What is "nowcast"

n. (context meteorology English) A weather forecast predicting the weather for a very short upcoming period, usually of a few hours.

The NowCast is a weighted average of hourly air monitoring data used by the United States Environmental Protection Agency for real-time reporting of the Air Quality Index (AQI) for PM (PM or PM) or Ozone data.

The PM NowCast is computed from the most recent 12 hours of PM monitoring data, but the NowCast weights the most recent hours of data more heavily than an ordinary 12-hour average when pollutant levels are changing. The PM NowCast is used in lieu of a 24-hour average PM concentration in the calculation of the AQI until an entire calendar day of hourly concentrations has been monitored.

Similarly, the Ozone NowCast is computed from the most recent 8 hours of ozone monitoring data, but weights the most recent hours of data more heavily than an eight hour average when pollutant levels are changing. The Ozone NowCast is used in lieu of a forward rolling 8-hour average for the calculation of the AQI until an entire calendar day of data hourly data is available.