Accuracy of Forecasts

Not very accurate at all, very rarley did forecast match up as to what snow was actually falling in Ellmau at beggining of this year. Already for this season the snow level at Ellmau is said to be 25cm upper 5cm lower, wrong there is not even a wurst.

johntin wrote:rarley did forecast match up as to what snow was actually falling in Ellmau at beggining of this year. Already for this season the snow level at Ellmau is said to be 25cm upper 5cm lower

Hi John, feedback is all welcome - especially when we get it wrong. Are you based in Ellmau? It would be useful to have your feedback later in the season to see if you notice any improvement...

From the webcams it does look as if our forecast for today of "Fair" is closer to reality than the local forecast (of "very cloudy") currently on the official TO website...

We've invested a lot of time and effort in analysing and improving our forecasts over the summer, and we'll shortly switch (at some expense) to a higher-grade (higher resolution) data source for most of our forecasts. We've already rolled out changes to our calculations that should result in more conservative snowfall estimates for lower altitudes in particular.

That said, long-range mountain forecasts are - for the foreseeable future - never going to be 100% reliable for all the reasons discussed earlier in this thread.

Our forecast snow depths can, similarly, never be more than rough averages due to the extreme variations in falls from one part of a mountain to another; variations caused by subtle changes in any and all of wind, temperature, humidity, etc. and topography. But we do our best... 8)

As for the current snow report - that's evidently incorrect and I will feed that back to our data provider today.

We hope to complete the roll out of our improved forecasts late this week, and I'll post a note to that effect when we do.

Hi Admin
sorry not based in Ellmau, but skied there many times, and always took note of forecasts and actual snow cover, and yes have viewed webcam which is always a good indication to start of season, which has a fantastic view of the top of the Hartkaiser funicular.
Wish I was based In Ellmau though :)

It's actually very easy.

You look up your resort report on J2ski, and then cross check with the local tourist office web site forecast. They are never the same so add them and divide by two to get the mean.

Then Multiply by two, and separately divide by two. Write both results down.

Also, timeshift by two days in each direction.

And there you have it.

Can I have my £5.00 now?

Dave Mac wrote:Can I have my £5.00 now?

No. You forgot to divide by the number you first thought of, and to add your favourite colour.

Anyway - how did you figure out our new forecast model so quickly? 8)

Hi All
I wanted to update this thread. I use snow forecasts on a regular basis when I am in resort. I know that predicting high mountain weather is very difficult but I would have thought that the overall 'theme' would be correct. Last week in Tignes, the prediction for the whole week from J2Ski was that it was going to snow every day with a total new snow of 174cms! In actuality it snowed once with a fall of about 15cms. I wrote to J2Ski about this vast difference and in particular with comparison to some other forecast sites but have had no reply. This is not a one off by any means as I mentioned here a few years ago. At that time Admin said that there were going to be changes made to their system. The accuracy of weather forecast is very useful and often important, and notwithstanding the difficulty in making forecasts surely the overall 'theme' should be correct. I would very much hope that someone from J2Ski forecast team is able to comment on this please.

Idkwia,

You need to start here :-

Every J2Ski Snow Report wrote:Mountain Weather Systems are notoriously chaotic and any forecast beyond 48 hours is only an indication. Snow Forecasts, Snow Depths and current local Skiing Conditions change rapidly

We put that on every snow mail - because it's true.

idkwia wrote:I know that predicting high mountain weather is very difficult

That's right. 8)

idkwia wrote:I would have thought that the overall 'theme' would be correct.

And that's very wrong.

When the 'theme' is a nice stable high pressure you'd be right. But that means no snow, so you'd probably not notice that most of the forecasts will be pretty 'accurate' (and very similar) in those situations.

Snow tends to fall when it all gets mixed up though - and the biggest snow falls often happen over relatively localized areas as weather systems collide. If you're under a storm track you'll get dumped on; if you're two miles away you might not get a flake.

At the start of last week the models had the Tarentaise in the sights of the weekends weather; but that was revised daily and most of the big snow went further North (e.g. Zermatt got plastered).

idkwia wrote:the prediction for the whole week

Expecting the forecast, any forecast, to be static is a fundamental misunderstanding. You need to watch the forecast develop and change every day.

All the weather models suffer from "compounding" of errors; if they're a bit out for Monday, then Tuesday's forecast will be further out, Wednesday even worse, and so on.

There is no "accurate, reliable" weather model. End of. If there was one, then we'd all be using it. Weather is chaotic and we try to keep up. Sometimes the models we use win out, other times Snow Forecast or Meteo France will get it right. And where you happen to be on any particular day will determine which you think is best.

Take a look at the ensemble graph ("The Wigglies" linked from this page); the different coloured lines represent runs of the model with very slightly varied parameters - they diverge rapidly as time progresses... for 7 days ahead they're currently giving predicted daytime temperatures (for Geneva) varying from 0C to 13C.

idkwia wrote:it snowed once with a fall of about 15cms.

Well the Tourist Office report the upper depths have gone from 190cm to 210cm so they think they've had a fair bit more than that... 8)

There's not a lot more I can say TBH; if weather actually was "predictable" we'd have a lot less to talk about... :lol:

The Admin Man - I appreciate your detailed and very considered reply which is very useful. I would have checked your forecast every day last week and despite it being very wrong for a couple of days the "theme" of the original forecast remained - that is my recollection which I accept could have got skewed due to the amount of reports I looked at. However, my overall perception of the J2Ski forecasting over the past 6 years is that they tend to over estimate the amount of snow when compared to other sites and certainly when compared to reality. Also, I don't understand why your forecasts for a particular day can be so wrong even when it is say 8am in the morning of that very day; maybe you could explain please?