GENERAL FORECAST TERMINOLOGY & 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

Describes the predominant/average sky condition based upon tenths of the sky covered by opaque (not transparent) clouds.

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

Used to describe the prevailing direction from which the wind is blowing, with speeds given in miles per hour. The numbers may vary in other parts of the country due to such variables as terrain and elevation.


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

Used to describe the forecast maximum and minimum temperature, or in some cases, the temperature expected at a specific time.

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
Uncertainty

Equivalent Areal Qualifiers
(convective only)

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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