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!
