You may see terms in a National Weather Service Zone Forecast that might not be easy to understand. The following defines much of the terminology we use in a Zone Forecast Product.
PRECIPITATION
Technically, the Probability of Precipitation (often referred to as a "POP") is defined as the likelihood of occurrence (in percent) of a measurable amount of liquid precipitation (or water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area.
The following are precipitation probabilities used by the National Weather Service.
POP Percentage | Expression/Uncertainty | Areal Qualifiers |
0 Percent | None | None |
10 Percent | ISOLATED or None | ISOLATED or None |
20 Percent | SLIGHT CHANCE | ISOLATED |
30-50 Percent | CHANCE | SCATTERED |
60-70 Percent | LIKELY | NUMEROUS |
80-100 Percent | (none) | OCCASIONAL or PERIODS OF |
SKY CONDITION
Forecasts normally include a sky condition unless it is implied from another part of the forecast. Below are terms we use to describe the sky condition:
Descriptive Term | Predominant or Average Cloud Cover |
CLEAR or SUNNY | No clouds |
MOSTLY SUNNY or MOSTLY CLEAR | 1/8 to 2/8 clouds |
PARTLY CLOUDY or PARTLY SUNNY | 3/8 to 5/8 clouds |
MOSTLY CLOUDY (sometimes CONSIDERABLE CLOUDINESS) | 6/8 to 7/8 clouds |
CLOUDY | 8/8 clouds |
Also used: INCREASING CLOUDS DECREASING CLOUDS |
WIND
Each Zone Forecast conditions three periods of wind information. The direction given is the direction from which the wind blows. Speeds are rounded to the nearest 5 mph, and often a range is given.
For speeds less than 5 mph, we often say "LIGHT WIND."
For speeds of 15-25 mph, we often say "BREEZY."
For speeds of 20-30 mph, we often say "WINDY."
For speeds of 30-40 mph, we often say "VERY WINDY."
For speeds greater than 40 mph, we often say "HIGH," "STRONG," OR "DANGEROUS."