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Re: Nice Weather We're Having



On Wed, 8 Mar 2000, David T. Marx wrote:

[snip]
Local meteorologists often express their predictions in percentages,
"today's forecast - 40% chance of rain." To me, this means that for the given
conditions, 4 out of 10 such days, it will rain. Does anyone know how they
really estimate the likelihood?

The POP or "probability of precipitation" forecast is NOT obtained from
the computer simulation. Forecasts are NOT made by comparing current
conditions with similar conditions seen in the past. A 40% POP does NOT
mean that the meteorologist is only 40% confident in the forecast. The
percentage is only created, as far as I can tell, in response to the
public's desire to have a probability forecast. The public doesn't want to
hear something as vague as "scattered showers".

In other words, it is the meteorologist's subjective guess (based upon
experience and understanding of the physics) for how likely you will get
wet given the meteorological conditions forecasted. For example, when
there are isolated storms or showers, it can be difficult to determine
when and where these showers may develop because the scale of the showers
is smaller than the resolution of the observation network on which the
prediction is based. The forecast might also depend on uncertainties in
the observations. The POP is then used to indicate the uncertainty.

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| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
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