The "Climatology" Technique

• a forecast that is based on "climatology" or average weather
• example:
  • Lets say the climatological records show that it rains only 1 day out of 100 during the summer months in LA.
  • Then, if you forecast "no rain" for any day in July and August, the probability you will be wrong is 1/100.
  • Therefore, the probability you will be right is about 99%
• NOTE: precip forecasts are usually given as probabilities
  • ex: "there is a 60% chance of rain today"
• this means that there is a 60% chance of measurable precip at any random place in the forecast area

Accuracy and skill in forecasting

• What is an accurate forecast?
  • your forecast for tonights min temp is 0°F
  • if the actual min was 1°F, is in inaccurate?
  • if the actual min was 10°F, is in inaccurate?
  • Accuracy (in forecasting) is arbitrary and relative - it's not clearly, objectively defined
• How does a forecast show skill?
  • must be more accurate than utilizing persistence or climatology
  • persistence is "hard to beat" on a time frame of minutes to hours
  • skill is often shown on time frame of hours to a few days
  • Beyond 10 days, forecasts are generally no better than those generated by climatology

QUESTION FOR THOUGHT:

Climatology shows that the skies over Vancouver, Canada are cloudy 327 days a year. If you were to forecast cloudy skies for every day of the year, how accurate will your forecasts be? Do your forecasts show skill? Explain.