This 10 part series will detail forecasting tricks that can be used to try to outforecast MOS. Outforecasting MOS is an important skill for a forecaster. MOS stands for Model Output Statistics and they are used as a guide for temperature prediction and precipitation prediction by forecasters. Model consensus is the average of the high temperatures, low temperatures or precipitation amount predicted by several forecast models.
PART 7: DON'T FRET OVER LOW POP AND LOW QPF
The models will often throw in some low POP (Probability of Precipitation) especially when there are clouds in the model forecast. When precipitation is very unlikely but clouds are likely you will often see 6 and 12 hour POPs in the single digits or teens. The advice is to not fret over these numbers. When the POP is less than 30% it is very unlikely it will precipitate. After looking over other data you may conclude to not even put any chance of precipitation in the forecast at all. If you notice the QPF (Quantitative Precipitation Forecast) is 0 and there is a POP value given such as 12% according to the model, then that is a clue the precipitation threat is very minimal. This is especially true if there is no instability and thus no chance for isolated afternoon thunderstorms.
If there is instability then you may want to include a chance for isolated thunderstorms when the POP and QPF are low (but not too low on POP). The models do not predict isolated thunderstorm convection very well. The model will spread the rain over the forecast area and decrease the QPF amounts since the model has no way of knowing exactly where the thunderstorms will develop. In reality, most of the forecast area will end up with 0 QPF while isolated areas can get quite a bit of rainfall if the thunderstorms develop.
Model forecasted QPFs of 0 (no precip), 1 (less than 10th inch) and the low end of 2 (10th to a quarter inch) are not very significant amounts. When these type QPF numbers occur with a POP of less than 40% it is very unlikely it will rain. However, you must look at all data available to make certain (all models and all weather data). Rules of thumb can always bust and the model may not be picking up on the precipitation event as well as it should. Again, if there is instability then that is a clue it could be an isolated or scattered thunderstorm situation.
When both the POP gets over 40% and the QPF is 3 or higher on several models, then a significant precipitation event is looking much more favorable.