Graphical AIRMETs (G-AIRMETs) are routinely issued at 0245 UTC, 0845 UTC, 1445 UTC and 2045 UTC. Each issuance produces 5 snapshots to include the Initial, 3 hr, 6 hr, 9 hr and 12 hr forecasts. For example, the 0245 UTC issuance produces a suite of forecasts valid at 0300 UTC (Initial), 0600 UTC (3 hr), 0900 UTC (6 hr), 1200 UTC (9 hr) and 1500 UTC (12 hr). These valid times are fixed and do not change regardless of the current time.
If the forecast is amended, the chart will show the issued time for when the amendment was made, but the valid time will remain unchanged. For example, in the Tango image below, this was the 6-hour forecast from the 0245 UTC issuance valid at 0900 UTC. This 6-hour forecast was amended at 0445 UTC, but is still valid at 0900 UTC.
Image 1. An updated G-AIRMET Tango that was issued at 0245 UTC, updated at 0445 UTC, but is still valid at 0900 UTC.