Six-Hour Thunderstorm Forecast

The six-hour thunderstorm forecasts are given as a pair of probabilities, separated by slashes. The number to the left of the slash represents the probability of a thunderstorm at or near the station during the preceding six-hour period. The number to the right of the slash represents the chance of a severe thunderstorm, if a thunderstorm occurs. For example, the probability of a thunderstorm between 1800 UTC 16 October and 0000 UTC 17 October is given as 9%, and the probability of any thunderstorms which occur being severe is given as 5%.

The sets of numbers are right-justified. Even though the thunderstorm probability is listed under 03, 15, etc., it applies to the same period as the severe thunderstorm conditional probabilities, or the periods ending at 06, 18, etc.

A severe thunderstorm is one in which the winds exceed 50 knots, hail exceeds 5/8 of an inch, or a tornado occurs.

The severe thunderstorm probability is what's known as a conditional probability: the chance that something will happen if something else happens. For example, in the final period, TSV06 is listed as 12/16. Thus, according to the forecast, there's a 12% chance that a thunderstorm will occur. If a thunderstorm does occur, there's a 16% chance (or about one chance in six) that it will be severe. Thus, out of 100 days like the one being forecasted, 12 of them will have thunderstorms. Of those twelve, one sixth, or 2 days, will have severe thunderstorms.

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

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