It bears repeating …

There seems to be a flood of ZOMG SNOW IS COMING! graphics and posts on social media already. Seriously?

Let’s be clear about the difference between a forecast, an outlook, a seasonal climatology outlook, and (insert bad words here) that seem to be flooding the intertubes.

We can forecast the weather at 48 hours reasonably confidently, and out to around 5-7 days with some confidence as to conditions but the timing starts to become inaccurate as you go out in time. And for snow/ice, that’s a problem unless you are far enough north where it’s below freezing all day for days on end, , because if the precipitation arrives overnight or early morning when it’s still cold, it’s ice/snow, but 6-8 hours later, it’s freezing rain or just cold water. And that’s about it.

Here on the coast, it’s sometimes even worse than that, because the conditions that will cause snow or prevent it are so finely balanced as to temperature, moisture, and other factors it’s really hard to forecast more than 12 to 24 hours out.

The GFS and ECM primary models go well beyond 7 days now, out to two weeks. But beyond a week more or less, these are at best outlooks. They can show patterns, but specific details like snow or rain quantities are worthless.

Beyond 15 days is getting in to the realm of climatology and “subseasonal forecasts”. Again, to be brutally clear, beyond a week it’s no longer a forecast, it’s an outlook. It may use a “forecast model” but it really doesn’t deserve that term. Again, it’s looking at patterns and probabilities.

Seasonal outlooks (30-90 days or beyond) are basically looking at climatology. Any one specific simulation run is worthless. You have to statistically analyze dozens or more runs made over the course of several initial times (up to a week or more when looking beyond 30 days) to get a handle on them.

So yes, it will be colder in December than August, with a higher chance of snow/ice. How much on any given day we’ll have to wait and see. If you see somebody “forecasting” anything more than a week away, take this advice:

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I’ll be doing more winter commentary this year, as well as spring storm season.

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