How To Read Computer Forecast Model Maps

WARNING: This is not your father's how-to. I'm going to hit you with some knowledge.

I'm going to take some flak from some of the meteorologists here, and if you want to be a meteorologist, yes, you have to go to at 4-year college and suffer through many, many semesters of Calculus to give you an "appreciation" for how the atmosphere works.

But if you want to be an armchair forecaster, here's the truth: It's not has hard as it used to be. When I see an old-school forecaster's furrowing brow staring at a 500 millibar chart of North America I sometimes flash back to the 1990s and think "why bother." When I was in school, that was all you had. The computer models only predicted upper-air conditions that the average person had never heard of: Vorticity, Heights, and Thickness. These days, the computer models predict simple stuff that we can all understand: Snow, Ice, Rain, Humidity, Cloud Cover and Simulated Radar. So, you can find out the models' opinions on what the weather will be, without involving any rocket science.

That said, if you want to understand those older charts to get a better appreciation of atmospheric science, feel free, there are a lot of great resources out there, just Google away. Habyhints has a lot of good stuff.

I'll give a very brief explanation here. The 500-millibar chart is the one that meteorologists look at most often, because it is the activity at this level that drives most weather at the surface (meteorologists rarely look at surface maps, they'll tell you, and the models still don't do a good job of interpreting where fronts are going to be anyway).

At 500 mb you are concerned about two things:

Heights: The height of various pressure levels, such as the "540 mb or hPA (hectopascals)" line which was used in the past as an approximate value to divide rain and snow at the surface. When I say "height of a pressure level" I mean that meteorologists look at the 3-d atmosphere not with lines of height (like pilots) but as lines of pressure (at different heights). Where pressures are low, it's called a trough and that's cooler air pressing down towards the earth. The opposite is a ridge. Troughs generally bring bad/cool weather, ridges clear/warm (that's really an overgeneralization fut it's sufficient).

Vorticity: This is how much upward motion is in the atmosphere at any given point. Upward motion at 500 mb generally translates to heavy precipitation down on the surface, though the levels in the atmosphere can be tilted to where the vorticity doesn't line up with the precip, which makes things more difficult.

Other levels, closer to the surface such as 700 mb (pressure gets higher as you go down) can be used to determine cloud cover (humidity) and at 850 mb you can get a good feel for temperatures without the rapidly changing geographic effects that you have at the surface. Way up, at 250 mb or so, you can look at the winds and see the jet stream. Looking at the "thickness" of the pressure levels (one common measure is the 1000mb-500mb thickness) can also help a lot with generalizing surface temperatures. Those are the key pressure levels that most models have, and most have Heights, Vorticity, Thickness, Relative Humidity and Winds.

But in day-to-day forecasting it's easier to use the newer charts and you shouldn't feel guilty... yes, the older meteorologists are going to tell you that the other charts are more accurate when interpreted correctly, and that may be true to a small extent but a lot of time and work

went into these new charts and they've taken all the guesswork out of forecasting. Some of my favorite charts on the Pro site are as follows (note that meteorologists have given them obscure names to make you think they're more complicated than they really are):

- 6-Hour 2-Meter Minimum/Maximum Temperature:Low and High temperatures on a map. Simple.- Combined Conditionals & Amounts: Maps showing areas and intensities of Snow, Freezing Rain, Sleet, and Rain. Familiar.

- Simulated Radar: This is golden - looks like a radar map, but goes out several days. Only one warning with these - they typically cut off at lower levels than you would see in thunderstorms, just because they aren't high enough resolution to see the extreme pockets of rain or hail, but they still give a good indication of storm placement, timing, and strength. The highest resolution models, such as the 4-KM WRF, can even pick out individual thunderstorm cells.

- Severe Weather Indices: This gets a little more complicated because there are a lot of these, but they beat figuring out how a 500 mb map spells "thunderstorms." You can read more here.

If you've got the maps above, what more could you need? Add to that the fact that you don't have to go blind looking at a map of North America -- on the Pro site, we've got maps of your state! (see temp & sim radar examples above)

Other things to remember:

1. There are over 50 computer forecast models and the meteorologist's job is to decide which will be right in which situation, and it's always a different one than you'd think. So even armed with the best maps, you're still going to be wrong until you get used to the model biases

, which may be different than your mom's house upstate - and all of this will vary from season to season and year to year.

2. Higher resolution models are better - in general. For reasons that I discussed in "The Secrets of Weather Forecast Models, Exposed", higher resolution models are generally better than lower-resolution ones, but they can still be highly inaccurate. They also don't go out very far in time.

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