It's never really too late at this time of the year, but it can be tricky to get the seed to germinate in cold weather. Lettuce seed will even germinate when soil temperatures are at 32 degrees, but it will take it about 7 weeks. Of course, during that 7 weeks, there's always a chance the seed will blow away, wash away or be eaten by critters before it can germinate. So, waiting until the soil is warmer (or manipulating the soil to make it warmer) will get you faster germination before the seed can disappear.

Check out the chart on the linked page that shows you the different germination rates of lettuce (and other) seed at various soil temperatures. To get the best germination right now, I'd start the seeds in pots in a warm location and then move the pots back outside after the worst of this coming cold spell passes. Or, after the cold spell passes, you could transplant the young seedlings into the ground.

Tom Clothier's Data Showing Link Between Temp/Seed Germination

Technically, it is both a little too late to sow seeds for fall/winter lettuce (OSU recommends Aug 1-15 )and a little early to sow seeds for spring/early summer lettuce (OSU recommends Feb 15-Mar 10). You probably could have gotten away with sowing it in a raised bed sometime over the last few weeks while we've been abnormally warm, but with all the wild weather now bearing down on us and bringing substantially colder and wetter weather very soon, your best bet, realistically, would be to wait a few weeks, or sow some in containers indoors where the warmth will aid in germination.

Dawn