The Jet Stream

[Sam Kennedy]

On a chase day, in relation to the jet stream, generally where do you want to be? North or south of the jet stream, or under the right entrance/left exit regions? I can think of reasons for being in each of the positions, but are any incorrect?

North of the jet stream:
When the jet stream curves in a trough pattern, the area inside the bend provides lift.

Right Entrance/Left Exit regions:
These are the areas of greatest upper level divergence, so act as a lifting mechanism.

South of the jet stream:
Warmer temperatures are found south of the jet stream so there is more potential for convection.

So which points am I correct with and what have I missed? Where should you position yourself on a chase day in relation to the jet stream?

 

[Jeff Duda]

You are right about the right entrance and left exit regions favoring upward motion due to upper level divergence, but only when the jet streak is linear and aligned with the upper level height contours. Odd things can happen if the isotach maximum is skewed from the angle of the height contours (there's a COMET module that covers this).

North or south of the jet stream isn't as important as the areas relative to the jet streaks that are favored for upward motion. When a jet streak is curved (sometimes called a "banana jet") the vertical motions on the inside of the curve are enhanced, so both the upward and downward motions on the inside of the curve are enhanced, not just upward motion. On the outside of the curve, upward motions are indeterminate from jet streak theory alone.

Being under the jet stream can also have advantages due to providing very intense wind speeds aloft, which can increase deep layer wind shear.

 

[Matt Gingery]

Alot of this would depend on whether or not the trough has a positive tilt, or a slightly negative tilt. For the sake of chasing tornadoes the SE side of the area of strongest jet streak in the warm sector would fair well. Depending on the tilt. If the jet is slightly negatively tilted SE of the streak is best in the warm sector where the gravity wave aloft is strongest. In a jet that is positively tilted it would seem best to be located directly under the strongest area of jet streak or a bit south along the boundary. The only events that really produce the greatest # of tornadoes are the slightly negative tilt trough south and east of the jet streak.

 

[John Peters]

For a strait zonal jet, north of the velocity maximum velocity decreases producing cyclonic vorticity (if you were to insert a pinwheel into this flow, higher velocities to the south would result in a clockwise rotation). This positions the "left exit" region downstream of the cyclonic relative vorticity maximum, hence cyclonic vorticity advection and ascent. To the south of the jet maximum the "pinwheel" would rotate the opposite way, due to an increasing velocity tendency to the north. This would orient the "right entrance" region upstream of an anticyclonic relative vorticity max, hence cyclonic vorticity advection and ascent. The same logic can be used to explain UL convergence and descent in the remaining quadrants.