Why is observed weather always different than a forecast


  • Administrator

    Wind

    Weather stations measure wind usually 2m above ground while forecast models (especially ECMWF) forecast wind 10m above the ground. Surrounding terrains, mountains, forests event buildings can completely change direction and force of the wind.

    Temperature, dew point, humidity

    Forecasted temperature, does not take in account a lot of factors. Weather stations placed on the top of the mountain or in deep valleys always report different values than a forecast. Forecast models with low resolution can not compete with high resolution models especially in mountains.


  • Meteorologist

    @ivo
    sorry,this is wrong.
    always wind measurements takes & report (metar,synoptic etc) 10m above the ground ;)
    awos.jpg


  • Meteorologist

    Source;
    WMO's "Guide to Meteorological Instruments and Methods of Observation" book

    PART 1 : Measurement of meteorological variables / Chapter 5 - Measurement of surface wind
    https://library.wmo.int/doc_num.php?explnum_id=3177


  • Sailor

    @regetr
    @ivo
    Right, WMO weather stations measure wind speed at 10m, but MADIS and other weather stations at 2m for most of them.


  • Meteorologist

    Hmm, I found this MADIS site.but I have no idea what it says :)

    Notes 5 - MADIS Surface Variables
    "Wind speed and direction are reported and stored in the database. The user can optionally select u & v wind components and those will be calculated and returned. The QC results from speed and direction will be used In either case, and if one of the map projections has been selected via the MSETDOM call, the winds will be rotated to match the projection. The documentation in the COOP cdl was changed to remove the height of the anemometer. All anemometers are at 10 meters heights except for (GYX, TANM3, and STYN6) which are at 3 meters."



  • Yep, 10m.

    Most airports and met stations will have a 10m mast, but when you're short of one, there is a formula you can use to (crudely) account for the drag due to being closer to the Earth, and therefore calculate the 10m based on the 2m wind.

    Do also bear in mind that it SEEMS like the NWP forecast is always wrong, but try watching it consistently for a few days. In some areas (where terrain, ocean, etc. do not create significant biases that the resolution can't cope with), it can be scary accurate.

    Much love Lewis F. Richardson ✌️


  • Meteorologist


  • Administrator

    @regetr thx for info did not know that


  • Administrator

    Well seems that I do not know this problematic. I wanted to make a info page for our new monster feature, we are finishing right now. And it seems that you understand the matter more than I do. Thx for all the comments.


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