Using Upper Winds to Predict Surface Weather

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Last week we took a look at why we have such significant temperature swings at this time of year and during the winter months. This is all due to an active jet stream. Jets form at the boundary between different air masses, essentially large temperature changes. Temperatures generally vary the most at this time of year, as the northern hemisphere moves away from the sun. This allows the jet stream to become much more active as it gets stronger. Active jet streams also impact surface weather in other ways, such as the creation of new weather systems.

Check out last week’s KSN Storm Track 3 Digital Extra on the jets here.

In jet streams, we have areas of maximum wind, also called jet trails. These jet sequences typically have winds ranging from 115 to 275 miles per hour! The jet sequence itself has the greatest impact on weather conditions on the surface. Keep in mind that the jets are above 30,000 feet.

Certain regions of the jet series are more conducive to the creation of weather conditions on the surface. That implies convergence and divergence. Convergence occurs when two zones of air meet and are forced upward. Divergence is exactly the opposite as the air zones move apart from each other.

In order to create storms, clouds and new weather conditions, we need convergence on the surface. Exactly the opposite is needed at higher levels. Remember that a higher level divergence is needed to create a new time.

When we look at an example of a basic throw, we see air entering from the left side and exiting from the right. Air flows from left to right through a jet. When air enters the trail, it accelerates. When he leaves, he slows down. These speed changes are crucial because they allow air to accumulate (converge) and then spread (diverge).

We can divide the jet sequence into four quadrants. Regions 2 and 3 are our top level divergence regions. When we watch a sequence of jets as meteorologists, these are the areas that we think will have some sort of active weather on the surface. The right inlet and left outlet regions are important in this model. Zones 1 and 4 do not generally facilitate new surface development and the weather can be calm in these regions.

A real-time example shows a sequence of jets in our overall jet stream. This jet sequence is shown in red where our winds are strongest. This specific model data product that we are looking at is often the first step in forecasting. It examines the predicted wind speeds and orientation of the jet stream around 30,000 to 40,000 feet. The sequence of jets in red has our right entry and left exit regions noted in pink asterisks. We will look and see if active weather is forecast at the surface.

As we take a look at our surface precipitation forecast product, at exactly the same time as above, we see a large storm system developing along the asterisk near Canada. The south asterisk is also associated with precipitation. This model that we use verifies with our technique of forecasting jet sequences.

Sometimes there will be no precipitation present under a sequence of streams. However, we can often at least see an increase in cloud cover. Depending on the ingredients available at the surface, cloud cover could be a sign of a developing storm system or it could significantly alter temperatures.

While there are many other factors that go into forecasting, it is often an initial diagnostic tool that meteorologists use to understand the atmosphere.

-Meteorologist Warren Sears

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