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How To Speak "Forecasting"

Understanding Forecast Terminology

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Most of us consult our local weather on a daily basis and have done so since memory serves. But when it comes down to it, do we really hold an accurate understanding of the information presented to us? Put your knowledge of these 3 weather variables to the test.

1. Probability of Precipitation (PoP)

Next to temperature, precipitation is perhaps the most important of weather variables; but what exactly does the phrase “chance of precipitation” mean? If there’s a 30% chance of precipitation for the day, does this means there’s a 30% chance that it will rain and a 70% chance that it will not? Will precipitation fall 30% of the day? The answer lies within the realm of probability.

Probability: “Probability” means the chance, or likelihood, that an event will occur. In forecasting, probability of precipitation is a two-part concept which includes:

  • The percentage chance that a measurable amount of precipitation will fall at any location within a forecast area during a specific forecast period
  • How much of an area the precipitation (should it occur) will affect

Table 1 gives a list of PoP ranges and equivalent terms often used in media forecasts. Notice that for 80-100%, the forecast has no qualifying expression; this is because precipitation is considered imminent at such high probabilities. For these events, expect to see terms which express expected duration, such as: brief, periods of, occasional, intermittent, and frequent.

Applying these expressions to the example of a 30% PoP above, the forecast could read in any of the following ways (they all mean the same thing):

  • A 30% chance of showers
  • A chance of showers
  • Scattered showers

The appropriate interpretation of this forecast is:
There is a 30% chance of precipitation at any one point within your forecast area, and a 70% chance that no measurable precipitation will fall at any location within the forecast area.

Precipitation Intensity: If precipitation is included in your local forecast, be sure to look for the following terms which give an indication of rainfall rates:
  • Very Light: Expect less than 0.01 inch hourly
  • Light: Expect 0.01 to 0.1 inch hourly
  • Moderate: Expect 0.1 to 0.03 inches hourly
  • Heavy: Expect over 0.3 inches hourly

2. Sky Condition/Cloud Cover

The presence (or lack thereof) of clouds determines how much of the sun’s energy reaches the Earth’s surface. For example, opaque, or dense clouds, block sunrays while wispy cirrus clouds allow heat to penetrate and warm the atmosphere. (In addition to interacting with the sun during daytime hours, clouds also influence nighttime temperatures through the processes of radiative heating/cooling.)

Sky condition is reported by visually dividing the sky into eighths, estimating the amount of sky obscured by opaque clouds, then expressing this number as a fraction of eight. (Table 2)

3. Winds

Wind measurements always include speed and direction. Remember: direction is based on the four cardinal and intermediate points of the 8-point compass rose and always describe where winds are blowing from -- not blowing to! (Table 3)

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