Jet Streams Supportive of Severe Weather

An upper-level trough is the synoptic scale pattern most supportive of severe weather. This trough can be identified by a strong dip in the jet stream, which is the flow of fast-moving air around the 250 millibar level in the atmosphere. These dips in the jet stream are more favorable for surface low pressure systems to form, and those surface low pressure systems are important for generating the converging flow and rising air leading to convection that can produce severe weather. The surface low pressure systems tend to form on the front side of the upper-level trough, where winds are moving back towards the north after dipping farther south. This forward flank of the upper-level trough is the favored region for severe weather development, but it is not the only factor affecting how and where severe weather occurs. One common approach would be to look at and analyze a 250 millibar map of upper-air observations and determine if there is a significant upper-level trough with a strong jet stream rounding around it.

Above is an example of an upper-level pattern which could be supported of severe weather. In this image, the strong upper-level trough is present over the western United States. The favored region for surface low pressure development and possible severe weather would be on the forward flank of the upper-level trough in the central United States. In this region, there is divergent flow aloft which supports rising motion to fill in the upper atmosphere as air separates in the upper-level flow. Imagine throwing a pebble into a pond and it creating a ripple that goes out in all directions. As that water is displaced at the top of the pond, water would have to come up from lower levels to fill in where the water used to be as the ripple extends outward. This upper-level support for rising air combines with the expected low-level support of rising air with the expected developing surface low pressure to create increased chances for thunderstorm development with strong updrafts. Positive feedback from the strengthening surface low pressure system could help strengthen the upper-level divergence and the upper-level support for rising air, which in turn strengthens the surface low pressure system and so on.