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 have tomorrow's date, but it still 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 4 p.m. PST the previous day, 1200 UTC is 4 a.m. PST the same day), 7 hours ahead of Pacific Daylight Time (e.g. 0000 UTC is 5 p.m. PDT the previous day, 1200 UTC is 5 a.m. PDT the same day).
Some of the data may be labelled GMT, but it really mean 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.