The cross section of a tree reveals growth rings, the oldest records of rainfall. These rings show that on rare occasions, droughts in Utah have lasted longer than a decade.

But that's certainly not typical. The average length of a drought in this state is four years. And here we are in year No. 5.

What are the prospects for the future? A look at the past offers some of the best clues.

The accompanying graph tracking precipitation in Utah for the past 107 years shows we are having our second-worst dry period in more than a century.

Notice, though, how fast conditions can change and how steep the rise is back to above normal. That means when it does start to storm, it might be really wet for a few years.

An understanding of droughts hinges on an understanding of two jet streams that impact weather in our state.

Normally, the polar jet stream is found in Canada during the summer while the subtropical jet stream is near the desert Southwest.

Jet streams are rivers of air flowing 40,000 to 60,000 feet above the Earth. They are the steering wheels for the storms that move along the Earth's surface.

In the fall, both jet streams begin shifting south. The subtropical jet moves deeper into Mexico and has a reduced influence on U.S. weather.

The polar jet, on the other hand, begins moving into the northern part of the United States, bringing an upsurge in storminess to northern Utah. By winter, the jet stream has reached its deepest penetration into the United States with storms now reaching St. George and even to Phoenix. The colder northern latitude air brings snow even to the valleys.

By spring the shift is northward, and the storm track and jet stream seem to frequently pass above Salt Lake City. For much of northern Utah, March through May is the wettest time of the year.

The rotation of the earth causes complex interactions with hot air rising from the equatorial areas and cold air sinking in the polar areas.

Each hemisphere has two jet streams that develop, one about a third of the way north from the equator and another about a third of the way south from the pole. The exact position of the jet stream seems to be determined by the temperature distribution across the oceans and the location of land masses around the earth. There is probably something else going on in the upper atmosphere in response to changes in solar luminosity, but we don't know how to incorporate it into our forecasts.

The polar jet stream can take on one of three primary modes. The first is called zonal flow. This is when the jet stream moves pretty much from west to east without much north or south swinging.

The second pattern is called meridional flow. In this mode, the jet stream undulates strongly from north to south on its journey around the hemisphere. When the jet plunges southward, it brings arctic air very far south, often setting records.

On the other hand, when the jet surges northward it takes heat very far north.

The third pattern is when the jet stream splits into two branches. Sometimes the two streams diverge widely from each other, and at other locations the two streams merge back together. The split jet stream presents one of the most difficult weather forecast challenges.

During much of the past five years we have either had the meridional type of jet stream with Utah stuck under the northern bulge, or the split type with the jet going around Utah.

And that is why Utah is in a drought.

We don't know why these patterns get locked in for such long periods of time, but they will eventually shift.

The official National Weather Service long-range forecasts are based primarily on sea-surface temperatures. That method shows next winter, the winter of 2003-04, has a 67 percent chance of being normal or wetter than normal.

Logic would tell us we should be pulling out of the dry cycle soon, but when it comes to figuring out what nature is going to do, we are babes in the woods.


E-mail: meubank@ksl.com