General Forecast
Terminology and 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 Conditions
Describes the predominate 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 variations 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
Wind Advisory usually in effect at this point.
30-40 mph Very Windy
40-58 mph Strong, potentially damaging.
58 mph or greater Severe
Severe Thunderstorm or 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 39 to 41.
Around 85 or About 85 A range of temperatures from 84 to 86.
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 a.m. until around 6 p.m.
Tonight Sunset to sunrise; around 6 p.m. until around 6 a.m.
This Morning Sunrise to noon; around 6 a.m. until around noon.
This Afternoon Around noon until around 6 p.m.
This Evening From around 6 p.m. until midnight; 6 p.m. until 10 or 11 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 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 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-11am 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-5am



Wind Chill Chart
The following table shows the cooling effect of wind on humans and animals. Winds of more than 45 mph add little to the chilling. Wind chills should be calculated with sustained wind speeds. Dangerous wind chills begin at -35° F.
Wind Speed (mph) Equivalent Temperature (°F)
calm 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35
5 33 27 21 16 12 7 0 -5 -10 -15 -21 -26 -31 -36 -42
10 22 16 10 3 -3 -9 -15 -22 -27 -34 -40 -46 -52 -58 -64
15 16 9 2 -5 -11 -18 -25 -31 -38 -45 -51 -58 -65 -72 -78
20 12 4 -3 -10 -17 -24 -31 -39 -46 -53 -60 -67 -75 -81 -88
25 8 1 -7 -15 -22 -29 -36 -44 -51 -59 -66 -74 -81 -88 -96
30 6 -2 -10 -18 -25 -33 -41 -49 -56 -64 -71 -79 -86 -93 -101
35 4 -4 -12 -20 -27 -35 -43 -52 -58 -67 -74 -82 -89 -97 -105
40 3 -5 -13 -21 -29 -37 -45 -53 -60 -69 -76 -84 -92 -100 -107
45 2 -6 -14 -22 -30 -38 -46 -54 -62 -70 -78 -85 -93 -102 -109



Heat Index Chart
The table below shows the combined effect of high temperatures and dewpoint temperature that produce a higher heat index value. Danger begins at 105° F.
  Dewpoint (°F)
Temp. (°F) 62 64 66 68 70 72 74 76 78
88 90 91 92 93 94 96 97 99 101
90 92 93 94 95 97 98 100 102 104
92 94 95 96 98 99 101 103 105 108
94 96 98 99 100 102 104 106 108 110
96 99 100 101 103 104 106 108 111 113
98 101 102 104 105 107 109 111 114 116
100 103 105 106 108 110 112 114 116 119
102 106 107 109 110 112 114 116 119 122
104 108 109 111 113 115 117 119 122 125
106 110 112 113 115 117 119 122 124 127
108 113 114 116 117 119 122 124 127 130



UTC time conversion chart
What is UTC time? It stands for Universal Time Coordinate. Also known as GMT-Greenwich Mean Time or Zulu time. Zulu is the military word for the letter "Z". Zulu is an abbreviation for Longitude Zero, where at that longitude is Greenwich, England. 

Why use UTC...Zulu, or "Z" time? All issuance times on products from the National Weather Service are coordinated on Z time. Since users of National Weather Service products are across the globe, all local time zones can not be accounted for when a product is issued. Therefore to eliminate confusion, Z time, which is the same time worldwide is used.

Below is a chart that converts Zulu time to the four local time zones that cover the lower 48 United States.

 

* One hour must be added to local time when daylight savings time is in effect *
 
Time(Z) EST CST MST PST
00:00 7:00 pm 6:00 pm 5:00 pm 4:00 pm
01:00 8:00 pm 7:00 pm 6:00 pm 5:00 pm
02:00 9:00 pm 8:00 pm 7:00 pm 6:00 pm
03:00 10:00 pm 9:00 pm 8:00 pm 7:00 pm
04:00 11:00 pm 10:00 pm 9:00 pm 8:00 pm
05:00 12:00 am 11:00 pm 10:00 pm 9:00 pm
06:00 1:00 am 12:00 am 11:00 pm 10:00 pm
07:00 2:00 am 1:00 am 12:00 am 11:00 pm
08:00 3:00 am 2:00 am 1:00 am 12:00 am
09:00 4:00 am 3:00 am 2:00 am 1:00 am
10:00 5:00 am 4:00 am 3:00 am 2:00 am
11:00 6:00 am 5:00 am 4:00 am 3:00 am
12:00 7:00 am 6:00 am 5:00 am 4:00 am
13:00 8:00 am 7:00 am 6:00 am 5:00 am
14:00 9:00 am 8:00 am 7:00 am 6:00 am
15:00 10:00 am 9:00 am 8:00 am 7:00 am
16:00 11:00 am 10:00 am 9:00 am 8:00 am
17:00 12:00 pm 11:00 am 10:00 am 9:00 am
18:00 1:00 pm 12:00 pm 11:00 am 10:00 am
19:00 2:00 pm 1:00 pm 12:00 pm 11:00 am
20:00 3:00 pm 2:00 pm 1:00 pm 12:00 pm
21:00 4:00 pm 3:00 pm 2:00 pm 1:00 pm
22:00 5:00 pm 4:00 pm 3:00 pm 2:00 pm
23:00 6:00 pm 5:00 pm 4:00 pm 3:00 pm

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