PoPs and the population complication
The WBRZ Weather team works all year long to pinpoint and explain those pesky PoPs-as they are such a crucial forecast aspect of often showery South Louisiana summer afternoons.
There are several ways to interpret PoPs, only a few are correct, but now we'll introduce a new wrinkle to the PoP puzzle.
To review, the National Weather Service defines a PoP as "the probability of precipitation for a given point over a specified time" (usually one day or 24 hour period). Let us examine that definition in a number of ways. When a broadcast meteorologist issues a 30% chance of rain on a given day, they believe there is a 3/10 or 30% chance rain will fall at your home. Another way to interpret a PoP is geographically-30% of the local "viewing area" will collect measurable rain for the same forecast. One way to check or verify this is by examining the actual rainfall totals that occurred on that day (see attached image).
NOTE: A common misconception is that the PoP percentage indicates for how much of the day it will rain. A 30% PoP DOES NOT MEAN it will rain for 30% of the day. In fact, 30% is often the type of percentage associated with a pop-up afternoon shower or thunderstorm with much of the day finding some sunshine and dry conditions. Even a 100% PoP could mean that the day will be sunny until a line of thunderstorms passes through in the afternoon bringing rain to all. If the meteorologist is doing a thorough job, this should be explained in the forecast on a number of platforms.
An example of this very misconception can be noted in the attached Facebook screenshot taken just over two weeks ago. Especially in this age of social media, information can spread rapidly and easily become confusing. Weather forecasts are under more intense scrutiny and at times, unjustifiably blasted due to a data consumer's misinterpretation.
The added wrinkle, to this already tricky PoP challenge for meteorologists, is population.
Using WBRZ for instance, the "viewing area" consists of East Baton Rouge, West Baton Rouge, Ascension, Assumption, East Feliciana, Iberville, Livingston, Pointe Coupee, St. Helena, Tangipahoa, St. Mary and West Feliciana Parishes along with Amite, Pike and Wilkinson Counties in South Mississippi. On Monday, the forecast called for a 50% chance of rain. Examining the rain totals graphic, geographically speaking, between 30-40% of the area received measurable rainfall-not a slam-dunk of a forecast, but not a major bust either.
But the population factor complicates the forecast's verification. The areas that received rain have an approximate population of 87,000 in a viewing area of over 856,000. That means only about 10% of WBRZ viewers saw rain, making "bust" a seemingly much more appropriate adjective for the forecast. Expanding upon this, with a population over 440,000, East Baton Rouge Parish contains half of the WBRZ viewership that may interpret whether or not a forecast was validated. This large number of residents is in only about 10% of the geographical area to which WBRZ weather forecasts are directed. It's a forecasting headache dealt with by meteorologists everywhere. However, when it comes to life-altering natural events such as weather, all must be considered. A forecaster cannot simply ignore a rural Parish because there are fewer viewers there.
The blatant truth is when the mode of showers and thunderstorms is pop-up, isolated or even scattered in nature, the science of meteorology cannot yet predict exactly where it will rain more than 2-3 hours in advance.
A 7-day forecast graphic, a weather bulletins board or even one forecast model snapshot does not tell the entire weather story. A broadcast presentation, online stories, blogs and posts to Twitter, Facebook and Google+ offer up the important details of exactly how those PoPs will affect your day on an hour-by-hour basis. That being said, Monday's explanation of 50% PoPs proves that with access to the right information, the dreaded weather percentages don't have to mean complication for the population.
You can get forecasts from Meteorologist Josh Eachus weekdays on 2une-In from 5-7am and News 2 at Noon from 12-1pm. Additionally, you can get the fastest and latest forecasts and weather news by checking in with wbrz.com/weather and following Josh on Twitter and Google+.