Weather observations and forecasts are usually reported in UTC. This allows weather observations the world over to have the same time stamp. Since UTC is ahead of local time in the United States, sometimes weather data will appear to have tomorrow's date, when it actually represents today's data (we are not THAT good at forecasting the weather).
UTC is 8 hours ahead of Pacific Standard Time (e.g., 0000 UTC is 1600 PST the previous day, while 1200 UTC is 0400 PST the same day), and 7 hours ahead of Pacific Daylight Time (e.g., 0000 UTC is 1700 PDT the previous day, while 1200 UTC is 0500 PDT the same day). We also use a 24 hour clock, so 1600 PST is the same as 4:00 pm PST.
Some of the data may be labelled GMT (Greenwich Mean Time), which is an older abbreviation, now deprecated in favor of UTC.
P.S. Why is it not CUT (Coordinated Universal Time)? The French wanted their own word order, or TUC, to be implemented, so a compromise was chosen. Please see the time FAQ for more information.