I thought I’d offer a behind the scenes look at my forecast to give you a better understanding of our storm chances for the next several days. Did you know that the thickness of the atmosphere relative to the Earth is comparable to the skin of an apple? Meteorologists break the sky down to many different levels for observational and forecasting purposes. We focus on conditions at the surface (we live there, after all) all the way up to 30,000 feet around the level of the jet stream. The weather maps in the extended entry depict forecast conditions around 18,000 feet (roughly the half-way point) and offer a good glimpse of the overall predicted weather pattern. The solid black lines are contours of equal height and indicate the presence of a ridge of high pressure or a trough of low pressure. The areas of red are upper air disturbances (shortwaves). With heat and humidity in place (i.e., summer in Tennessee) a small upper air disturbance is usually enough to encourage upward air motion and trigger a few thunderstorms. I don’t know why, but it feels good to boil four years of calculus-based meteorology into a couple of sentences! Now to the computer maps and the misty distant future.

The map above is valid for Sunday early afternoon (we’re looking at the GFS model but that’s not important for our purposes). An upper level ridge of high pressure is predicted for the Mid-South and the eastern third of the nation this weekend. Note there are shortwaves from Wisconsin to Iowa to Missouri and Arkansas, but none over Middle Tennessee. Our storm chances this weekend are relatively small due to a lack of upper air support. Isolated storms will still pop Saturday and Sunday afternoon, but should be few and far between.

The second map (above) is valid Tuesday early afternoon. On Monday and Tuesday the upper level ridge of high pressure moves east. The flow around a ridge is clockwise; as the ridge pushes to the East Coast we’ll see increased flow from the south (the Gulf of Mexico) and that will pull more moisture into the atmosphere. Also note the presence of a few shortwaves over the Mid-South. More moisture and some upper level support means a better chance of thundershowers on Monday and Tuesday.

The third map is valid early Thursday afternoon. On Wednesday and Thursday our thundershower chances go up even more. The dip in the black lines over the Upper Midwest and the Great Lakes depicts an upper level trough of low pressure. As opposed to small disturbances (shortwaves) these bigger dips are known as longwave troughs and are made up of cool air overhead. Cool air aloft means instability and an even better chance for unsettled weather. We’re on the southern fringe of this trough during the middle of the week; with a juicy atmosphere and some organized upper air support in addition to a surface front…Wednesday and Thursday should bring the most numerous showers and thundershowers to the area.

All of this thinking is reflected in my seven day forecast. Instead of percentages (which I believe confuse the public more than anything else) I like to use small t’shower icons for small t’shower chances and larger t’shower icons for larger t’shower chances. This is just one part of composing a forecast. There are a number of other levels in the atmosphere to keep an eye on, but now you know a little more about what goes into making a forecast. I should mention that these maps come from my favorite web site for weather forecasting: the Penn State e-Wall.

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