At 0600 UTC there are 21.3 inches of snow on the ground and no precipitation fell during the 6-hour period. What should be reported in remarks?

In this scenario, when there is no precipitation reported during the specified 6-hour period, it is not necessary to include any specific remark about the snow depth. The key point is that since no new precipitation has fallen during this timeframe, reporting additional remarks regarding snow accumulation is not mandated.

Standard practice in weather reporting indicates that if the observed conditions remain stable without any changes, such as no new snow accumulating over the specified time frame, no further remarks are required. Therefore, the absence of precipitation during the 6-hour period supports the conclusion that there is nothing new to report that would alter the existing conditions already observed.

This decision is in accordance with established reporting guidelines whereby remarks are typically reserved for significant changes or new observations that would provide additional context to the existing conditions. Hence, it's appropriate to conclude that no specific remark is warranted in this case.