Understanding precipitation probabilities or POPs

Often we get accused of covering the bases with the use of precipitation probabilities. But it is the best way to convey the potential for a specific location to get wet.Here’s the definition as used by the National Weather Service Offices.

Below is a table that defines our PoP ranges, their associated qualifying terms and the equivalent areal term we use in the forecast to try to describe the coverage of precipitation events for convective events.

PoP Value Qualifying Term Equivalent Areal Term

20 slight chance/ isolated or widely scattered

30,40, 50 chance/ scattered

60, 70 likely/ numerous

80, 90, 100 none/ none

When I was in Indianapolis I tracked the validity of precipitation proabilities and plotted a reliability curve. Over the meteorological summer months of June, July and August the forecast of rain chances and the actual rain occurrence was nearly on the money. There was one notable exception.

When forecasters called for a 70 precent chance or rain, there was measurable rain about 80 percent of the time. There was a slight bias to pull back a little on the POPs, particularly twenty-four hours out, due to the possibility that the anticipated convective precipitation might steer slightly north or south of a specific location.

When you hear a probability or rain it means the chance of getting measurable precipitation at your location during a particular twelve hour period. It really has little to do with intensity or duration. Climatology (over a long period of years) suggests that on any given day in July there is about a 30 percent chance for showers in the Twin Cities.

Last evening was a great example of a 30 percent probability in the Metro area. Around 330PM thunderstorms developed and were moving southeast from just south of St. Cloud to near Blaine. Thunderstorms split Target Field. One produced a downpour in the western suburbs. The other, a stronger storm, resulted in a Severe Thunderstorm Warning for parts of Ramsey and Washington Counties.

Measureable rain was reported at St. Paul Holman Field, while the International Airport didn’t see a drop of rain. There was a report to the National Weather Service in Chanhassen of a brief tornado touchdown on Coon Lake in Anoka County.

Despite the forecast of relatively low probabilities of rain on Thursday and Thursday evening, it does appear that a couple of spots will get wet. Here’s the national graphic for potential rainfall on Thursday and Thursday night.

thurngtpcpn.gif

Enjoy today’s sunshine and comfortable temperatures.

CE

  • bsimon

    Great explanation. I was driving home last evening when Tom (I think it was Tom) interrupted Marketplace with the severe weather warning. Meanwhile we were hosting a party/barbecue a mile north of MSP airport. There was a brief time when the winds picked up & I thought we’d have to retreat inside – but we never got a drop of rain.

  • martin kappeyne

    looking at the defintions I have the sense that 80-90-100% is incorrectly stated.

    20 slight chance/ isolated or widely scattered

    30,40, 50 chance/ scattered

    60, 70 likely/ numerous

    80, 90, 100 none/ none

    Wouldn’t it be appropriate to say widespread/almost certain

    with 0-10% being none/none

  • Craig

    Your suggestion of the use of widespread is well taken. Some have turned this suggestion on me by taking the phrase to mean spread widely. Which is not correct interpretation or the intention of the wording in the forecast.

    Currently when the POP is 80 percent or greater the forecast just says RAIN, SNOW or SHOWERS and THUNDERSTORMS.

    Thanks for jumping in on the discussion.

    Craig