PROBABILITY of PRECIPITATION (POP) is the likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. The NWS uses two different methods to indicate the chance of precipitation for a specific area: numerical or in non- numerical terms. The "Expression of Uncertainty" category is used for widespread precipitation and the "Equivalent Areal Coverage" for convective (i.e., showery) events. Below is a table of these two methods with the corresponding POP.
POP Expression of Uncertainty Equivalent Areal CoverageThere are other qualifying terms which are used with the above non-numerical expressions. For example:
0% None Used None Used 10% Slight Chance (seldom used) Isolated or few 20% Slight Chance Widely Scattered 30-50% Chance Scattered 60-70% Likely Numerous 80-100% None Used None Used
For duration - brief, occasional, intermittent, frequent.
For intensity - very light, light, heavy, very heavy. VERY LIGHT <.01 inches LIGHT .01 to .10 inch per hour MODERATE .10 to .30 inch per hour HEAVY .30 inch per hour
WIND DIRECTION and SPEED A forecast wind (direction and speed) is included in the first two periods of the forecast. The wind is included in the third and/or fourth period if considered significant.
1. Wind direction is the direction where the wind is coming FROM and is based on an 8-point compass (NE, E, SE, etc.). Light wind (usually 5 mph or less) will be handled in the following ways:
LIGHT SOUTH WINDS (if direction is known), LIGHT AND VARIABLE WINDS , or LIGHT WINDS (where "light" implies a variable wind direction).2. Wind speed will be given in miles per hour. Following is a list of terms sometimes used to describe the wind speed.
Speed range TermsNote: A forecast can contain a peak wind speed in gusty situations. For example, "NORTHWEST WIND 20 TO 30 MPH WITH OCCASIONAL GUSTS TO 40 MPH.")
0-5 mph Light or Light and Variable 5-15 mph None used 15-25 mph Breezy (usually for mild weather) Brisk (usually for cold weather) 20-30 mph Windy 30-40 mph Very Windy 40 mph or greater Strong, Damaging, Dangerous, High
1. "Near," "around," or "about" a specific value rounded to the nearest five zero. Above 100°F or below 10°F, any number will be used. For example:
NEAR 40, AROUND 15, ABOUT 85, or NEAR 106.2. A general range where the terms are defined by the following:
LOWER 50's (50 - 54) MID 50's (53 - 57) UPPER 50's (56 - 59) 50's (50 - 59)3. A specific range rounded to the nearest five or zero (except ranges below 10°F or above 100°F, any number may be used). For example, 70 to 75 or 102 to 108.
4. Specific numbers for site-specific locations: TRI-CITIES 70/50/72
Term Opaque Coverage Aviation
Clear or Sunny < 1/10 Clear Mostly Clear/Mostly Sunny 1/10 to 2/10 Scattered Partly Cloudy/Partly Sunny 3/10 to 6/10 Scattered Mostly Cloudy 7/10 to 8/10 Broken Cloudy 9/10 to 10/10 opaque clouds Overcast
The wind chill index provided below shows the effective cooling on exposed skin. When the wind blows across the skin, it removes the insulating layer of warm air adjacent to the skin. When all factors are the same, the faster the wind blows, the greater the heat loss, which results in a colder feeling. Temperature (° F) 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 mph W 5 32 27 22 16 11 6 0 -5 -10 -15 -21 -26 -31 -36 -42 -47 I 10 22 16 10 3 -3 -9 -15 -22 -27 -34 -40 -46 -52 -58 -65 -71 N 15 16 9 2 -5 -11 -18 -25 -31 -38 -45 -51 -58 -65 -72 -78 -85 D 20 12 4 -3 -10 -17 -24 -31 -39 -46 -53 -60 -67 -74 -81 -88 -95 25 8 1 -7 -15 -22 -29 -36 -44 -51 -59 -66 -74 -81 -88 -96 -103 S 30 6 -2 -10 -18 -25 -33 -41 -49 -56 -64 -71 -79 -86 -93 -101 -109 P 35 4 -4 -12 -20 -27 -35 -43 -52 -58 -67 -74 -82 -89 -97 -105 -113 E 40 3 -5 -13 -21 -29 -37 -45 -53 -60 -69 -76 -84 -92 -100 -107 -115 E 45 2 -6 -14 -22 -30 -38 -46 -54 -62 -70 -78 -85 -93 -102 -109 -117 D
The NWS has devised the "HEAT INDEX" (HI), sometimes called the "apparent temperature." The HI is the temperature the body feels when the heat and humidity are combined. The table below is the Heat Index Chart. (Note: This chart is based upon shady, light wind conditions. Exposure to full sunshine can increase HI values by up to 15° F.) Relative Humidity (%) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 120 107 111 116 123 130 139 148 115 103 107 111 115 120 127 135 143 151 110 99 102 105 108 112 117 123 130 137 143 150 T 105 95 97 100 102 105 109 113 118 123 129 135 142 149 E 100 91 93 95 97 99 101 104 107 110 115 120 126 132 138 144 M 95 87 88 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136 P 90 83 84 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113 117 122 85 78 79 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97 99 102 105 108 80 73 74 75 76 77 77 78 79 79 80 81 81 82 83 85 86 86 87 88 89 91 75 69 69 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78 78 79 79 80 70 64 64 65 65 66 66 67 67 68 68 69 69 70 70 70 70 71 71 71 71 72 Heat Index Possible Heat Disorder 80°F to 90°F Fatigue possible with prolonged exposure and/or physical activity. 90°F to 105°F Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity. 105°F to 130°F Sunstroke, heat cramps, or heat exhaustion likely, and heatstroke possible with prolonged exposure and/or physical activity. 130°F or greater Heat stroke/sunstroke highly likely with continued exposure.