Weather Blog

How can forecasts vary among different sources?

How can forecasts vary among different sources?
With the crazy weather this week between thunderstorms to kick off the week and chances of lowland snow in the latter half of the week, one person asked how come forecasts from different vary when "we're all looking at the same charts?" 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. Meanwhile, other sites just spit out raw computer forecast model data without human interaction.

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 United Kingdom and European Union.

To top that off, the University of Washington runs their own models now (called the "MM5") that both run on a much higher resolution and provide short term (and up to 7-day) forecasts for the Pacific Northwest.

All of the models are available to everyone in the world (except the Europeans hold a lot of their model outputs close to the vest and don't provide it for free). 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 nightmare. 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, but also has the highest demand for accuracy since it arguably has the most impact on the region.

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?

And thus since minor differences can lead to a great difference in impact, snow forecasts tend to have the most variety as each weather forecaster tries to nail the minute details as best they can.