Jet Streak Dynamics

Intermediate Severe Thunderstorm Forecasting Jet Streak Dynamics

Jet streaks are areas of particularly strong flow in the upper-level jet stream. This strong flow typically has wind speeds of at least 50 knots, or about 58 miles per hour. Jet streaks are an important synoptic scale feature when determining upper-level support for thunderstorm formation. They will typically add even more increased support for rising air in certain favored regions of the jet stream. The two typical areas of the jet streak which are most favorable for severe weather formation are the left exit region and right entrance region of the jet streak. If winds in the jet streak are flowing from west to east, then the left exit region would be on the northeast side of the jet streak where winds are exiting on the left side of the flow. Likewise, the right entrance region on the southwest side of the jet streak, where winds are entering on the right side of the flow.

Example of a jet streak outlined in blue shading. The favored regions for severe weather would be the left exit and right entrance regions. The left and right, as well as the entrance and exit sides can be determined by the direction of the flow.

Within a jet streak, the above image shows the favored regions for severe weather development. These two regions are favored because they dynamically support more upper-level divergence and rising air. The reasoning for these dynamics is not explored in this lesson. However, these two regions can be identified on upper-level observation maps to key in on particular areas which may be susceptible to severe weather.

Example of a jet streak circled in pink centered over the central US. This image is from the previous lesson on jet streams. The favored regions for severe weather associated with this jet streak would be the left exit region in the area where the states of Colorado, Kansas, and Nebraska meet, as well as the right entrance region in central Texas.

Local storm reports from the same day as the above jet streak image. Notice the cluster of severe weather reports in Kansas, Colorado, and Nebraska, as well as the cluster of severe weather reports in west-central Texas.

The above two images show an example where severe weather occurred in both the left exit and right entrance regions of an observed jet streak. Note how tornadoes, large hail, and damaging winds occurred in both regions, but more widespread severe weather occurred in the left exit region.

Curved Jet Streaks

Jet streaks which are curved instead of straight add additional upper-level support for upper-level divergence, leading to rising air, which may lead to severe weather. When a jet streak takes a curved shape similar to a kidney bean shape, the downwind portion of the jet streak is more favorable than the upwind portion of the jet streak for rising motion. In other words, the forward side of the jet streak (where the left exit region is located) is more favorable for upward motion than the back side of the jet streak (where the right entrance region is located) if the jet streak is curved. In the above example, notice how there was a slight curve to the jet streak, which resulted in more severe weather being reported in the left exit region than the right entrance region.

Previous Lesson

Back to Course

Next Lesson