The era of dollar-a-gallon gasoline has finally arrived in Washington as a growing number of stations have begun taking advantage of new federal price rules to nudge their pump rates above the three-figure mark, surveys showed yesterday.

But while more expensive than ever, gasoline should at least be easier to find in the region this weekend.

The jump over the dollar mark was reflected in an American Automobile Association survey that found 16 local stations - double the number last week - charging more than $1 for premium grades of gasoline.

Similarly, a Washington Post survey of a dozen stations whose prices were all below the dollar mark last week showed that half had crossed that line for premium gasoline. The Sunoco station at 7313 Baltimore Blvd. in College Park had the highest price in the survey, charging nearly $1.03 a gallon for premium.

Many of the price increases were promted prompted a U.S. Department of Energy rule change last Monday, permitting service station operators to charge 15.4 cents a gallon over the wholesales price they pay. A spot check of dealers indicated that most of the 1,500 stations would raise their price 3 to 5 cents a gallon.

And there are indications that the full brunt of the rule change has not yet hit in the local area.

"There's some misunderstanding. Some dealers think if [the rule change] doesn't go into effect until Aug. 1, and they're waiting," said Roy Page of the Virginia Gasoline Dealers Association.

But despite the gloomy price news, officials in the District of Columbia and Virginia said yesterday that more gasoline stations would be open for longer hours over the weekend.

Nearly 90 stations will be open for some weekend hours in Fairfax County, Alexandria, Falls Church, Vienna and Herndon, according to a survey done by the Fairfax County government and the retailers association. That is more than double the number found open in a similar survey last week, according to county spokeswoman Jane Stern.

Chuck Clinton, director of the D.C. Energy Unit, said it appeared more stations would be open and for longer hours, based on monitoring his office has done.

In Maryland, energy officials predicted that consumers would find gasoline availability, the same as last weekend.

"People are still having difficulty getting gas late in the day, particularly Sunday evening," said Janet Shorey, of the state's energy office. "I advise them to purchase it as early as possible on Sunday."

Page said he believes the demand for fuel was not as great as anticipated this week, and operators are staying open longer hours in order to sell the quotas they had set for themselves as the week began.

"If sales are down, operators may stay open on Saturday or Sunday to even it out," Page said.

As far as price increases are concerned, Page said the majority of retailers would take advantage of the federal rule change.

The AAA survey Tuesday, one day after the rule change went into effect, showed the average price of premium gasoline at full-service pumps was 99 cents a gallon, compared to 95.7 cents last week.

That was the largest increase, although regular gasoline was also up to an average 92.1 cents a gallon, and unleaded to 96.6 cents.

One station in Prince George's County was selling premium gasoline for nearly $1.09 a gallon, and another was selling unleaded for nearly $1.08 - both the highest prices the AAA found.

At the self-service pumps, consumers still could do better, paying an average 90.2 for regular gasoline, 96.7 for premium and 94.5 for unleaded. But even when serving themselves, the AAA found, consumers would pay more than $1 for a gallon of premium at two service statoons.

The national AAA survey showed an average increase in gasoline prices of 1.5 cents, and a spokesman said he expects next week's survey to show the "full effects" of the federal rule change.

Meanwhile, leaders of the National Congress of Petroleum Retailers denounced the rule cahnge as "inadequate and discriminatory" and called on the Energy Department to amend it by Aug. 1.

The organization's board of directors argued that the change - permitting retailers to raise their profit ceiling - fails to provide adequate relief from the "ravages of inflation" and may even damage some retailers.

The group threatened a national protest in Washington if the rule is not changed. CAPTION: Chart, Daily Gasoline Guide, The Washington Post