GENERAL FORECAST TERMINOLOGY & TABLES
- Sky Condition
- Winds
- Temperatures
- General Time Periods
- Precipitation Probabilities
- Time Periods for Verification
- Wind Chill and Heat Index Tables
Forecasts for sky, wind and temperature conditions are an important part of weather information disseminated on a routine basis. Listed below are descriptions of regularly used weather terms that should aid in a better understanding of their meaning.
SKY CONDITION
| Sky Condition | Cloud Coverage |
|---|---|
| Cloudy | 9/10 to 10/10 |
| Mostly Cloudy, or Considerable Cloudiness |
7/10 to 8/10 |
| Partly Cloudy, or Partly Sunny |
3/10 to 6/10 |
| Mostly Clear, or Mostly Sunny |
1/10 to 3/10 |
| Clear, or Sunny |
1/10 or less |
| Fair (used mostly for nighttime periods) |
Less than 4/10 opaque clouds, no
precipitation, no extremes of visibility, temperature or winds. Describes generally pleasant weather conditions. |
WINDS
Sustained Wind Speed |
Descriptive Term |
|---|---|
| 0-5 mph | Light, or light and variable wind |
| 5-15 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 mph or greater | Strong, dangerous, damaging. High wind warning required. |
TEMPERATURES
Described as... |
Means... |
|---|---|
| Near 40 | Approaching 40, or a range of temperatures from 38 to 42. |
| Around 85 or About 85 | A range of temperatures from 83 to 87. |
| Lower 50's | temperatures of 50,51,52,53,54 |
| Middle 70's | temperatures of 73,74,75,76,77 |
| Upper 30's | temperatures of 36,37,38,39 |
| 60's | 60 to 69 |
GENERAL TIME PERIODS
The terms listed below are used in National Weather Service forecasts to delineate time periods (for Central Time Zone). Except for the term "this afternoon", two interpretations are given for each period. However, even these are open to further interpretation and will vary with location and time of the year.
Time Period |
Definition |
|---|---|
| Today | Sunrise to sunset; around 6:00 a.m. until around 6:00 p.m. |
| Tonight | Sunset to sunrise; around 6:00 p.m. until around 6:00 a.m. |
| This Morning | Sunrise to noon; around 6:00 a.m. until around noon. |
| This Afternoon | Around noon until around 6:00 p.m. |
| This Evening | From around 6:00 p.m. until midnight;
6:00 p.m. until 10:00 or 11:00 p.m. |
PRECIPITATION PROBABILITIES
Following are precipitation probabilities used in National Weather Service forecasts and a brief explanation of each. Technically, the Probability of Precipitation (PoP) is defined asthe 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 defined as equal to or greater than .01 inch or .2 mm. Normally, the period of time is 12 hours, unless specified otherwise. The forecast area, or zone, is generally considered to be a county. In some geographically unique areas (mountains), the forecast area/zone may consist of portions of a county or two counties. At times, some NWS forecasters will use occasional or periods of to describe a precipitation event that has a high probability of occurrence, i.e., they expect any given location in a forecast zone area to most likely have precipitation, but it will be of an on and off nature. Usually, away from the mountains, each and every county is a forecast zone area itself.
PoP Percent |
Expressions of |
Equivalent Areal Qualifiers |
|---|---|---|
| 10 percent | - | isolated, or 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 (categorical) |
(none used) | (none used) |
SPECIFIC NWS TIME PERIODS FOR VERIFICATION PURPOSES
The following time period table shows the explicit period breakdowns that the NWS uses for internal verification of its forecasts. The Probability of Precipitation (PoP) and temperatures are verified for the 12 hour periods of "TODAY" and "TONIGHT". Although these forecast elements are not verified for the 6 -hour periods of "THIS MORNING", "THIS AFTERNOON", or "THIS EVENING", you should be aware how the time periods referred to in NWS forecasts are internally defined. For most purposes, one should use the definitions of the forecast time periods as shown in the table "GENERAL TIME PERIODS".
PERIOD |
PST/PDT |
MST/MDT |
CST/CDT |
EST/EDT |
|---|---|---|---|---|
| TODAY | 4am-4pm/5am-5pm | 5am-5pm/6am-6pm | 6am-6pm/7am-7pm | 7am-7pm/8am-8pm |
| THIS MORNING | 4am-10am/5am-11pm | 5am-11am/6am-noon | 6am-noon/7am-1pm | 7am-noon/8am-2pm |
| THIS AFTERNOON | 10am-4pm/11am-5pm | 11am-5pm/noon-6pm | Noon-6pm/1pm-7pm | 1pm-7pm/2pm-8pm |
| THIS EVENING | 4pm-10pm/5pm-11pm | 5pm-11pm/6pm-mid | 6pm-mid/7pm-1am | 7pm-1am/8pm-2am |
| TONIGHT | 4pm-4am/5pm-5am | 5pm-5am/6pm-6am | 6pm-6am/7pm-7am | 7pm-7am/8pm-8am |
WIND CHILL & HEAT INDEX TABLES
For information on the wind chill or heat index, along with tables, follow these links:
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