[wxqc] Rain Forecast

[Victor Engel]

On 9/14/06, Evan Bookbinder <Evan.Bookbinder at noaa.gov> wrote:
>
> Victor,
>
> In terms of the NWS, the zone forecasts imply the probability that 0.01"
> (measurable) precipitation will occur in a 12 hour time frame covering
> that period (today, tonight, etc...). That time frame is understood to
> be 12Z to 00Z and 00Z to 12Z.
>
> With respect to the graphical forecasts used in the link below, those
> are 6 hourly pops covering time periods 00Z to 06Z, 06Z-12Z, 12Z-18Z,
> and 18Z-00Z. You'll notice the numbers remain constant in those 6 hour
> blocks. Again, the forecast is the chance that 0.01" (measurable)
> precipitation will be observed at that location in that 6 hour window.


OK. Thanks. That certainly changes things when the period of time covered is
different.


With respect to your calculations below, this is incorrect. This is a
> matter of statistics equations (which I unfortunately don not have handy
> at the moment). You're assuming the events are cumulative, which they
> are not.


I'm not sure what you mean by cumulative. Certainly you don't add them up.
That's not what I was doing.

Each 6 hour window is mutually exclusive from other events. In
> other words, the chance of rain in a 24 hour or 48 hour period is not
> the highest 6-hour number, not a sum of 6 hour numbers, nor an average
> of them.


Right. It's the formula I used. Think of it as the odds that it will NOT
rain. Then you can simply multiply the odds. If there is a 50% chance (0.5)
it will rain today and a 60% chance (0.6) it will rain tomorrow, then the
odds it will rain on either day is equal to 100% minus the odds that it
won't rain at all.

The odds that it won't rain at all is just the product of the odds it won't
rain on each day, or 50% times 40% (20%). So there is an 80% chance it will
rain on one of the two days. This is straightforward probability.

Victor
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