Windy Community

    • Register
    • Login
    • Search
    • Unread
    • Categories
    • Groups
    • Go to windy.com

    Why is observed weather always different than a forecast

    Frequently Asked Questions
    6
    10
    3944
    Loading More Posts
    • Oldest to Newest
    • Newest to Oldest
    • Most Votes
    Reply
    • Reply as topic
    Log in to reply
    This topic has been deleted. Only users with topic management privileges can see it.
    • ivo
      ivo Administrator last edited by

      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.

      R 1 Reply Last reply Reply Quote 0
      • R
        regetr Meteorologist @ivo last edited by

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

        ivo 1 Reply Last reply Reply Quote 3
        • R
          regetr Meteorologist last edited by regetr

          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

          idefix37 1 Reply Last reply Reply Quote 1
          • idefix37
            idefix37 Sailor Moderator @regetr last edited by

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

            1 Reply Last reply Reply Quote 0
            • R
              regetr Meteorologist last edited by

              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."

              1 Reply Last reply Reply Quote 0
              • johnckealy
                johnckealy last edited by

                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 ✌️

                1 Reply Last reply Reply Quote 0
                • R
                  regetr Meteorologist last edited by

                  for example;
                  https://confluence.ecmwf.int/display/FCST/Known+IFS+forecasting+issues

                  ECM İSSUE.PNG

                  1 Reply Last reply Reply Quote 0
                  • ivo
                    ivo Administrator @regetr last edited by

                    @regetr thx for info did not know that

                    1 Reply Last reply Reply Quote 0
                    • ivo
                      ivo Administrator last edited by

                      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.

                      1 Reply Last reply Reply Quote 1
                      • W
                        WxGuesser52 | Premium last edited by

                        FMH 3 (weather observers handbook) states that the anomometer should be 15' (5m) above the ground.

                        1 Reply Last reply Reply Quote 0
                        • First post
                          Last post
                        Windyty, S.E. - all rights reserved. Powered by excellent NodeBB
                        NodeBB & contributors, OSM & contributors, HERE maps
                        Terms and Conditions     Privacy Policy