Sitting on the couch watching the forecast wondering if an umbrella is going to be a required accessory is something we do quite frequently. And often we are confused by the call, "50% chance of rain". Does this mean that the forecasters just really have no idea whether it's going to rain or not? While that is what it sounds like, it's not wholly accurate.
We have our precipitation forecast served to us in two ways, in the form of percentages and descriptive phrases. Chance of showers, scattered showers, occasional thunderstorms... It can be confusing. So what does it all mean? Well, the National Weather Service calls it POP (or Probability of Precipitation). Let's assume we are talking about "showers" in this case, they break it down like so:
|Slight chance, widely scattered, isolated||= 10-20% chance|
|Chance or scattered||= 30-50% chance|
|Likely or Numerous||= 60-70% chance|
|Occasional, periods, or "it's gonna rain"||= 80-near 100% chance|
Computer models are used along with local forecasting experience to create the POP, just like the overall forecast.
Interesting that today's forecast on weather.com says a 30% chance of thunderstorms but describes the chance as "isolated". Well, just as new "comfort" measurements are being used more frequently like heat index, "real feel", sometimes we could settle for a forecast like "you're not going to get a cab", or "don't bother wearing the North Face"