Friday, October 28, 2022

How Do Meteorologists Define Precipitation Chances? Part 2

Part 1 of the Precipitation Percentage topic talked about the technical definition and my definition and usage.  What about on-air meteorologists?  How do they define precipitation percentage?  How do they come up their number for their forecast? Do they even use percentages?  Here are some answers from on-air meteorologists across the country?

  • "I actually don't include percentages in my forecasts as I feel they can be confusing to the audience. My understanding of them as issued by the NWS is that percentage od the forecast area would receive any precip."

  • "On our 12 hour planners the pops are specific to the city of choice. On the 7 day its the pops for our viewing area."

  • "There's an art to this, it seems, but like NOAA, we use coverage and confidence as our main factors in determining POPs for our zones. I tell our customers, "You should expect to cross with rain at least six of every ten times you hear us forecast a '60% chance of rain'."

  • "My working definition for precipitation percentages is the percent of my viewing that will see rain. So, if the rain chance is 70%, then about 70% of the viewing area would see rain and about 30% of the DMA would not. However, if that 70% sees rain, then there is a 100% chance of rain for them.  I typically used MOS guidance to help me out a bit on rainfall percentage. I also just use model guidance and common sense, especially if I’ve seen a similar weather pattern on the models or maps. Sometimes the models will say it’s a big chance, but other variables keep the chance down....confidence plays a huge role in choosing POPs. I want was much information as I can to make the best forecast I can with precip. I do also believe that rainfall intensity sometimes does play a factor. If I see potential for heavy rain, that will affect my decision making. Also, the major counties or cities in our DMA. If there is a better bet for rain in those location, I have, at times, adjusted the forecast to reflect the population center of our viewing area."

  • "I fill out a spreadsheet with all the model data to get an analytical view of the numbers. That has high/low/PoPs to compare and see trends from day to day. Also helps to compare anomalies with models that are trending drier/wetter/colder/hotter than the rest."

  • "We don't use POPs on-air, or convey rain as a percentage on our forecasts. However, we do use "isolated" "few", "scattered", and "widespread" to describe rain coverage."

  • "We use POPs as defined by confidence in the forecast times and the coverage of the rainfall. While we use this traditional method in calculating the POP we do tailor them more to the audience as strictly the chance of seeing rain for the forecast area."

  • "We don't use pops on air because of the confusion you're likely looking to highlight.  We use words to qualify precipitation placement and likelihood.  There is a bit of POPs in the making of that, though.  Main variables being coverage and confidence of at least .01" rain in our area... Oh... also, we use coverage * confidence for a given time period."

  • "PoP to me is the percent of the populated area that will get rained on at some point during the forecast period. I know that is not technically right. But it works."

  • "...Percent chance of any one location seeing precipitation during the stated time period. Not areal coverage or duration. We show precipitation icons for any PoP 30% and above and do not use 10% in daily forecasts (we do in hourly forecasts). We do not use 50% for either daily or hourly forecasts as it invites “forecast skeptics” to make “50/50 guess”-type comments."

  • "I use it as a general number for chance of rain in our averaged area as a whole. It's tough to do broadly for a whole forecast area right? Since it's really more useful for a specific point."

  • "For instance I may put 50% chance of rain on a graphic, even knowing the chance is 10% on a specific city and 85% in Central parts of the state. You have to eyeball it a little...then explain further in your forecast who's actually most likely?"

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