At no time shall an observation be started more than how many minutes prior to the scheduled observation time?

The guideline specifies that an observation should not be initiated more than 15 minutes before the scheduled observation time to ensure that the data reflects the most accurate and timely weather conditions. This time frame allows observers to take recent and relevant measurements without introducing significant discrepancies that could arise from changes in weather conditions that may occur shortly after taking the observation. Starting an observation too early could lead to inconsistencies in reporting, especially in rapidly changing weather scenarios. Therefore, adhering to the 15-minute rule helps maintain the integrity and reliability of the weather data being recorded, which is critical for forecasting and analysis.