Okay, I always thought this meant pretty much what it says - basically, that this forecast is stating that there is a 30 % chance that it will rain (or snow) in my area.
The other day the meteorologist on our provincial news said that this ACTUALLY means if there was a grid of 100 squares placed over the area, there would be precipitation in 30 of those squares.
I understand the concept, but … it just doesn’t sound right.
Can someone clear this up for me?
Thanks tj!
Grrr - guess I should learn how to Search the archives better.
I THOUGHT the weatherguy was on crack, now I KNOW he was!
The NOAA seems to agree with your met man (and thus to disagree with Cecil, as cited by tj). This is from www.crh.noaa.gov:
“Technically, the Probability of Precipitation (PoP) is defined as the likelihood (expressed as a percent) of a measurable precipitation event at any given point within the area covered by the zone forecast during a specified time.”
Xema, can you post a direct link to that definition? I can’t seem to find it.
The part you quoted seems to agree with Cecil more than the OP’s meteorologist, but it’s hard to tell.
The NOAA definition is here (partway down the page): www.crh.noaa.gov/lmk/general.htm
I’m not sure the language is completely clear. But it seems to me that the NOAA definition is saying that PoP refers to the chance of rain at any one point in the area covered by a forecast. Thus, “30% chance of rain” translates as “about 30% of the area covered by our forecast will measure some rain.”
The alternative meaning is “on about 3 of 10 days with this kind of weather, rain will fall somewhere in the county” (forecast areas are often counties). This seems as if it would lead to much higher percentages – a brief shower in the NE corner of the county counts the same as a drenching rain that blankets the entire area.
Well the alternative meaning is correct.
Though I do love the Pacific Northwest Interpretation on Cecil’s article…
There was a prior thread on this subject, and Cecil was wrong. Take a look at the radar maps the next time a certain percentage chance of showers is forecast, and you will see that is the percentage of the area covered by showers that is moving into your area.
A 30% chance of showers means that showers will occur in 30% of your geographical area. This may mean that there is a 3 in 10 chance you will get rain, or it may mean that you will get rain 30% of the time.
Sometimes, especially here along the SE coast, the radar shows no rain at all, but a 30% chance will be predicted for the afternoon. This is based on certain meteorological factors that will provide a 30% chance of showers for the area, such as midday heating and resulting convection. If there are other favorable conditions, such as easterly winds or a disturbance in the ocean, the percentage will be raised to 40% or so. Hence a forecast of 30% or 40% can be based on factors that will provide this chance of rain later in the day.
So the meteorologist in your provincial news was correct.
[QUOTE]
*Originally posted by barbitu8 *
**There was a prior thread on this subject, and Cecil was wrong. **
[/quote[/]
Cecil wrong? Arooo?
I think it just means there is a 30% chance of rain anywhere in the forecast area. If anyone is out there forecasting for large areas then I want their job.
I could do the forecast for the US: Rain, with sunshine, too, and seasonal temperatures for most of the country. 
30% chance of rain = 70% chance it won’t
(Hope this helps…
)
The third sentence there does not follow logically from the second.
The definition is given as the “likelihood (expressed as a percent) of a measurable precipitation event at any give*(sic)* point within the area covered by the zone forecast during a specified time.” Any given point. So if there is a 30% chance of rain, there is a 30% chance that rain will fall at any given point within the zone.
I’m going to have to stick with Cecil on this one. I’m going to search for the previous thread and post a link here if I find it.
It does not conclude that Cecil is wrong. Someone suggests it, but fails to back the allegation with any sound argument.
No matter how the forecast is derived, 30% chance of rain means that 30% of any given area will receive rain, which means that if you divide the given area into 100 grids, 30 of them will get rain, as the meteorologist said. This also means that it may rain 30% of the day for any given area since the probabilities are the same.
Cecil puts this forecast solely on history: past conditions, performances, etc. However, the Weather Bureau is not an almanac. It derives its forecasts from current meteorological conditions, and also has half a dozen computers to help them predict the weather. There is a certain element of past conditions involved. For example, it is known that the midday heat will cause air to rise (convection) and this will tend to produce pop-up showers if other conditions are present. It knows this from past performances. But then any science can be based on past performances since theories and hypotheses are supported by actual events. However, meteorology is a science not an almanac.
Right. “30% chance of rain at any given point” seems to be saying that 30% of the points will get rain and 70% of them won’t. If the wording were “at some point within the area” or “somewhere in the area” then I’d draw a different conclusion.