Definitions of Terms Used in the Zone Forecast Products

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."

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