Weather Blog

There is rhyme and reason to weather forecast terms

There is rhyme and reason to weather forecast terms

Sometimes a forecast is for scattered showers. Then there is "isolated showers" and "numerous showers". Are we just pulling names from a hat?

Actually, no -- at least, not where the National Weather Service is concerned. When you look at a National Weather Service-issued forecast the terms used have very specific meanings.

For example, did you know that "isolated showers" means you have a better chance of staying dry than a forecast of "scattered showers"? And that there is a specific definition of "breezy" versus "windy" versus "very windy"?

I found this handy chart on the National Weather Service website that details quite a few of the strict definitions their forecasters must use.

From the NWS: (My notes in the brackets):


"For rainfall, technically, the probability of precipitation (PoP) is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount (.01 inch or more) of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Forecasts are normally issued for 12-hour time periods. Descriptive terms for uncertainty and areal coverage are used as follows:"

PoP Expressions of
Uncertainty
Areal
Coverage
0% none used none used
10% none used isolated
20% slight chance isolated
30-50% chance scattered
60-70% likely numerous
80-100% none used none used

The following terms of duration imply a high probability (80-100%) of occurrence;
brief, periods of, occasional, intermittent, frequent.

[When they say "none used" that means the forecast would just read "rain" or "showers" as opposed to "showers likely" or "scattered showers." Note that "rain" is steady, long lasting, widespread precipitation while "showers" are shorter in duration and vary in geographical coverage.]

SKY CONDITION  

Term Predominant or Average
Sky Condition
Cloudy 95 to 100% opaque cloud cover

Mostly Cloudy or Considerable Cloudiness

70 to 95% opaque cloud cover
Partly Cloudy or Partly Sunny 30 to 70% opaque cloud cover
Mostly Clear or Mostly Sunny 5 to 30% opaque cloud cover
Clear or Sunny 0 to 5% opaque cloud cover
Fair Less than 40% opaque cloud cover, no precipitation and no extremes of temperature, visibility, or wind.

WIND  

Sustained Wind Speed Descriptive Term
0 to 5 mph light or light and variable
5 to 15 mph none used
10 to 20 mph none used
15 to 25 mph breezy (mild weather) or brisk, blustery (cold weather)
20 to 30 mph or 25 to 35 mph windy
30 to 40 mph or 35 to 45 mph very windy
40 to 73 mph high, strong, damaging or dangerous winds

[None used would mean just "South Wind 10 to 15 mph" as opposed to: "Breezy. South Wind 15-25 mph"]


Note that in our own forecasts here, we don't have to adhere to such strict interpretations -- but we do tend to generally follow these guidelines.

In tomorrow's blog, we'll go into how other counties give out their own forecasts -- some are more colorful than others!