The six-hour Probability Of Precipitation (POP06) is the forecasted percentage area of the region which will receive measurable precipitation during a given six-hour period. An alternative interpretation, the chance that a given location will receive measurable precipitation, is not correct but turns out to be pretty close.
The probabilities are given every six hours, and cover the period ending at the time listed. For example, the forecast below states that there is a 27% chance of measurable precipitation between 0600 UTC October 17 and 1200 UTC October 17.
The POP06 and POP12 forecasts are computed differently, and will not necessarily be consistent with each other.
DTW EC NGM MOS GUIDANCE 10/16/96 1200 UTC DAY /OCT 16 /OCT 17 /OCT 18 / HOUR 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 MN/MX 57 72 54 60 TEMP 70 70 66 62 61 59 59 65 69 69 64 61 59 57 55 56 57 55 51 DEWPT 56 56 57 57 56 55 55 56 56 55 55 54 52 51 50 49 46 44 42 CLDS SC SC SC SC BK OV OV OV OV OV OV OV OV OV OV OV OV OV OV WDIR 23 23 17 18 19 18 17 17 18 17 15 15 16 19 20 21 23 23 23 WSPD 11 10 05 06 06 07 08 12 16 14 11 14 15 15 16 19 23 25 18 POP06 8 36 27 20 22 55 66 62 50 POP12 45 39 80 75 QPF 0/ 1/ 0/1 0/ 0/0 2/ 3/3 2/ 1/2 TSV06 9/ 5 23/ 3 11/ 5 7/ 4 12/10 24/ 6 20/ 7 13/ 3 12/16 TSV12 27/ 7 15/ 8 30/13 26/ 9 PTYPE R R R R R R R R R R R R R R R POZP 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 POSN 0 0 0 0 0 0 0 0 0 0 0 2 6 10 21 SNOW 0/ 0/ 0/0 0/ 0/0 0/ 0/0 0/ 0/0 CIG 7 7 7 7 6 6 6 6 6 7 7 6 4 VIS 5 5 5 5 4 4 4 4 5 5 5 5 4 OBVIS H H H H F F H H N N N N F