First, an explanation of "the ETA model"
The science of weather prediction is the science of data analysis. Meteorologists receive data from weather reporting stations worldwide, and from that data, they attempt to predict the future, using the laws of physics as well as assumptions and educated guesses.
Educated guesses?! Say what?!
That's right. Weather is one of the original chaotic systems, and although the weather obeys physical laws, we can't gather enough information about the current state of the atmosphere to predict what will happen in the long term with any degree of accuracy.
Bummer. So what are all the computers for?
Well, we've found that supercomputers can predict the weather out to a few days ahead with a reasonable degree of accuracy, even without perfect information. They do this by applying educated guesses and rules of thumb. Using different assumptions gives a different forecast model. Some of the more well-known models are the GFS, the NAM, and the EMCWF. The ETA is one of the lesser-used models.
Great. So what does "based on the current ETA model" mean?
Simple. It means that the SIGWX chart gets created based on the most recent data that the ETA supercomputer churned out. If the chart is supposed to come out at 1800 UTC, but the ETA model run doesn't complete until 1845, then that new forecast data doesn't make it into that chart.
Enough. Just answer my questions.
which SIGWX charts are relevant for future weather developments (at
All the SIGWX charts are relevant, but the newer charts will have the newer forecast data. That doesn't necessarily mean that they're going to be more accurate, but they probably will be.
To use your example, it's 1100Z on Monday morning. The 0000 chart says "IFR" but the 0600 chart says "No IFR". There might still be IFR out your window - nothing says that the supercomputer will get it right. Maybe the older one was more correct than the newer one. Doesn't happen often, but is it possible? Sure.
So is it fair to say that "all" SIGWX charts are updated when a SIGWX
chart is published (at 0000, 0600, 1200 and 1800)?
No, as soon as a chart comes out, it becomes part of history. Charts are not updated with new weather information; they remain as a historical record of what was predicted to occur, given the best information possible at the time (which, for the forecasters, can be just as valuable if not more valuable, to see where they went wrong in the past).