Will there be precipitation between 12Z tomorrow and 12Z the next day?

Try consulting the NWS point forecast, which provides probabilities of precipitation for periods between 6 am and 6 pm and between 6 pm and 6 am local time. On Central Standard Time, these times happen to coincide exactly to 12Z to 00Z and 00Z to 12Z, respectively—that is, two 12-hour periods that together span the period from 12Z to 12Z. (When KOKC is on Central Daylight Time, 12Z is 7am, which is close enough to 6 am that we won't worry about the difference.)

To get a pretty good estimate of the probability of precipitation for the full 24-hour period from 6 am tomorrow to 6 am the next day, we use the NWS's probabilities for each of the back-to-back 12-hour periods. The approach is based on the common-sense idea that during the 24-hour period, either it will rain (or snow, or whatever) or it won't. The probability that either it will rain or it won't rain, is just the sum of the separate probabilities of each. That, of course, is 100% because there aren't any other possibilities! That is, there is a 100% probability that it either will rain or it won't rain. (A probability of 100%, expressed as a simple fraction instead, is 1.0.)

Since the separate probabilities that it will rain and that it won't rain must add up to 100% (or 1.0), it's straightforward to determine the probability that it will rain simply by subtracting the probability that it won't rain from 100% (or 1.0). For example, if the probability that it will rain during a 12-hour period is 30%, then the probability that it won't rain during that period must be 100% - 30% = 70%, or expressed as simple fractions, 1.0 - 0.3 = 0.7.

Our tactic here will be first to determine the probabilities that it won't rain in each of two successive 12-hour periods starting at 12Z tomorrow (from 12Z to 00Z and from 00Z to 12Z the next day), use them to determine the probability that there won't be precipitation in the full 24-hour period, and subtract that from 100% (or 1.0) to get the probability that there will be precipitation during the full 24-hour period. Here's how:

  1. Calculate the separate probabilities that it won't rain in each of the 12-hour periods from 6 am to 6 pm tomorrow and from 6 pm tomorrow to 6 am the next day, by converting each of the probabilities that it will rain (provided by the NWS point forecast) to fractions and subtracting each of them from 1.0.

    [For example, if the probability of precipitation during the first 12-hour period is 30% and during the the second 12-hour period it is 40%, convert each to a fraction (0.3 and 0.4, respectively), and subtract each from 1.0. The probability that there won't be precipitation in the first period is 1.0 - 0.3 = 0.7 (or 70%), and in the second 12-hour period it is 1.0 - 0.4 = 0.6 (or 60%).]

  2. The probability that two independent events will both happen, is simply their separate probabilities (expressed as fractions) multiplied together. Hence, the probability that it won't precipitate during either 12-hour period (and hence won't precipitate at all during the entire 24-hour period) is simply the probabilities that it won't precipitate in each 12-hour period multiplied together.

    [To continue the example above, the probability that it won't precipitate during the entire 24-hour period would be 0.7 × 0.6 = 0.42, or 42%.]

  3. Now that we know the probability that it won't precipitate during the 24-hour period (expressed as a fraction), we can easily calculate the probability that it will precipitate, simply by subtracting the "won't precipitate" probability from 1.0.

    [To finish the example above, this would be 1.0 − 0.42 = 0.58, or 58%. So, the probability that there will be precipitation during the 24-hour period is greater than 50%, even though the odds that there will be precipitation in either 12-hour period individually is less than 50%!]
If the probability of precipitation during the 24-hour period exceeds 50%, you'd probably go ahead and forecast that precipitation will occur. (If you don't think there will be precipitation even when the probability is greater than 50%, or if you think there will be precipitation even when the probability is less than 50%, then you'd probably want to have a good reason.)

As an additional refinement to the tactic described above, you can also consult the NWS surface forecast maps, which show not only forecast frontal locations and types but also areas of precipitation (a hatched area bounded by a thick green border) and type of precipitation (various "present weather" symbols). These maps include forecasts for 00Z , 06Z, 12Z, and 18Z.

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