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Which Model for rainfall - A laymans guide ?

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    #32
    Su Campu
    Closed Accounts Posts: 4,279 ✭✭✭Su Campu


    redsunset wrote: »
    Not by me anyway,I think Met Eireann do a fantastic job and yes i do often use the Atlantic charts on their site to get a feel for what lies ahead.

    No ME bashing here.
    Sorry Red, I didn't mean you, though it may have come across like that!


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    #33
    Discodog
    Registered Users Posts: 15,822 ✭✭✭✭Discodog


    When do Weatheronline update - they are still showing the 12 noon chart. Shouldn't the 1800 be up by now ?

    OK now this is getting really confusing. Both Yr.no. & DMI use HIRLAM yet yr.no show Galway as dry tomorrow & DMI show it as wet !


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    #34
    Harps
    Closed Accounts Posts: 3,548 ✭✭✭Harps


    Assume you mean the GFS charts? The 18z isn't released until about 9.30pm, takes about an hour for the whole thing to come out


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    #35
    Su Campu
    Closed Accounts Posts: 4,279 ✭✭✭Su Campu


    Discodog wrote: »
    When do Weatheronline update - they are still showing the 12 noon chart. Shouldn't the 1800 be up by now ?

    OK now this is getting really confusing. Both Yr.no. & DMI use HIRLAM yet yr.no show Galway as dry tomorrow & DMI show it as wet !

    The yr model seems to be an earlier run than the DMI one, but taking their output for midday tomorrow, they're pretty similar.

    Yr.no
    182870.png

    DMI
    182869.gif


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    #36
    Discodog
    Registered Users Posts: 15,822 ✭✭✭✭Discodog


    This, issued at 7pm, gives a different picture. A little rain midday but basically dry till 1800. XC (gfs) gives a similar forecast whilst NAE has it as wet. I am going to wake up & see !

    meteogram-1.png


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    #37
    Discodog
    Registered Users Posts: 15,822 ✭✭✭✭Discodog


    On the GFS & NAE charts they state that the numbers refer to how much rain will fall in a 6 hour period. When does the period run from in relation to the forecast time ?


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    #38
    Su Campu
    Closed Accounts Posts: 4,279 ✭✭✭Su Campu


    If the chart is for 12UTC, then it means the total rainfall that will have fallen in the 6 hours up to then (i.e. from 0600 to 1200).


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    #39
    Deep Easterly
    Closed Accounts Posts: 5,150 ✭✭✭Deep Easterly


    Discodog wrote: »
    This, issued at 7pm, gives a different picture. A little rain midday but basically dry till 1800. XC (gfs) gives a similar forecast whilst NAE has it as wet. I am going to wake up & see !

    Usually when yr.no update around 7pm or 7am it is the 'rest of the world' data that has been updated (ie, that latest ECMWF run). Those hourly graphs are what the HIRLAM model they use are forecasting and I think are only updated twice a day.

    Bottom line is though that you can look too much into something. Rainfall models will never ever be totally spot on so it is fruitless trying to find one particular model that does. They only serve as a guide and an estimation of where rain, and how heavy it may be, at any one time. This is why we need human input and interpretation of these such as we see on weather forecasts etc. People may criticize weather forecasts for being to general but that is the best they can ever be because it is not a precise science!

    If it is any help, the DMI 6hr maps give a pretty good idea of where rainfall may occur during each 6 hour period with a 24hr period:

    http://www.dmi.dk/dmi/index/danmark/vejrkort/eurprec1.htm


    PS, Since you do work outdoors, it is no harm actually looking at the sky also, always gives a good idea if it about to rain or not. Better than any hi res model!!


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    #40
    Discodog
    Registered Users Posts: 15,822 ✭✭✭✭Discodog


    A big thank you to everyone for your detailed answers - I should have posted this thread years ago. Hopefully it will guide others. I do study the Sky & cloud patterns etc but especially on the West Coast things move so fast. Also a lot of systems seem to slide up almost parallel to the coast so it can be touch & go whether Galway is wet or dry. Without the radar I would have no idea whether the rain is a 20 min shower or a 2 hour burst !.


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    #41
    Su Campu
    Closed Accounts Posts: 4,279 ✭✭✭Su Campu


    Another way of predicting rainfall extent is by using the 700 hPa relative humidity charts. For frontal rain anyway, the rain usually occurs within areas where the 700 hPa RH is 90% or greater.

    Case in point - this evening:

    182957.gif

    182956.png


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    #42
    Discodog
    Registered Users Posts: 15,822 ✭✭✭✭Discodog


    I emailed yr.no & asked about the discrepancy between their map & the pictogram. I got a refreshingly honest reply:

    Hello.

    Yes, this is not good!
    The maps are a composite of different model runs, and there is a delay in these data. So while the meteograms are updated frequently, the precipitation in these maps are at certain times from an older model.

    In the near future we'll change all our maps at yr.no, and we'll try to sort this out.

    Best regards
    Havard Larsen
    met.no


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