Anatomy of a Forecast

The National Weather Service issues forecasts for sky condition, temperature, wind, and precipitation probability on a routine basis. Because the weather is always changing, the terminology used in these forecasts is also quite variable. Listed below are descriptions of regularly used weather terms that should give you a better understanding of each forecast.

SKY CONDITION
The sky condition describes the predominant/average sky condition based on eighths 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
Cloudy 7/8 to 8/8
Mostly Cloudy / Considerable Cloudiness 5/8 to 7/8
Partly Cloudy / Partly Sunny 3/8 to 5/8
Mostly Clear / Mostly Sunny 1/8 to 3/8
Clear / Sunny 1/8 or less

TEMPERATURE
The temperature in a forecast is used to describe the forecast maximum and minimum temperature, or in some cases, the temperature expected at a specific time.

Temperature Description Forecast Meaning
Around 40 / Near 40 / About 40 A range of temperatures from 38 to 42
Lower 40s Temperatures of 40, 41, 42, 43, 44
Middle 40s Temperatures of 43, 44, 45, 46, 47
Upper 40s Temperatures of 46, 47, 48, 49
40s A range of temperatures from 40 to 49
40 to 45 Temperatures of 40, 41, 42, 43, 44, 45

WIND
The wind describes the prevailing direction from which the wind is blowing, with speeds in miles per hour. Wind velocity is normally included in the first five periods of a zone forecast, although descriptive terms may be used in all periods.

Sustained Wind Speed Descriptive Term
0-5 mph Light / Light & variable wind
5-10 mph / 10-15 mph / 10-20 mph None
15-25 mph Breezy (mild weather)
Brisk or Blustery (cold weather)
20-30 mph Windy
30-40 mph Very windy
40-73 mph Strong, dangerous, high, damaging
(High Wind Warning criteria)
74 mph or greater Hurricane force

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”. Unless specified otherwise, the time period is normally 12 hours. The forecast area, or zone, is generally considered to be a county. NWS forecasters 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 Area Qualifier (convective use only)
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

TIME PERIODS

Forecast Time Period NWS Definition (Local Standard Time)
Today 6AM-6PM
This Morning 6AM-Noon
This Afternoon Noon-6PM
This Evening 6PM-Midnight
Overnight Midnight-6AM
Tonight 6PM-6AM