Why do forecasts vary among weather sources?

The best way to think about it is if you're standing in an art gallery, and there are 5 people looking at a painting of abstract art, and if you were to ask each one what they though the painting was representative of, you might get five different answers.

There are a host of different forecast models that are put out by various institutions. The National Weather Service puts out a few different computer forecast models -- Some are more designed for short-term forecasts, some are designed for more long-term forecasts. The Canadian government has their own models, as does the European Union.

To top that off, the University of Washington runs their own model now, and we have one that's exclusive to us (That "Futurecast" product we air) -- they both run on a much higher resolution and provide short term forecasts for the Pacific Northwest.

All of the models are available to everyone in the world (except the Futurecast, which is just for KOMO.) Each model takes a slightly different tact in trying to compute the atmosphere and say what's going to happen next.

Now, most of the time, the models are in general agreement, with just a few different tweaks here and there.

Think of it as asking four people how to get from West Seattle to a restaurant in Auburn. They might have minor differences in what streets they take to get there, but they'll usually at least get you to the restaurant, and then it's up to you to rely on experience and expertise as to which is the shortest route, based on traffic, etc.

With the models, you learn over time which ones perform better in different situations.

There are some days, and some weather patterns when it can become a meteorological. That's because none of the models are on the same page -- it's utter chaos.

It'd be like you want to go to Auburn, but one person gives you directions to Tacoma, the other to Arlington, the third to Bremerton, etc.

In that case, now you have my first example. Everyone in the weather community looking at a piece of abstract art and trying to figure out a forecast amid the model chaos. Some may be taking model A, some may be going for B, etc. Some may be going on just looking at a satellite photo and throwing the models out of the equation completely.

And to mix things up, sometimes the models can vary wildly from model run to model run.

You may notice differences especially during snow forecasts. Snow is the most difficult weather to forecast around here, since the area waters tend to fight the cooling needed to snow.

And when it's mostly cold enough to snow, people demand much more accuracy in where and when it will precipitate. During rain events, the difference between .05" of rain and .15" of rain is no big deal. But with snow, that could be the difference between a dusting and 4", and everyone wants to know if it will happen at their house, and how much, and if not, why not, and why did you miss it, and how come Everett got 6" and we got rain here in Federal Way?