Lines of motorists waiting to buy gasoline shrank dramatically yesterday, but service station operators warned that the let-up was only temporary and added that gasoline will cost about a penny a gallon more this weekend.

Yesterday's shorter lines contrasted sharply with near-record lines on the Fourth of July. But dealers surveyed by the American Automobile Association predicted that motorists will line up today as usual to buy gasoline for the weekend.

The AAA surveyed weekend hours and prices at 68 service stations.

The weekend outlook for gasoline buying is expected to be much the same as it was last weekend. About 62 percent of the stations checked by AAA said they will open Saturday and a majority of that group will remain open until 6 p.m. Only about 5 percent are expected to pump gasoline on Sunday, however.

Motorists buying gasoline this weekend will pay an average of 90.2 cents a gallon for regular at full service pumps, according to the AAA survey. That is 1.2 cents more than last week's price.

Premium now averages 96 cents, up from 95.7 cents.

The largest increase will be for unleaded gasoline, which now averages 94.4 cents a gallon. That is 1.6 cents over last week's price.

Actually gasoline prices vary widely from one station to another. The price spread may be more than 10 cents a gallon.

Premium gasoline, for example, sold for 88.9 cents a gallon Thursday at the Crown Self-Service Station, 3216 Jefferson Davis Hwy., Arlington. But at Ralph's Sunoco, 801 N. Washington, Alexandria, it was a fraction less than $1.02.

The difference in the lines at the two stations was as dramatic as the difference in prices. Fifteen cars were lined up waiting to buy Crown's bargain fuel; no one was waiting at Ralph's.

Stations did have long lines in some areas when they opened early Thursday morning, Post reporters found. At Van Ness BP, 4444 Connecticut Ave., 69 cars stretched over five blocks at 6:10 a.m. But by 9:24 a.m., all the customers had been served and the line had disappeared.

"If people don't see a line, they figure we're closed," said station attendant Dennis Kenhoe, 17.

Van Ness stayed open until after lunch to sell its daily allotment of gasoline of 4,600 gallons - far beyond its usual closing time.

"One reason for the improvement Thursday might be that people bought gas for July outings and then changed their minds because of the rain," said AAA spokesman Glenn Lashley.

He said the even-odd plans also contributed to the short lines. "Thursday was the third day of the week that odd-day licenses were eligible for gas," because Wednesday was a holiday when anyone could buy gas, he said.

EVENING OPERATIONS TODAY

Here is a list of gasoline stations in the District of Columbia that a city energy official said will sell gasoline between 7 p.m. and 9 p.m. today.

Pennsylvania Avenue Gulf, 2300 Pennysylvania Ave. SE; Hillcrest Shell, 2721 Naylor Rd. SE; Bill's Guld Servicenter, 5120 Georgia Ave. NW; Jamison's Sunoco, 1253 Ninth St. NW; Melby's Gulf, 3124 Mount Pleasant St. NW; MacArthur Service Center Amoco Station, 5101 MacArthur Blvd. NW; Parkway Shell, 1944 Bladensburg Rd. NE, and Webb's Exxon Servicenter, 4100 Hunt Pl. NE.

Here is a list of gasoline stations that have agreed to open Sunday in Maryland in exchange for 4,000 gallons of gasoline each from the state's reserves.

HOWARD COUNTY

Gulf, U.S. Rte. 1 & Maryland Rte. 175, Elkridge; Amoco, U.S. Rte. 40 and St. Johns Lane, Ellicott City; Mobil, Craddock Way, Columbia, and Sunoco, Twin River Road, Columbia.

ANNE ARUNDEL COUNTY

Hess, 400 Baltimore-Annapolis Boulevard, Glen Burnie; Crown, Forest Drive and Bay Ridge Road, Annapolis; Shell, Benfield Road, Severna Park; Amoco, Rte. 165 and Higgins Avenue, Odenton; Texaco, 2006 West St., Annapolis; Texaco, 5570 Shadyside Rd., Churchton; Texaco, U.S. Rte. 50 and Old Mill Bottom Road, Annapolis; Gulf, Maryland Rte. 175 & Maryland Rte. 178, Crownsville; Exxon, 6701 Ritchie Hwy., Glen Burnie; Exxon, 2791 Solomons Island Rd., Edgewater, and Mobil, 2914 Mountain Rd., Pasadena.

CARROLL COUNTY

Amoco, 1525 West Main St., Hampstead; Gulf, Pennsylvania and Hersch avenues, Westminister; Gulf, Keymar, and Crown, Libert and Ridge roads, Eldersburg.

PRINCE GEORGE'S COUNTY

Amoco, 311 Washington Blvd., Laurel; Amoco, 7101 Allentown Rd., Oxon Hill; Amoco, 10604 Campus Way, South Largo; Chevron, 6900 Marlboro Pike, Forestville; Gulf, U.S. Rte. 301, Mitchellville; Gulf, 10621 Baltimore Ave., Beltsville; Gulf, 6100 Oxon Hill Rd., Oxon Hill; Sunoco, 5541 Marlboro Pike, District Heights; Sunoco, 5715 Old Branch Ave., Camp Springs; Sunoco, 7313 Baltimore Blvd., College Park; Sunoco, 9499 Lanham Severn Rd., Seabrook; Shell, 4201 Kenilworth Ave., Bladensburg; Exxon, 5915 George Palmer Hwy., Seat Pleasant; Exxon, Maryland Rte. 197 and Baltimore-Washington Parkway, Laurel; Texaco, 8014 Old Branch Ave., Hyattsville; Texaco, 14323 Indian Head Hwy., Accokeek, and Phillips 66, 5818 Riggs Rd., Chillum.

MONTGOMERY COUNTY

BP, 10010 Darnestown Rd., Rockville; BP, 13990 Georgia Ave., Wheaton; Exxon, 13400 Old Columbia Pike, Silver Spring; Exxon, 17651 Redland Rd., Potomac; Exxon, 5001 Bradley Blvd., Chevy Chase; Southern States, 11111 Georgia Ave., Wheaton; Amoco, 596 North Frederick Pike, Gaithersburg; Amoco, 7725 Old Georgetown Rd., Bethesda; Amoco, 18635 Germantown Rd., Germantown; Amoco, 3700 University Blvd., Kensington; Amoco, 8550 Connecticut Ave., Chevy Chase; Texaco, 19300 Montgomery Village Ave., Gaithersburg; Texaco, 5335 Westbard Ave., Bethesda; Texaco, 7224 Carroll Ave., Takoma Park; Shell, 10201 New Hampshire Ave., Silver Spring; Citgo, 15821 Frederick Rd., Deerfield, and Citgo, 10810 Colesville Rd., Silver Spring.

ST. MARYS COUNTY

Sunoco, Mechanicsville, and Sunoco, Maryland Rte. 2, Leonardtown.

CHARLES COUNTY

Exxon, Rte. E-4, Walforf.

CALVERT COUNTY

Amoco, Prince Frederick, and Mobil, Chesapeake Beach.

Telephone Help

Following are telephone numbers of agencies providing information to deal with gasoline emergencies:

District of Columbia "hot line"; 628-3181. An official said it will be staffed from 9 a.m. to 8 p.m.

Prince George's County "hot line": 779-1151. An official said it would be staffed around the clock.

Bowie area gasoline hotline: 262-6262. It is scheduled to begin operation at 5 p.m. today. It will feature a recorded message listing names, locations and hours of gasoline stations.

Frederick County motel/gas hotline: (301) 663-8687 or 662-2126. A spokesman said it is staffed 24 hours a day.

Anne Arundel County "hot line": 263-2681. An official said it would be staffed from 6 a.m. to 9 p.m. daily.

American Automobile Association: 222.5000. Officials said AAA members may obtain small quantities of gas or be towed to a nearby station if they are stranded.

Flag System

Washington area service stations have been asked to display special flags, noting the availability of gasoline

A green flag means leaded and unleaded gasoline are available.

A yellow flag means only leaded gasoline can be purchased.

A red flag means a station's pumps are closed.

Gasoline station owners in the District of Columbia can pick up flags at the Munsey Building, 1329 E St. NW, Room 1258. CAPTION: Chart, Daily Gasoline Guide, The Washington Post