Here is an image showing how much snow is on the ground (as of April 3, 2012) compared to normal for this date. This isn’t necessarily comparing this year’s total snowfall to an average year, but is rather showing how much snow is on the ground right now compared to average. Good news to the north, not-so-good news to the south. Pretty typical of a La Nina season. It’s the areas in the middle — Tahoe, Utah, Colorado — that could go either way during a La Nina. Last year, these areas did very, very well. This year, not so much. Oh, weather…why do you tease us so?
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The last few blog posts showed a contrast of different computer model forecasts for the Sunday night storm in Colorado. The European was the most consistent and therefore believable.
Well, today we have evidence as to why I generally wait until the three-day forecast before feeling like I’ve “nailed” the storm. While there were big differences between the models with the six day, five day, and four day forecasts, now looking at the three-day forecast — all the models basically agree. Again, the blue line is the the leading edge of the main part of the storm.
Now, compare these three-day forecasts to six-day forecasts, and you can easily see how the models were all in disagreement six days out and then “magically” came into agreement when the storm was 72 hours (three days) away.
When planning your life around storms and snow, you can begin to get a sense of what will happen 5-8 days away — we knew there would be a storm Sunday into Monday, even a week out. However, to get the details right, you often need to wait until the storm is just 2-3 days away. But think about. Even if weather forecasting isn’t perfect, this is still an amazing feat of science! Enjoy the snow, Colorado. Looks like the cold air arrives between about 6pm-9pm Sunday night and most mountains get a few inches. The eastern foothills and southern mountains may do the best with 3-6″ if they’re lucky.
Here’s an update on the Sunday storm. It looks like the European model is the most consistent from run-to-run (notice how closely the blue lines are packed together), though with each forecast the storm is slowing down a bit (further west). The American model now looks a lot like the European, which gives more confidence that the European will be correct. The Navy and Canadian models are just out to lunch…
As I wrote about yesterday, there was some serious uncertainty between the models concerning the forecast for the Sunday storm.
Today, the picture is a little clearer. There will be much colder air (back to normal temperatures) moving into the northern half of Colorado on Sunday, with some snow.
Here’s the model comparison for the forecasted weather at 6pm on Sunday. This forecast was made Tuesday night (a 5-day forecast), and the blue line shows the leading edge of the colder air. If the blue line is further south, the storm is stronger.
But how do we choose between models? The European is usually the best and shows more cold air and storminess than the American GFS model for this storm.
One way to extend our analysis is to see which forecast model has been most consistent in its forecast. Consistency doesn’t always correlate to accuracy, but if a forecast model is consistent, it gives me more confidence in its forecast. Think of it like an NBA player. You never know when they’ll have a great shooting game and when they’ll miss most of their shots — life can be random sometimes. But if you pick the player that has the most consistent performance night after night, chances are you’ll have an easier time predicting how well they’ll do over the next few games.
So, how consistent are the models? The graphic below shows the 5-day forecast made on Tuesday night (thick blue line) and the 6-day forecast made on Monday night, 24 hours earlier (lighter blue dashed line). The closer these lines are together, the more consistent the forecast. The further they are apart, the more “all over the place” the model is and the less I trust it.
Well, sure enough the European model is the most consistent. The worst model might be the American, as it forecasted a strong, cold storm yesterday and today’s forecast now shows a much weaker storm.
The summary? I think the storm will come in like the European shows, with a decent amount of cold air for northern Colorado on Sunday and perhaps a few inches of snow.