Forecast Terms

The National Weather Service issued forecasts for sky condition, temperature, wind and precipitation on a routine basis. The terminology used in these forecasts is quite variable. Listed below are descriptors of regularly used weather terms and their meanings to help give a better understanding of each forecast.

Sky Condition

The sky condition describes the predominant/average sky cover based on percent of the sky covered by opaque (not transparent) clouds. If a high probability of precipitation (60% or greater) is expected, then the sky condition may be omitted since it is inferred from the precipitation forecast.

Sky Condition	Opaque Cloud Coverage
Clear/Sunny	1/8 or less
Mostly Clear/Mostly Sunny	1/8 to 3/8
Partly Cloudy/Partly Sunny	3/8 to 5/8
Mostly Cloudy	5/8 to 7/8
Cloudy	7/8 to 8/8

Wind

The wind describes the prevailing direction from which the wind is blowing with speeds in miles per hour. The wind forecast is included in the first three periods of the zone forecast.

Sustained Wind	Speed Descriptive Term
0-5 mph	Light/ light and variable wind
5-10 mph/ 10-15 mph/ 10-20 mph	None
15-25 mph	Breezy
20-30 mph	Windy
30-40 mph	Very Windy
40 mph or greater	Strong, dangerous, high, damaging (High Wind Warning Criteria)

Precipitation Probability

The probability of precipitation (POP), is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is equal to or greater than 0.01 inches. Unless specified otherwise, the time period is normally 12 hours. NWS forecasts use such categorical terms as occasional, intermittent, or periods of to describe a precipitation event that has a high probability of occurrence (80%+), but is expected to be of an "on and off" nature.

POP Percent	Expression of Uncertainty	Equivalent Areal Qualifier
10 percent	(none used)	Isolated/ Few
20 percent	Slight Chance	Widely Scattered
30, 40, & 50 percent	Chance	Scattered
60 & 70 percent	Likely	Numerous (or none used)
80, 90, & 100 percent	(None used)	Occasional, periods of, or none used

Time Periods

Forecast Time Period	NWS Definition (LST)
Today	6am-6pm
This Morning	6am-noon
This Afternoon	Noon-6pm
This Evening	6pm-midnight
Overnight	Midnight-6am
Tonight	6pm-6am

American Samoa's Tropical Cyclone Outlook 2024-2025

Forecast Terms

Sky Condition

Wind

Precipitation Probability

Time Periods