Scattered summer showers hard to plan around this time of year


(WOWK) — Scattered showers and storms over the next week will make things complicated for several people involved in several kinds of business who rely on long stretches of dry weather.

We do have high chances of rain happening overall and we do have high chances that many areas will see that rain in the next few days but the rain won’t last too long in any area.

That brings us to the notion of what the chances of rain mean. What do those percentages you see on the forecast mean?

Below is the description of how the National Weather Service defines those percentages. Of course the StormTracker 13 meteorologists make their own forecast so the percentages may change, and on TV we are trying to express the larger idea about rain chances to an entire viewing area so the numbers will likely differ from the numbers you see even on the StormTracker 13 app which are designed for specific areas.

It can be difficult to utilize the numbers to convey what models are indicating when you see instances of scattered storms like the following image from Predictor:

Predictor model output for Tuesday evening

The NWS definition of what the percentages mean is as follows:

The probability of precipitation forecast is one of the most least understood elements of the
weather forecast. The probability of precipitation has the following features:
….. The likelihood of occurrence of precipitation is stated as a percentage
….. A measurable amount is defined as 0.01″ (one hundredth of an inch) or more
(usually produces enough runoff for puddles to form)
….. The measurement is of liquid precipitation or the water equivalent of frozen
….. The probability is for a specified time period (i.e., today, this afternoon, tonight,
….. The probability forecast is for any given point in the forecast area

To summarize, the probability of precipitation is simply a statistical probability of 0.01″ inch of
more of precipitation at a given area in the given forecast area in the time period specified. Using
a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be
covered by precipitation at given time in the given forecast area or (2) that you will be seeing
precipitation 40% of the time in the given forecast area for the given forecast time period.

Let’s look at an example of what the probability does mean. If a forecast for a given county says
that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point
in the county from noon to 6 p.m. local time.

This point probability of precipitation is predetermined and arrived at by the forecaster by
multiplying two factors:
Forecaster certainty that precipitation will form or move into the area
X (multiplied by)
Areal coverage of precipitation that is expected
(and then moving the decimal point two places to the left)

Using this, here are two examples giving the same statistical result:
(1) If the forecaster was 80% certain that rain would develop but only expected to cover 50% of
the forecast area, then the forecast would read “a 40% chance of rain” for any given location.
(2) If the forecaster expected a widespread area of precipitation with 100% coverage to
approach, but he/she was only 40% certain that it would reach the forecast area, this would, as
well, result in a “40% chance of rain” at any given location in the forecast area.

You can always get the forecast that includes those location specific rain chances on the StormTracker 13 app. It’s free and you can download it here.

Follow Spencer Adkins on Facebook and Twitter for the latest weather developments.

For local forecasts, weather alerts, live VIPIR Radar and more, download the FREE StormTracker 13 Weather App from the Apple App Store or the Google Play Store.

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