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  • NeverDie
    replied
    Thanks for the link.

    Looks as though the forecast for tomorrow there has already shifted: "day":12, "month":10, "year":2014, "yday":284, "hour":19, "min":"00", "sec":0, "isdst":"1", "monthname":"October", "monthname_short":"Oct", "weekday_short":"Sun", "weekday":"Sunday", "ampm":"PM", "tz_short":"CDT", "tz_long":"America/Chicago" }, "period":2, "high": { "fahrenheit":"64", "celsius":"18" }, "low": { "fahrenheit":"55", "celsius":"13" }, "conditions":"Overcast", "icon":"cloudy", "icon_url":"http://icons.wxug.com/i/c/k/cloudy.gif", "skyicon":"", "pop":10, "qpf_allday": { "in": 0.61, "mm": 15 }, "qpf_day": { "in": 0.00, "mm": 0 }, "qpf_night": { "in": 0.61, "mm": 15 }

    Anyone please do jump in to correct me, but I'm guessing the interpretation of the revised forecast might be: There's a 10% chance of rainfall >= 0.01" tomorrow. If it does rain, then the expected amount of precipitation is 0.61", all at night and none during the day.

    Come to think of it, maybe the earlier pop of zero that you reported wasn't really zero in the sense of 0.0000. Maybe it was a float that was greater than zero but less than 0.5, and it got rounded to the nearest integer (zero) for reporting purposes.
    Last edited by NeverDie; October 11, 2014, 11:52 PM.

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  • frankc
    replied
    I agree. Error on the side of water conservation.

    And, yes, the stuff I pasted came from here:

    http://api.wunderground.com/api/---your_key_here ---/forecast/q/KS/SHAWNEE_MISSION.json

    You don't need a key to get it, but, without the key, the URL format is a little different. But the keys are free and give you 500 hits a day.

    That is the place Python goes to to get the forecast. And I've looked at it several times in the past and the 0% probability and then significant amount of precip happens often. I am sure it makes sense to people who understand what it actually means.

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  • NeverDie
    replied
    Originally posted by frankc View Post
    But, I am not using the qpf figure. I need to fix that. Maybe reduce the amount of irrigation by that qpf figure?
    That's the key question, and I would also like to know the answer too. As a starting point, it's why I thought it worthwhile to investigate how accurate the forecasts actually are. For instance, early today WU was predicting 0.25" total precipitation for today, but we actually had 1.48". I'd say that's a pretty large variance between forecast and actual. If that's typical of forecasts, then maybe a good strategy is to irrigate the bare minimum (and only if it's absolutely needed) to get through the next day or two to see how the rainfall actually pans out. Then once all the facts of how much rain actually precipitated are known, you could irrigate more to supplement, if needed. That's the sort of strategy that I'm currently thinking might waste the least amount of water, but I'd be very interested in a brainstorm on other possible strategies, of which you have already mentioned one. So, if anyone can suggest any other possibly worthwhile strategies, please do jump in and say it.
    Last edited by NeverDie; October 11, 2014, 11:03 PM.

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  • NeverDie
    replied
    Originally posted by frankc View Post
    Here is what I don't understand about pop and qpf:

    This is for tomorrow:
    "pop":0,
    "qpf_allday": {
    "in": 0.51,
    "mm": 13
    so there is a 0% chance of precip. But prediction is half an inch of rain.
    I agree it doesn't make sense. i.e. it would seem to be self-contradictory. Are those actual numbers that currently appear on the WU website?

    pop = probability of precipitation
    qpf = quantified precipitation forecast

    Just a wild guess, but I suppose it might come about as an infrequent glitch if WU used one model/methodology to compute pop and a completely different model/methodology to compute qpf. Maybe then, because the two models don't completely agree with each other, you could get anomalies like this. However, I don't see how it could happen if pop and qpf were derived from the same forecast model.

    I haven't myself run across an anomaly like the one you posted, but I had been looking at accuweather, not WU, because until today I didn't notice WU having a qpf.
    Last edited by NeverDie; October 11, 2014, 10:10 PM.

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  • frankc
    replied
    Here is what I don't understand about pop and qpf:

    This is for tomorrow:
    "pop":0,
    "qpf_allday": {
    "in": 0.51,
    "mm": 13
    so there is a 0% chance of precip. But prediction is half an inch of rain.

    For the next day:
    "pop":90,
    "qpf_allday": {
    "in": 1.57,
    "mm": 40
    That makes more sense to me: 90% chance and 1.57" predicted.
    ??

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