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LAKE MARY, Fla. - As we enter the rainy season here in Central Florida, you'll notice each day there is a chance for rain expressed in a percentage. For example, the forecast this past Saturday was for an "80% chance for rain."
Many interpreted that as rain which would fall for 80% of the day. It did not. What it meant was that 80% of our viewing area saw rain, and some heavier storms, after 8 p.m.
Those storms delivered quite the impact including some roof damage in one Orlando neighborhood, a house fire near Ocoee believed to have been sparked by lightning, home damage from trees down near Tavares, and even a tornado warning. However, during the daylight hours Saturday, there was virtually zero rain.
So, was it appropriate to call for an 80% chance? Many might say, "ff course not."
The equation for the "Probability of Precipitation" or, "PoPs" is simple: "C x A," where "C" represents the confidence of precipitation occurring in the area and "A" represents the percent of the area forecasted to receive precipitation. Confidence multiplied by the percentage of the area forecasted equals the "percentage of precipitation." So if there's a 100% confidence that 30% of the area will see rain, then it's a 30% chance [(1 x 0.3)100 = PoPs].
Less intuitive and less common, if there was a 30% confidence that 70% of the area could get rain, then it would be a 20% chance.
What if there was a 100% chance that 40% of the area would see rain all day? Would that warrant a 40% chance of rain for the people getting rain all day? And what about the 60% not getting rain at all that day?
You'll notice one glaring factor omitted from this equation: time.
If there's a 100% chance that 100% of the area will get rain from 9 p.m. to 11:59 p.m., should that constitute a "100% chance for rain?" That would mean all the daylight hours are dry, but most nighttime hours are wet. (What about that big golf tournament?)
How much of the day will it rain? When will it arrive? That's important for trying to plan the day.
On Friday last week, we were careful to tell people that the mornings would be fine but that after 3 p.m., all bets were off. But, what if someone was busy tending to children or some other distraction and only caught the last part of the weather broadcast when they saw the 7-day forecast map with the percentages? Would they cancel plans?
Indeed, time is not factored into the PoPs equation and that's where subjectivity can result. Subjectivity in this context means the forecast can vary a lot between meteorologists (typically between competing organizations). Often to account for this lack of, "duration of rain" or, "time of day rain arrives" considered into the equation, meteorologists will factor in a bit of that, "Je ne sais quoi" and offer what they feel is a more appropriate number based on people's lives. They'll wing it to try and make it more accurate for viewers' lives. Doing that can lead to many pitfalls.
Welcome to the conundrum of the rainy season, or the, "rainy season dilemma."Now, anyone who knows Florida knows that in the summer months, 99% of the daily rain comes after 3 o'clock but for the uninitiated, this could be a weather consumer's dilemma as well, as it may have been this weekend.
The PoPs equation needs to be finessed to include duration precipitation in any given day, plus whether or not it'll happen during the daylight or only at night, assuming most outdoor plans are made for the day and the evening. Any mathematicians out there willing to give it a stab?
