KSN Storm Track 3 Digital Extra: Using upper level winds to forecast weather at the surface

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Last week, we took a look at why we have such large swings in temperatures this time of year and through the winter months. It is all due to an active jet stream. Jets form on the boundary between differing air masses, essentially big changes in temperature. The temperatures typically vary the greatest at this time of year as the northern hemisphere tilts away from the sun. This allows the jet stream to become much more active as it strengthens. Active jet streams also impact the weather at the surface in other ways, like creating new weather systems.

Check out last week’s KSN Storm Track 3 Digital Extra about jet streams here.

Within jet streams we have areas of maximum winds, also known as jet streaks. These jet streaks generally have winds upwards of 115 to 275 miles per hour! The jet streak itself has the biggest impact on weather patterns at the surface. Keep in mind, jets are found above 30,000 feet.

Certain regions within the jet streak are more favorable for creating weather at the surface. This involves convergence and divergence. Convergence occurs when two areas of air meet and are forced upwards. Divergence is the exact opposite as areas of air depart from one another.

In order to create storms, clouds and new weather patterns, we need convergence at the surface. The exact opposite is needed at the upper levels. Remember that upper-level divergence is needed to create new weather.

As we look at a basic jet streak example, we see air entering in from the left-hand side and exiting on the right. Air flows left to right through a jet streak. When air enters the streak, it speeds up. When it leaves, it slows down. These speed changes are crucial as they allow air to pile up (converge) and then spread out (diverge).

We can split the jet streak into four quadrants. Regions 2 and 3 are our upper-level divergence regions. When we look at a jet streak as meteorologists, those are the areas we think will have some sort of active weather at the surface. The right entrance and left exit regions are important in this model. Areas 1 and 4 generally do not assist in new development at the surface and weather can be quiet under those regions.

A real-time example shows a jet streak within our overall jet stream. This jet streak is indicated in red, where our winds are the greatest. This specific model data product we are looking at is often the first step in forecasting. It looks at the forecast wind speeds and jet stream orientation around 30,000 to 40,000 feet. The jet streak in red has our right entrance and left exit regions noted in the pink asterisks. We will look and see if any active weather is forecasted at the surface.

As we take a look at our surface precipitation forecast product, at the exact same time as above, we see a big storm system developing along the asterisk near Canada. The southern asterisk also has some precipitation associated with it as well. This model we are using checks out with our jet streak forecasting technique.

Sometimes, there will not be precipitation present under a jet streak. However, we can oftentimes at least see an increase in cloud coverage. Depending on what ingredients are available at the surface, cloud coverage could be a sign of a developing storm system or it could significantly alter temperatures.

While there are plenty of other factors that go into forecasting, this is often an initial diagnosis tool that meteorologists use to get an understanding of the atmosphere.

-Meteorologist Warren Sears

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