personal belongings began streaming down Echo Valley Road onto the Alaska Highway.

Christine Paradis at the Emergency Measures Organization said about 130 people have registered with it as part of a head count of people in that area.

"A few people are staying in the area," she said. "Not that many."

They have informed RCMP of their decision, she said.

Whitehorse reverberated to the steady drone of water bombers heading back and forth, back and forth to the fire from the retardant station at Whitehorse Airport.

The huge bombers were lined up on the runway to get pumped up with reddish fire retardant, a combination of salt and fertilizer.

"We’re getting it, we’re getting under control," said one pilot as he wiped down his engine before filling up with retardant.

But then, an eery silence fell across the city as the bombers were called off. The 20-storey-high flames could be seen by Riverdale residents from their back yards.

In Crestview, anxious residents put on evacuation alert paced their lawns through the evening, watching flames leap 60 metres (200 feet) into the air just 2.5 kilometers from their homes.

The entire neighborhood was awake overnight, with lights on and televisions glimmering through open doorways and windows.

Billowing clouds of black, white and brown smoke drifted through the sky, settling into a thick, dull mist which spread to Carcross and beyond.

The city of Whitehorse was organizing the head count for evacuees from Echo Valley. They were urged to head downtown to the community college campus.

Once they checked in, the evacuees went to numerous downtown hotels, who operators were offering free accommodation.