I don't have to look at the freezing level chart to know where the lowest freezing levels are going to be. I can also look at the 500 mb chart which shows me the height of the 500 mb surface. The lower the 500 mb surface, the colder the air is in that region below 500 mb.
Here's an example of a 500 mb chart with a trough centered just off the southwestern coast of California.
The lowest freezing levels are going to be in the center of this trough as you can see from the 8-hr freezing level forecast below. Normally a height of 5400 meters (540 line) equates to a freezing level near mean sea level. Nevertheless, the 500 mb chart isn't a way to identify an absolute freezing level (such as 3,000 feet), but freezing level forecasts as shown above only go out to 12 hours on ADDS (18 hours for those that have access to the Internet Wx Brief Roadmap). So if you are looking two or three days in advance to get a sense of the freezing level along your route, try looking at the 500 mb chart.
CFI & former NWS meteorologist
Administrator of AvWxWorkshops.com
Most pilots are weatherwise, but some are otherwise™
Edited 2 time(s). Last edit at 12/02/2011 07:01AM by Scott Dennstaedt.