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Old 04-24-2017, 04:47 PM
watchwolf49 watchwolf49 is offline
Join Date: Dec 2009
Location: State of Jefferson
Posts: 8,067
Originally Posted by Chronos View Post
Best I can interpret is that the all-day number is the probability that it'll be raining at any randomly-selected point during the day. Thus, if it were known with 100% certainty that 100% of the area would be dry all morning, and rain from the stroke of noon to the end of the day, then the daily number would be 50%.

But then, that interpretation is inconsistent with the numbers in post #8.
The NWS publishes forecasts every 6 hours around here ... and the rain probabilities are given in those time intervals ... and it really does take that much time to run all the computer models they use; plus analysing the data, conferring with neighboring forecast offices, going outside and looking up and typing out the copy ...

In the case Chronos presents is somewhat fanciful ... we've taken the 6 am weather data, plugged into the computer model and let 6 hours pass in the model and our result is rain starts right at noon ... upon consulting the historical record we'd have to find that under the 6 am conditions, it has never rained earlier than expected in the past 100 years of record keeping ... because if in 10% of the historical cases it did rain earlier than expected ... then the forecast would say "10% chance of rain before noon, then 100% chance of rain after" ... things really do change that quickly ...

One thing the NWS does that no one else seems to bother with is they provide a detail discussion of the forecast, where the forecaster will state which of the models give what results and which model run the forecaster is relying on ... "The 12z GFS sped up precipitation timing closer to the 00z EC solution and suspect most of the region will be wet by at least late Tuesday afternoon...if not sooner."

We have to have a crew of professional forecasters to make these predictions by the hour ... and racks of powerful computers to get the results ... I honestly don't think The Weather Channel has invested that much money ...

Last edited by watchwolf49; 04-24-2017 at 04:50 PM.