# Presentation: 734896

## POP-ClimoTrainingNational Weather Service Western Region

- July 2006
- Developed by Western Region POP-Climo Team under direction from the WR Digital Service Management Team

## Section 1:  Goals of Training

- Improve the accuracy and usefulness of WR NDFD POP grids through the application of
- probabilistic forecasting
- verification feedback
- climatological POPs.

## Section 2:  Our Current Situation, Is It Cause For Concern?

- National NDFD verification stats indicate that a significant dry bias exists in Western Region by day 5.

## Section 2:  Our Current Situation, Is It Cause For Concern?

- A human low-POP bias is contributing to lower scores for the NDFD grids, especially for longer forecast ranges.
- Our partners and customers are not well served when a recognizable and correctable bias is not removed from one of our forecast systems
- Through the correct use of climatological PoPs and MOS, this low-PoP bias can be significantly reduced or even removed.

## Section 3:  What Is A POP?

- 12 hour NDFD POP (NWS Directive 10-506 Digital Data Products/ Services Specification)
- 12-hour Probability of Precipitation (PoP12) is the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch) at a grid point during the valid period. The 12-hour periods in NDFD begin and end at 0000 and 1200 UTC.

## Section 3: What Is A POP?

- Climatological POP
- Represents a long term average POP, which includes both wet and dry periods or upper level ridges and trough patterns.  Conceptually, it can be compared to a long term temperature average.

## Section 5:  The Transition From Deterministic To Probabilistic Forecasting

- As lead time increases, probabilities:
- tend to be less extreme
- the use of the very low and especially the very high values is less frequent
- Eventually (5-10 days) the range has shrunk to the single value of the _**climate**_ frequency or _**climate POP**_.

## Section 5:  The Transition From Deterministic To Probabilistic Forecasting

- One effective way to estimate the uncertainty for a given forecast, and the forecast period past which it is no longer reasonable to make forecasts deterministically, is to use ensembles.
- Ensemble forecasting yields information about the magnitude and nature of the uncertainty in a forecast (Wilks, 1995).
- For more information about the best way to use ensemble forecasts, visit the following excellent training sites:
- _[http://www.hpc.ncep.noaa.gov/ensembletraining](http://www.hpc.ncep.noaa.gov/ensembletraining)_
- _[http://meted.ucar.edu/nwp/pcu1/ensemble](http://meted.ucar.edu/nwp/pcu1/ensemble/)_

## Section 6:  What Is A Good POP?

- Forecasters _should_ strive for POPs that are accurate and unbiased
- So far as the scientific problem of weather forecasting is concerned, the forecaster’s duty ends with providing accurate and _unbiased_ estimates of the probabilities of different weather situations. (Brier, 1944, Hughes, 1980)
- POPs _should_ reflect both uncertainty and knowledge about future weather so customers can make appropriate decisions based on risk. (Wilks, 1995)
- POP grids _should_ reflect weather conditions, e.g., elevation differences should be much more distinct during convective or upslope precipitation events and less distinct during stratiform periods.
- Instead of trending towards 0% during a _period of forecast uncertainty_ in the longer term forecasts, trend towards climatology (unless climatology is represented by zero).  This does not mean simply populate directly with climatology, but as a rough starting point.  Using collaboration with surrounding offices, trend above climo if the models are hinting at a long wave trough, or below climo if the models are hinting at a ridge.

## Section 7:  Final Recommendations  

**Use Climatological Pops as a “first-guess field” when there is uncertainty, especially in the longer range forecast period.**
**The range of usable POP values narrows with forecast time, eventually shrinking to the climatological POP at some time in the future.**
**Use references to Climatological PoP grids in 12Planet chats to make collaboration more efficient.   **
**MOS and Climatological POPs are not a perfect forecast.  However, significant deviations in the longer term should be limited to known model bias periods and other known local confidence factors.   **
**Ensemble forecasts provide a means to help determine confidence.**
**Know your skill, verify your forecasts, adjust accordingly.  **