What are They?
A shortwave trough has a wavelength (distance between center points of two troughs) of less than 3700 mi. For a refresher of what a trough is check out this previous episode of Weather Word of the Week!
These upper level waves are associated with an upper level front or cold pool. So take a cold front and move it up from the surface to around 10,000 ft and you basically have a shortwave.
Movement of Shortwaves
Shortwaves typically follow the height lines of an upper level chart just like train cars follow the track. The height lines are the solid lines on any upper level chart such as 850mb, 500mb, 300mb, etc. In the image below the shortwave is represented by the solid black line cutting across the panhandles of TX and OK.
When these waves become embedded in a longwave trough they can significantly increase the likelihood of seeing precipitation. The heaviest precipitation usually occurs when the shortwave passes overhead of the area as it enhances the lift.
These disturbances move along at around 23 mph during the summer months and can increase their forward speed up to 35 mph in the winter time. If you will recall shortwaves are upper level cold pools. The winter months produce colder air masses due to a lower sun angle and lack of daylight. The colder temperatures strengthen the cold pool aloft, and increases the temperature contrast in the upper levels. The larger contrast in temperatures leads to a stronger polar jet with faster wind speeds.
How Do They Affect Our Weather?
A shortwave acts like a cold front in the upper levels. As it swings through the region it forces air ahead of it to rise. Depending on the strength of the wave and the presence of moisture, a shortwave can produce anything from clouds, rain, and even severe storms.
In fact, oftentimes for severe weather events the ingredients of shear, instability, and moisture are already in place. The only ingredient missing is a source of lift which the shortwave provides as it moves into the region. This acts as the matchstick dropped on top of the waiting powderkeg! KA-BOOM!
Pay attention to the weather the next time we mention a shortwave is headed our way, and observe what type of weather occurs in its wake. To learn more about other weather topics, check out the other episodes of Weather Word of the Week.