4 ms·

50% means 50% of models predict rain. They run multiple models (- dozen roughly) and.probability of rain is thr mix of all models.

by guepe 4y ago

maxerickson 4y ago
NWS predicts precipitation amounts in addition to probability, by the hour, I don't think they are just reporting what a set of models predict. https://forecast.weather.gov/MapClick.php?lat=40.6924&lon=-89.588&unit=0&lg=english&FcstType=graphical https://forecast.weather.gov/MapClick.php?lat=40.6924&lon=-8... (the narrative forecasts are more compact than the hourly forecast, but it seems like they would at least have overlapping methodology)
seadan83 4y ago
That is a common misconception. Chance of rain = (chance you will be somewhere in the forecast area where it is raining) x (chance it will rain at all) [1] The two are independent variables, the overall probability of two independent events both occurring is the product of either event occurring. The chance of rain is the product of two independent events. So, 50% could mean a 50% chance it will rain everywhere, or a 100% chance it will rain in 50% of the forecast area, or it could be anything in between. (Eg, a 1 mile square cloud that is dropping rain travelling through a 10 square mile area, or a 10% chance that a rain forming cloud will develop over an entire area, or anything in between is a 10% chance of rain. The first case can often be observed with radar and visually, an hourly forecast can be very accurate to know that one cloud will still be raining in an hour. Hence the size of rain area and size of forecast area are variables that come into play) As a few counterpoints: - Multiple models do not mean each model is equally likely. Nor do multiple models mean we cover the set of all possibilities. For chance of rain to be the fraction of models predicting rain, those two statements would have to be true - models themselves each give a chance of rain. These different values are not combined in any way, forecasters pick a model Some implications and interesting points about forecast area: - if a forecast area is "zoomed" out, should that alter the chance of rain? [Yes, this implies that chance of rain has forecast area as a dependent variable]. Hence, if you "zoom in" on a forecast area far enough, the chance of rain developing becomes the dominant variable for chance of rain. One interesting thing is a forecast assumes equal probability that you will be at any given location within that forecast area. Which means the percent area forecasted to receive rain is the same probability that you will be at any given location within a forecast area receiving rain [1] https://www.discovery.com/science/chance-of-rain https://www.discovery.com/science/chance-of-rain
- deleted 4y ago
  [deleted]
- ec109685 4y ago
  The national weather service simplifies it: https://www.weather.gov/ffc/pop https://www.weather.gov/ffc/pop They pick a point and base it on the chance of rain there.

50% means 50% of models predict rain. They run multiple models (- dozen roughly) and.probability of rain is thr mix of all models.

by guepe 4y ago

50% means 50% of models predict rain. They run multiple models (- dozen roughly) and.probability of rain is thr mix of all models.

maxerickson 4y ago

NWS predicts precipitation amounts in addition to probability, by the hour, I don't think they are just reporting what a set of models predict. https://forecast.weather.gov/MapClick.php?lat=40.6924&lon=-89.588&unit=0&lg=english&FcstType=graphical https://forecast.weather.gov/MapClick.php?lat=40.6924&lon=-8... (the narrative forecasts are more compact than the hourly forecast, but it seems like they would at least have overlapping methodology)

seadan83 4y ago

That is a common misconception. Chance of rain = (chance you will be somewhere in the forecast area where it is raining) x (chance it will rain at all) [1] The two are independent variables, the overall probability of two independent events both occurring is the product of either event occurring. The chance of rain is the product of two independent events. So, 50% could mean a 50% chance it will rain everywhere, or a 100% chance it will rain in 50% of the forecast area, or it could be anything in between. (Eg, a 1 mile square cloud that is dropping rain travelling through a 10 square mile area, or a 10% chance that a rain forming cloud will develop over an entire area, or anything in between is a 10% chance of rain. The first case can often be observed with radar and visually, an hourly forecast can be very accurate to know that one cloud will still be raining in an hour. Hence the size of rain area and size of forecast area are variables that come into play) As a few counterpoints: - Multiple models do not mean each model is equally likely. Nor do multiple models mean we cover the set of all possibilities. For chance of rain to be the fraction of models predicting rain, those two statements would have to be true - models themselves each give a chance of rain. These different values are not combined in any way, forecasters pick a model Some implications and interesting points about forecast area: - if a forecast area is "zoomed" out, should that alter the chance of rain? [Yes, this implies that chance of rain has forecast area as a dependent variable]. Hence, if you "zoom in" on a forecast area far enough, the chance of rain developing becomes the dominant variable for chance of rain. One interesting thing is a forecast assumes equal probability that you will be at any given location within that forecast area. Which means the percent area forecasted to receive rain is the same probability that you will be at any given location within a forecast area receiving rain [1] https://www.discovery.com/science/chance-of-rain https://www.discovery.com/science/chance-of-rain

deleted 4y ago
[deleted]
ec109685 4y ago
The national weather service simplifies it: https://www.weather.gov/ffc/pop https://www.weather.gov/ffc/pop They pick a point and base it on the chance of rain there.