At precisely 7:30 Monday night the 40-story First National Bank tower here went dark, the blinking marques advertising "All the President's Men" on the downtown Fox movie theater suddenly electric lights snapped off.

The 20 minute voluntary cutoff was an unsual effort to alert Portland's 382,000 residents to what many officials in the drought-plagued Pacific Northwest fear may be a common plce even by the end of summer - the rolling blackout.

Reservoirs and the mountain snowpacks that feed them have dropped so low that federal and state power officials says if the drought continues there won't be enough that runs this region.

Nearly 90 per cent of the electricity in Oregon, Washington and Idaho comes from 28 dams on the mighty Columbia River and lesser waterways.

In Seattle Monday, Mayor Wes Uhlman held a news conference to announce that his city is likely to face mandatory electric cutbacks this summer. Gordon Vickery, superintendent of Seattle's City Light Co. said in an interview later that the city is spending $1 million a week - twenty times the normal cost - to buy electricity it can't generate because of low water levels.

Vickery said Seattle electric users face up to a 50 per cent surcharge on their bills to pay for the power. The city is also quietly putting together a 70 page emergency plan for rolling blackouts - staggered cutoffs by neighborhood - that it may have to impose this summer.

South of Seattle, energy officials in the industrial port of Tacoma are preparing to ask the city council this month for permission to tack a [WORD ILLEGIBLE] per cent surcharge on to electric bills to cover the cost of outside purchases.

In Idaho, where the drought is describe as the worst since 1931, the Public Utilities Commission last month declared an energy emergency and ordered the state's three investorowned power companies and 31 electrical coops and municipal utilities to submit plans to handle mandatory cuts in electricity for their areas.

"Unless there's a complete turnaround in the weather this summer, a mandatory cut in supplies is inevitable." said Idaho PUC spokesman Garth Andrews. "We have a 75 per cent hydroelectric base and the water just isn't there this year."

Washington and Idaho have additional drought-rated energy problems. Farmers in both states have jumped the gun on traditional irrigation schedules, imposing an added strain on water and thus on power supplies. In Oregon, water reserves have dwindled to the point where energy officials must decided whether to spill water over - instead of through - the Columbia dams, thus losing it for power generation, or to leave river levels too low for fish to return to the sea from their upstream spawing grounds.

Portland's voluntary balckout this week was initiated by Gov. Robert Straub. Even skeptics, who viewed his gesture as a political public relations move, were surprised by its initial effectiveness. Most large businesses shut down early and electrical consumption dropped by 20 per cent.

Straub, who viewed the blackout with reporters, elatedly announced afterward that the event "proved that Oregonians are willing to make sacrifices and turn out their lights if they understand what's at sake."

Others here, however, are less enthusiastic about the effectiveness of voluntary cutbacks. Last month when the Bonneville Power Administration called for a voluntary 10 per cent cut in electrical use it got only 1 per cent.

Ray Foleen, BPA's deputy director, estimated there is a 50-50 chance that the entire Northwest may be forced to take a 10 per cent power cut by August.

Water levels behind BPA's dams are already below normal, but Foleen and other osnowpack. This year's is estimated to be 57 million acre-feet of water - about half the normal runoff and the lowest ever recorded in the Columbia basin.

BPA said is expects to lose 13 billion kilowatt hours from the reduced runoff, enough to light the city of Portland for eight to 10 years.

Another year of drought could force a 50 per cent cut in electric supplies, a ccording to BPA. The agency already has cut by 25 cent its supply to the Northwest aluminum industry, a move BPA predicted could ultimately cost 25,000 jobs. Nearly one-third of the nation's aluminum is produced in the Northwest, ironically because of the abundance of cheap hydroelectric power.

James Vann, operations manager at Alcoa's big Vancouver, Wash., aluminum facility, estimated that the power cut has already reduced the nation's aluminum supply by 7 1/2 per cent. A second drought year would have "a substantial national impact," he said.

In Idaho, two big phosphate operations have also lost 25 per cent of their power because of cutbacks. The phosphate that the FMC plant in Pocatello and the Monsanto plant in Soda Spring mine and process is used in fertilizer.

With additional cuts in sight, north-western states are already jockeying behind the scenes for electrical supplies. Some energy experts here expect that is a 10 per cent mandatory cut goes into effect this summer it will generate fierce competition between industrial and residential consumer groups.

But Vann said he expected the burden to be shared equally by both. "It doesn't do much good to cut industry back heavily if the remaining power goes to someone who is home out of work," he said.