NOAA weather forecast maps often include a valid time period, e.g: "valid from time1 thru time2." However, many maps only indicate a single time, e.g: "valid time1." In the absence of other information, does this typically (or officially) mean valid up until time1, or valid after time1?
Short answer: Valid just means the expected conditions at that certain time.
In your example the map shows a forecast of 00 hours which is valid for 0600 UTC. Basically it means that the forecast/simulation is initialized on 0600 UTC and because the forecast time is 00 hours this forecast is also valid for 0600 UTC.
NOAA also provides forecasts for several hours (or days depending on the product). In this case, your simulation may be initialized on 0600 UTC but if you want to see what the conditions on 1500 UTC are you have a forecast of 09 hours which is valid for 1500 UTC. See this example.
Since the question was referencing an aviation weather product (links no longer functional), there is a contextual answer as well. The aircraft dispatcher for an airline flight must plan the flight according to FAA regulations, some of which apply to weather information. Only specific sources of weather observations and forecast products are authorized, and the "VALID" period must also be taken into account.
If I'm planning a flight that will depart at 1400Z and arrive at 1755Z, I am not authorized to use a forecast product that was issued at 00Z and is valid at 1800Z. Even though this forecast product was issued prior to me starting to plan the flight, and thus is possibly the "most recently published product available", the product was produced for a specific valid period - namely, from 1800Z onward. I would have to reference as different product that was "valid" during my scheduled flight window.
The accepted answer correctly describes the meaning of the valid time for point forecasts, where a forecast is initialized at a particular time, and describes (is valid for) a second time. In this case, as the answer states, a "valid" time is the point in time that the forecast describes.
But this is not applicable to all NOAA products. Several forecast products describe a range of time, e.g. 3 hr precipitation forecasts, and these tend to be valid for the period ending at the valid time.
For example, the current 3 hour precipitation forecast from GFS states that it is valid at "THU 04/11/24 15UTC". It looks like this:
If you load this forecast through the Model Guidance interface, you'll have access to the product information for each product. For the 3hr precipitation forecast, this information reads as follows:
Accumulated precipitation in inches for the 3 hour period prior to the forecast hour expressed as color fill. A reference bar of values for each color is located on the left side of the image. [emphasis mine]
The same appears to hold for most other products that describe a period of time rather than a point. However, I would recommend always checking the product description and model guidance documents where available, as they will often clarify the relevant meaning of "valid" and other terms in the forecast.
