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John Moosie
climber
Beautiful California
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Topic Author's Original Post - Jan 22, 2009 - 08:44pm PT
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This morning I woke up and it was raining. It had been raining all night. I look at the forecast for Yosemite, and it said 80 percent chance of rain. It says 50 percent chance of rain tonight. It is still raining. The radar looks like it will clear out fairly soon.
My question is, why 80 percent. The mornings forecast was posted at about 7 am. It was raining. It had been raining for a number of hours.
Can someone explain the percentages?
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Toker Villain
Big Wall climber
Toquerville, Utah
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Jan 22, 2009 - 08:54pm PT
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Forecasts are often not locally based but generated from computer models that process multiple data sources.
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TGT
Social climber
So Cal
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Jan 22, 2009 - 09:06pm PT
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My understanding (might be mislead) is that the % number is the predicted coverage for a given forecast area and probability, so 80% means that most of a given forecast area has a high probability it will get significant rain. 20 % of that area could still get nothing.
A combination of both probability and coverage of a given area.
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Reilly
Mountain climber
Monrovia, CA
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Jan 22, 2009 - 09:07pm PT
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"why 80 percent"
'cause that's what it said on the bottle he was hittin' off of
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Captain...or Skully
Social climber
North of the Owyhees
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Jan 22, 2009 - 10:07pm PT
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Uh, They don't have windows at the weather center........
Maybe.
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John Moosie
climber
Beautiful California
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Topic Author's Reply - Jan 22, 2009 - 10:17pm PT
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Thats what I thought too skully. haha. I use to work on an orchard and we had a contract with a meteorologist. He said he would always go outside before making his forecast for the day.
I think Piton and TGT have it basically. It is a computer giving a percentage over a given area. What is strange is that Yosemite has multiple data points for rainfall totals and it seems like a computer would have read these and said.. aha... its raining there. Ipso facto.. 100 percent chance of rain today.
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hagerty
Social climber
A Sandy Area South of a Salty Lake
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Jan 23, 2009 - 01:01am PT
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A probability of precipitation or POP of 40% doesn't mean that it will rain 40% of the day, or that rain will fall on 40% of your area or that it rained 40 times out of 100 on that particular day in the past (that is a climatological forecast). It also says nothing about how much rain or snow will occur.
A POP of 40% means that the forecasters have calculated that in 100 similar weather situations, rain has fallen 40 times in the forecast area. POP is for any point in your forecast area, not the whole area. So, for instance, a POP of 90% for rain means that 9 times out of 10 when this weather situation is predicted, you ought to get rain somewhere in the forecast area, e.g., at your home, playground or at the airport.
Mathematically, PoP is defined as follows:
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measurable precipitation, if it occurs at all.
So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)
But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )
In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.
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