When we talk about precipitation forecasting, we classify it into two
categories, PoP(Probability of Precipitation) and QPF(Quantitative
Precipitation Forecast). PoP and QPF have very little to do with each
other. A high PoP does not represent high QPF. However, high QPF
sometimes relates to a high PoP.
First, let's look at PoP. PoP is the probability that precipitation
will be reported at a certain location during a specified period of
time. Wunderground takes various numerical weather prediction models,
such as GFS(Global Forecast System), GEM(Global Environmental
Multiscale) and other regional models, and makes a consensus forecast
for Pop. If numerous models predict precipitation, the higher the PoP
Next, what is QPF? A spatial and temporal precipitation forecast that
will predict the potential amount of future precipitation for a
specified region, or area. Currently, Wunderground uses NDFD(National
Digital Forecast Database ) for US cities and GFS for international
cities. We don't have any data for the measure precision at this point
The quality of the forecast is usually good during the first 72 hours.
The error gets larger farther into the future. Both radar and
satellite data are integrated into the model data assimilation to
improve its analysis and forecasts. Also, convection effect is taken into account by the model.