Those of you who follow me on Twitter or Instagram (@PGLocal4) periodically see me post one of my hand-analyzed maps…I usually do this if an impactful storm is approaching.
There’s a lot of interest in these maps, and also a lot of questions. I promised that I’d write an article sometime to explain these, and here it is!
Let’s first take a look at this first map (the one above). See the individual groups of computer-generated numbers at various cities across the region? That’s meteorological shorthand for that hour’s weather observation at an airport. The top left number is temperature, the bottom left number is dewpoint temperature, the top right number is a shorthand for the pressure, and the lower right number is how much the pressure has changed in the past three hours.
That line with the little sticks on the end is the wind direction and speed (a big stick on the end represents 10 knots and a half-stick is 5 knots…or roughly 10 and 5 mph). Think of the line as an arrow, with those little sticks representing feathers on the tail of the arrow. You can now discern the direction the wind is blowing toward.
The hand-drawn lines are called isobars, which are lines of equal pressure. On this particular map, the pressure each isobar represents gets lower and lower as they encircle a low-pressure area in Illinois. In addition to the isobars clearly identifying where the low is, you can also see the counterclockwise circulation around the low by looking at the wind.
That wind also can help identify where a front is, such as the warm from just south of Michigan, where the wind is flowing generally from the east north of the front but shifts to the southeast south of it.
Same with the cold front extending southward from the low…you can see the southeast winds ahead of it shift behind the front. On some occasions, the wind shift isn’t very different before and behind a front, so I need to use the temperature, dewpoint, or isobars to locate it.
In certain situations, I will highlight a freezing line (where the temperature reaches freezing), or a particular dewpoint threshold. And sometimes, those isobars will identify a trough of low pressure…not a front, but a feature that could trigger anything from summer storms to winter snow squalls.
So why do I do this? Well, for one, I love to! Yes, some of you have seen tweets of these maps on my day off when a good storm is headed our way. If there’s something important on the weather map, I’m going to hand-analyze one of these, whether I’m at work or not. But there’s a value to these much greater than my own enjoyment.
Take a look at the map below. That’s a computer-generated national surface weather map available to us meteorologists on a daily basis…this map is from roughly the same time as the hand-analyzed one I did above.
My maps have considerably more data and detail. In this case, there isn’t much difference in the placement of the low and fronts. But sometimes, I find smaller scale weather features on my map that the national map doesn’t pick up on, and some of these features are very important.
And sometimes, I need a very close-up view because we have weather features almost right on top of us. On the two maps below, done eight hours apart, I didn’t need the isobars…I just looked at the wind barbs and pressures and eyeballed where the low and fronts were.
You can easily see the change in wind direction and temperature ahead of and behind the fronts, as well as the change in the low’s and fronts’ positions.
So now you know what these maps are, and why I do them. I may be a little “old school” but, in meteorology, old school used with the new technology yields the best forecasts.
And as an aside, I should probably mention that every weather intern I’ve had in the Local 4 weather office, some of which are now TV meteorologists around the state and country, all had to hand-analyze a surface map as the first thing they did every day they were in the office with me.
So this old school technique has spread to some in the new generation of TV meteorologists!