Optimistic forecasts

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  fonta op 29 November 2017 · 08:52

  Is it me of i notice a sort of over-optimistic drift in the snow forecasts here?
  I kinda noticed (from the measured data) that the amount of snow that actually fell, quite often is less than the forecast amount.
  
  Example...let's see what will happen in the next 6 days here in my birthplace:
  

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  Expert
  meteomorris op 29 November 2017 · 11:18

  @Fonta,
  
  Thanks for your post. Good thing to compare forecasts with actual snowfall. When you do so, keep two things in mind.
  
  1. 6 day forecasts are never very accurate on micro scale. 6 day forecasts will give you an idea what might happen on macro and sometimes meso scale. Forecasters do work with what they call the forecast funnel. For the long term (5 days and further) they will express their thought on a macro level (in this case the Alps and sometimes on a more specific level: northern Alps, southern Alps, western Alps), for the mid term (2 tot 5 days) they will express their thoughts on a regional scale or even bigger (in this case northern Alps, southern Alps, western Alps, but also Lombardia, Vorarlberg, southern Piemonte etc..). For the short term 0-48 hours they will get more specific on a micro scale.
  
  So if you want to compare actual vs forecasted snowfall on a micro scale you should use the 24 hours forecast maps and compare them with weather stations (and webcams) in the neighborhood. For example these:
  * http://www.arpa.veneto.it/bollettini/meteo/h24/img15/Graf_392.htm?sens...
  * http://www.arpa.veneto.it/bollettini/meteo/h24/img15/Graf_393.htm?sens...
  
  And these webcams: http://wepowder.com/nl/webcam/1452341914
  
  2. Actual fallen snow and accumulated snow are never the samen. Snow settlers over time and the amount fallen is always more than the amount measured since it settles. Read also: http://wepowder.com/en/forum/topic/233471
  
  The actual forecast is based on a storm that is expected to move in on friday-saturday. This storm is accompanied by very cold temperatures and the warm Mediterranean will boost the snowfall. But since we are 36-48 hours away from the moment the storm moves in and the fact that this storm is not directed by a strong and dominant jetstream the exact storm track is still uncertain. So if you want to compare forecast and actual snowfall on a micro scale please use the 24hour or the 48hour maps.
  
  Does this help?

  May the powder be with you.
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  fonta op 29 November 2017 · 11:41

  Thanks for the accurate answer, of course all what you said is correct and needs to be taken into account.
  I'm not a meteorologist, but as a geophysicist I luckily have some minimal basic knowledge of forecast models and their time-dependent accuracy, and since I am really interested in weather dynamics I always try to learn more 😃
  
  My point was more something like: given the intrinsic uncertainty in the forecasts, I would expect errors (difference forecast-reality) to be normally distributed. My feeling (just a feeling i din't analyze data) is that the error distribution might be negative-skewed, which would mean "in doubt overestimate the precipitation a bit".
  
  Of course as you said accumulated is not the same as fallen...question...on your maps do you display fallen or accumulated snow?
  
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  Expert
  meteomorris op 29 November 2017 · 12:15

  @fonta, thanks. Our maps do you show expected snowfall (so fallen snow). Calculating settled snowmaps is one of the biggest challenges for Science at the moment.
  
  1 Run does show their normal out of 30 ensemble model runs. But storms in northern Italy are pretty though to predict and maps do show some eclectic colors from time to time.I do think that the map from this morning might such an eclectic spike. We will see what the maps will show us tomorrow.

  May the powder be with you.
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  Expert
  meteomorris op 30 November 2017 · 16:16

  @fonta, it looks like it was an eclectic spike.

  May the powder be with you.
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  fonta op 4 December 2017 · 10:36

  @fonta, it looks like it was an eclectic spike.meteomorris op 30 Nov 2017 16:16
  
  Yep, we had 5-20 cm in average.
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  Expert
  meteomorris op 4 December 2017 · 13:29

  @fonta, thanks. Luckily the Piemonte forecast was spot on 😉

  May the powder be with you.
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  Beginner
  rand24 op 5 December 2017 · 16:22

  i notice on the site there is a feature on the maps that says -3 days....is this a radar derived estimate of fallen snow and do you have anything like that on the site?
  
  Also, CAn i overlay my favorites of the top resorts in the forecast on the map - often as i zoom in on the colored maps i see nothing and have to start again. My son and I are coming to the alps for a 12 day powder tour - expecting to be looking at austria carefully and want to keep as many small funky option open as possible. tx for a great and fun website.
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  Expert
  meteomorris op 6 December 2017 · 10:51

  @rand24, the -3 days is based on the forecast. So it is not a radar derived estimate. Best thing to do for Austria is checking the links to the weather stations nearby the ski resorts to see how much snow has fallen in the last few days. You can find these on the skiresortpage (like for example ski arlberg: http://wepowder.com/en/ski-arlberg)
  
  Please read this article for more help: http://wepowder.com/en/forum/topic/249613
  
  Also helpful are the maps that are constructed by the Austrian weather office: http://www.zamg.ac.at/incaanalyse/
  For the time being these are the only reliable sources. We are working/studying on a new project but this may take some time.
  
  Regarding your second question.. I just listed it on our wish list for future development. Thanks!

  May the powder be with you.

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