90% chance of what and where?

Here’s what that percentage means

Rainy weather
Rainy weather

Rain chances today and tonight for Southeast Texas are 90%, but this blog isn’t about tonight’s weather. We have plenty of articles on that. This blog is more to answer one of my viewer’s questions from Facebook:

“Decades ago, while pursuing my B.B.A., I had Richard Peterson, Ph.D. (Texas Tech University), for Atmospheric Science. He explained when a 30% chance of rain was in the forecast, about 30% of the viewing community would likely get rain. It was not a 30% chance of rain, in general. Does that make sense? So, is that explanation of rain forecast still the norm, or has it changed since 1990?” -- Debbi Whitten

That 30% is what we meteorologists call POPs, or probability of precipitation.

With all due respect, Dr. Peterson’s definition has never worked for me because it begs the question: “Which 30% of the viewing community exactly?” It just doesn’t work. Consider our 20% chance of thunderstorms in the summer which is for all of the viewing community and simply indicates we all have a 20% chance to get one of these storms, NOT a particular area of the community (again, exactly which 20%?). If it’s JUST a 20% chance for the coast, then we will say that, but it still means ALL of the coastal viewers have a 20% chance, not just 20% of the coast.

I’ve also heard people say this indicates that 20% of the day it will rain. We both know that definition doesn’t fly and begs another question: which 20% of the day?

So just what does a POPs forecast mean? First, here’s the National Weather Service definition and then I’ll sort it out:

“What does this ’40 percent’ mean? ... will it rain 40 percent of the time? ... will it rain over 40 percent of the area? The ‘Probability of Precipitation’ (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01″ of rain. So, in this example, there is a 40 percent probability for at least 0.01″ of rain at the specific forecast point of interest!” -- San Angelo NWS

So, using this definition, the “specific forecast point of interest” is the town or area you’re forecasting for and sometimes we are referring to all of Southeast Texas. We might break this down to 40% north of I-10 and 30% south of I-10 and 20% at the coast. We do pinpoint the forecast with the most likely place for rain, how much rain, when it arrives and when it ends.

Like tonight, our forecast is for 1-2″ of rain beginning in the late evening and ending in the early morning as the front crosses the area. All of us in SE TX have a 90% chance of this rain. Not just 90% of the area. Today’s weather on KPRC 2 will be spent pinpointing the exact time it arrives for your neighborhood, how long it lasts, how much rain we get, and what other severe threats there will be.

I hope this helps!


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