What the hinky hell is "probability of precipitation"?
My fine friends,
For decades, weather forecasts have included "probability of precipitation," which is expressed as a percentage. Today, for instance, there is a 30% chance of storms. Same percentage tomorrow (Wednesday). On Thursday and Friday, though, it's 60% during the day and 50% at night. More of the same percentage horseshit over the weekend.
(Expect temperatures to reach above 90 degrees Tuesday, Wednesday, and Thursday. We won't really see a break in the humidity until after the Independence Day holiday.)
Further complicating matters in forecasting precipiation are terms like "scattered," "isolated," "widespread," "severe," and "strong" when referring to storm potential.
Fear not: Uncle Monsoon is here to sort it all out for you.
(Wait. That sounds creepy, somehow. Let me try that again.)
Fear not: your special weather friend Monsoon is here to make everything al
(Even creepier.)
Fear not: I will explain.
First, the percentages. A 30% chance of storms doe not mean that it will storm 30% of the time. It also does not mean that 30% of the places in the forecast area will see storms.
Instead, the Probability of Precipitation (PoP) describes the chance of precipitation occurring at any point in the forecast area. It's a mathematical formula:
PoP = C x A (where C = the confidence that measurable precipitation will fall somewhere in the forecast area, and A = the percentage of the area that will receive measurable precipitation, if it gets any at all)
So, for example, with thunderstorms, a forecaster typically knows that they are going to occur (A = 100) but determines that two-fifths of the forecast area will receive precipitation. This is typical with pop-up thunderstorms (those that are spotty and arise on hot, humid days and unstable atmosphere) rather than storms associated with a cold front, which are much more likely to strike a larger area. In the example, then, the PoP is 40%.
Let's say that a forecaster is only half sure that precipitation will occur at all, making A = 50. But the forecaster believes that if it does rain, it will happen in 60% of the coverage area. So that would make PoP = .5 x .6 = 30%.
An more confusing explanation of this percentage is as follows: a 60% PoP means that if the weather conditions theoretically occurred ten times, it would rain six out of those ten times.
The bottom line: when there is any chance of thunderstorms expressed in the forecast and your plans include being outside at any point, keep an eye on the Doppler radar. Light green is light rain; medium rain is medium rain; dark green is chubby rain. Yellow is a deluge. Orange is a downpour. Light red is a storm. Dark red is a heavy storm. Pink is a really, super bad storm. Purple and violet indicate possible tornadic activity, so don't shit around with that. Get in the basement or a windowless room until it blows over.
The best radar, I think, is:
http://www.intellicast.com/National/Radar/Current.aspx
A quick note about the terminology you'll hear from meteorologists about the likelihood of rain or storms (isolated, scattered, widespread, etc.). Here's how to sort it all out:
Isolated means less than 15% of the forecast area will see this kind of weather.
Widely scattered means 15-24% will see it.
Scattered means 25-54%.
Numerous or widespread means 55 to 70% chance that the area will see this kind of weather.
Prevalent means 71-72%
Pervasive means 73-75%.
Ubiquitous means 76-85%.
Omnipresent means 86-95%.
Fully rife means 96-100%.
Alright, I may have made up the last five. But the other ones are spot-on.
In future installments here at http://monsoonmartin.squarespace.com, we will tackle the difference between "partly cloudy" and "partly sunny" (and other permutations denoting cloud cover) as well as the difference between "showers" and "rain."
Stay tuned!