Enter the number of ensemble members / forecast runs predicting rain and the total number of ensemble members used into the calculator to estimate a model-vote rain probability. (Note: simply counting different named models like GFS/ECMWF as “votes” is not an official Probability of Precipitation method because models are not independent or equally likely.) This calculator can also evaluate any of the variables given the others are known.
Rain Probability Formula
The following formula is used to calculate an ensemble-member (model-vote) rain probability estimate when you have T equally weighted ensemble members and N of them predict rain for the same location, time period, and precipitation threshold. Variables:
RP = (N / T) * 100
Variables:
- RP is the ensemble-member rain probability estimate (%)
- N is the number of ensemble members (or forecast runs) predicting rain
- T is the total number of ensemble members (or forecast runs) used
To calculate this ensemble-based estimate, divide the number of ensemble members predicting rain by the total number of ensemble members used. Multiply the result by 100 to convert the fraction to a percentage. This gives the fraction of ensemble members indicating rain for that location and time period. Public “chance of rain” forecasts are not always computed this way; many operational forecasts describe Probability of Precipitation (PoP) using PoP% = (C% × A%)/100, where C is forecast confidence and A is areal coverage (see the PoP Builder tab).
What is a Rain Probability?
Rain probability, often called the Probability of Precipitation (PoP) or “chance of rain,” describes the likelihood that a given location will receive measurable precipitation during a specified forecast period. It is usually expressed as a percentage. For example, a 50% PoP means there is a 50% chance of measurable precipitation at your location during that period (not that it will rain for 50% of the time, and not that it will rain over 50% of the area). In some contexts, an “ensemble probability” is estimated by the percentage of ensemble members predicting precipitation, which is what the Basic tab calculates.
How to Calculate Rain Probability?
The following steps outline how to calculate the Rain Probability (RP) using the given formula:
- First, determine the number of ensemble members (or forecast runs) predicting rain (N) for the same location, time period, and threshold.
- Next, determine the total number of ensemble members used (T).
- Next, use the formula RP = (N / T) * 100 to calculate the rain probability estimate.
- Finally, insert the values of N and T into the formula and compute RP.
- After calculating the result, check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge:
Number of ensemble members predicting rain (N) = 5
Total number of ensemble members used (T) = 10
Rain probability estimate (RP) = (5 / 10) × 100 = 50%
