The jet stream and jet streaks are commonly located on the 300-mb chart. High winds associated with the jet stream can also be analyzed at the 500-millibar level. The jet stream extends a significant vertical distance through the atmosphere. A jet streak is a higher segment of wind within the jet stream. A trough is a synoptic scale region of lower heights. The base of a trough is the bottom of the trough where maximum curvature is located (see diagram below).  A 500-millibar jet streak in the base of a trough will profoundly impact the value of vorticity to the north and to the south of the 500-millibar jet. A depiction of a 500-mb jet at the base of a trough is given below.  Relative vorticity is comprised of curvature and shear vorticity. A vorticity maximum will be located in the region experiencing the greatest combination of positive curvature and positive shear vorticity. Within the base of a trough, curvature vorticity is positive. In relation to a 500-mb jet streak in the base of a trough, curvature vorticity is positive BOTH north and south of the 500-mb jet streak. The diagram on the next page shows positive curvature vorticity both south and north of the 500-millibar jet streak.  The story is different with respect to shear vorticity. Shear vorticity is positive north of the 500-mb jet streak, however, shear is negative south of the 500-mb jet streak. The diagram below explains this.  This results in positive curvature AND positive shear to the north of the 500-mb jet streak in the base of a trough. It also results in positive curvature BUT negative shear to the south of the 500-mb jet streak in the base of a trough. The vorticity maximum will be located to the north of the jet streak because this region has the greatest combination of positive curvature and shear vorticity. The negative shear vorticity to the south of the 500-mb jet partially or completely offsets the positive curvature vorticity. Since the vorticity maximum is to the north of the jet streak, upper level divergence associated with PVA is more likely in this region. See diagram below for position of vorticity maximum.  Haby sez: When a 500-mb jet streak is in the base of a trough, the most significant vorticity maximum and the greatest potential for PVA and NVA will be north of the 500-mb jet streak. |