|FORECASTING TRICK SERIES:|
DON'T FRET OVER LOW POP AND LOW QPF
METEOROLOGIST JEFF HABY
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.