I can't recall them doing that in recent years. But I do remember one race (i can't remember where) when due to weather and lack of weekend's left in the season, they held one qualifying session on Sunday before eliminations. I believe it was a case where they almost had to complete the race that weekend, and due to rain they were only able to get one qualifying session in, and they didn't think it was fair to set a field, with championship races still at stake, on one session.
The only time I can recall them setting the field on one session was the rain at the finals a few years ago, but they only did that because the season had to be completed by sunday, or early monday, due to the awards ceremony being at the kodak theatre on that monday.
Again, if it is rained out today, and judging by Alan's report it's a good chance, and if Saturday is rained out as well, at least they have two open weekends to complete the event. As of now, there are six weekends left in the season to complete three events.