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Hi Branson,
I have a different understanding, but we may be thinking of different terms of art.
Dueling meteorologists:
METEOROLOGIST JEFF HABY
POP is the probability of any particular point location within a forecast area receiving measurable precipitation in a given time period. POP is determined by two parameters: The probability that any precipitation will occur in forecast area AND predicted areal coverage of precipitation if precipitation does occur. Thus, areal coverage is just one aspect of POP. The chance that any measurable precipitation will occur in the first place within the forecast area must also be considered. When referring to POP it is most accurate to say, "There is a ___% POP that any particular measurement station in the viewing area will get precipitation". For example, when averaged over many 30% POP days, a particular station should have precipitation 30% of the time if the forecaster or computer model is accurate.
Suppose during the course of a year that a Dallas measurement station has a 30% POP on 100 individual days. If the POP prediction is fairly accurate over the long term, it should have precipitated at the Dallas station on about 30 of those days. Thus, think of POP in terms of a long term average prediction. All to often, people assume a 20% chance of rain means 20% of forecast area will get rain on that one day. This is often not the case for any one day. Often no precipitation occurs at all in the forecast area on days with a 20% POP since one component of POP is the chance that any precipitation will develop.
The ultimate weather education website: http://www.theweatherprediction.com/
WRAL to which I have been referring is quoting POP.
I had another way of interpreting it than your way or POP so I am glad to have looked up what is really meant.
My working definition was that the percentage was not "areal," but strictly a probability. I would say, "the percentage is that given the predicted conditions there is this chance of precipitation somewhere - anywhere - in the area of the forecast.
It looks like POP is a combination of your way and mine. I guess there is some formula and a swag, but "knowing" the definition, it seems more nebulous than before. The whole bit is conflated by the last statement of the first paragraph - you only really get 30% after they make a series of right and wrong predictions and they average out - if the forecaster and models are accurate - otherwise is is more shit than shinola.
Anyway I am packing a rain coat in case I get cold. I will be just as wet in the coat as out due to sweat. I am taking tights too.
I got caught out in a storm 2 weekends ago and got pretty cold even though it was a hot day. It is a pain in the butt, but to enjoy the ride I need to be not too hot and not too cold :-(
M
--
Michael Ross
=================================
Cycling in Central North Carolina
Schwinn Voyageur 11.8
Linear LWB, Greenspeed GTO, BikeE CT, AT
I have a different understanding, but we may be thinking of different terms of art.
Dueling meteorologists:
POP is the probability of any particular point location within a forecast area receiving measurable precipitation in a given time period. POP is determined by two parameters: The probability that any precipitation will occur in forecast area AND predicted areal coverage of precipitation if precipitation does occur. Thus, areal coverage is just one aspect of POP. The chance that any measurable precipitation will occur in the first place within the forecast area must also be considered. When referring to POP it is most accurate to say, "There is a ___% POP that any particular measurement station in the viewing area will get precipitation". For example, when averaged over many 30% POP days, a particular station should have precipitation 30% of the time if the forecaster or computer model is accurate.
Suppose during the course of a year that a Dallas measurement station has a 30% POP on 100 individual days. If the POP prediction is fairly accurate over the long term, it should have precipitated at the Dallas station on about 30 of those days. Thus, think of POP in terms of a long term average prediction. All to often, people assume a 20% chance of rain means 20% of forecast area will get rain on that one day. This is often not the case for any one day. Often no precipitation occurs at all in the forecast area on days with a 20% POP since one component of POP is the chance that any precipitation will develop.
The ultimate weather education website: http://www.theweatherprediction.com/
WRAL to which I have been referring is quoting POP.
I had another way of interpreting it than your way or POP so I am glad to have looked up what is really meant.
My working definition was that the percentage was not "areal," but strictly a probability. I would say, "the percentage is that given the predicted conditions there is this chance of precipitation somewhere - anywhere - in the area of the forecast.
It looks like POP is a combination of your way and mine. I guess there is some formula and a swag, but "knowing" the definition, it seems more nebulous than before. The whole bit is conflated by the last statement of the first paragraph - you only really get 30% after they make a series of right and wrong predictions and they average out - if the forecaster and models are accurate - otherwise is is more shit than shinola.
Anyway I am packing a rain coat in case I get cold. I will be just as wet in the coat as out due to sweat. I am taking tights too.
I got caught out in a storm 2 weekends ago and got pretty cold even though it was a hot day. It is a pain in the butt, but to enjoy the ride I need to be not too hot and not too cold :-(
M
On Fri, Jul 18, 2008 at 8:55 AM, Branson <bullcitybiker@...> wrote:
That's a beautiful twist to a popular route.
A less than 50% chance of is noteworthy because even a 30% chance is
still a good chance. If I had a 30% chance to win the lottery, I'd be
plunking down some money and fast.
A meteorologist I work with also explained it this way: think of it as
30% of the area will have a 100% chance of measurable precipitation.
They just can't say it that way in their forecasts.
Be safe, have fun, and hope you win the Full Moon Lottery!
Branson
--- In ncrandoncyclists@yahoogroups.com, "Michael Ross"
<michael.e.ross@...> wrote:
> I am not sure why a less than 50% chance of rain is noteworthy,
> but the various weather sources treat it so...
--
Michael Ross
=================================
Cycling in Central North Carolina
Schwinn Voyageur 11.8
Linear LWB, Greenspeed GTO, BikeE CT, AT
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