WASHINGTON, Jan. 19--This jaded city is all astir over Lyndon Johnson's inaugural at noon today. Washington has never witnessed so expensive an inaugural--$2.1 million--nor one so cold. The huge crowd lining the streets to watch the parade will have to stand in 20-degree weather if the official forecast proves accurate.

The President's inaugural speech remains a mystery. Presidential aides Bill Moyers and Richard Goodwin were absent from most of the pre-inaugural festivities. These two men, along with the President, have been chiefly responsible for the speech, and they were said to be at work on it late last night.

The theme of the speech is believed to be a close definition of Johnson's "Great Society." It will deal primarily with the Society's "challenges" since the State of the Union speech two weeks ago described the "promises."

Work on the address began weeks ago, but Johnson is apparently still working overtime on it. He is not expected to repeat any of the themes of the State of the Union speech. As one Washingtonian put it. "He can't afford to miss badly with this one, because of the inevitable comparison with Kennedy."

The inauguration of four years ago has not been forgotten. Official Washington remembered it by bringing in Army bulldozers and snowplows to be ready for a snowy disaster like that of four years ago.

The crowds of inaugural visitors have not forgotten either. Huge lines snaked across the hill to the site of John Kennedy's grave yesterday despite sub-freezing temperatures.

It could not be said, however, that Washington lacks exuberance. It has been supplied by more than 5000 Texans, some of whom have swaggered into the capital with cowboy boots, ten-gallon hats, and at least one tiara that says "Howdy, I'm from Texas" in lights when its wearer pushes a button. More than 3000 visitors jammed a coffee hour given by Sen. Ralph Yarborough (D-Tex.) in a tiny Capitol Hill room yesterday morning.

The Texans have made life miserable for their elected officials by demanding tickets to all the packed inaugural events. "I'm at the end of my rope," said one Lone Star congressman, "but if I don't help these people, there are probably enough of them here to defeat me for re-election."