A meteorologist was talking about the possible 3/30 severe weather event, and a possible jet stream split. I've heard about this before, and I am curious what causes the jet to split, and why it increases the risk of severe weather for locations that are in between the jet stream split, if that is actually true. Does wind shear become stronger, or is it something totally different I'm not getting here? :blink: [/b]
I think they were probably talking not about actually "split jet" (which does not typically favor large-scale severe weather outbreaks), but rather a "splitting" of the jet, as in strong difluence. The term "split jet" is usually used to describe the times when the polar and subtropical jets are seperated. Such a synoptic/global configuration does not tend to favor strong cyclogenesis and large severe weather events, since the forcing aloft is usually more marginal. When the polar and subtrop jets "merge" (or phase), you'll usually see the polar jet dive southward, with the subtropical jet supplying ample mid-upper level moisture. These flows tend to be more amplified, with strong large-scale ascent, and can be associated with strong cyclogenesis and the "synotpically-evident" severe weather events.
Difluence is one component of divergence (speed differential is the other -- e.g. speed convergence or divergence). Difluence occurs when the winds / jet streaks "fan out", which is not entirely uncommon in the exit region of jet streaks. Imagine a narrow stream opening into a larger river. Strong difluence is often seen on days with widespread severe weather. Now, it's important to remember that difluence does not directly imply divergence! {Aside: one can usually assume upward motion under areas of upper-level horizontal or quasi-horiztontal divergence per mass continuity, but remember that air rises along isentropic surfaces when not saturated, and such surface aren't vertical on the large-scale -- they are slanted)}. For example, areas that experience difluence, as noted above, are often in the exit region of jets (on the "nose" of jet streaks), which are also areas of speed CONvergence (imagine a highway slowing from 70mph to 55mph). The divergence contributed from difluence does not necessary outweigh the convergence contributed by the speed differential (speed convergence). Of course, with a "typical" / climo Plains severe weather outbreak, you'll see the jet streak rounding the base of the trough and eject into the plains. On the nose of this jet streak, there's often difluence, in addition to divergence associated with transverse circulation. However, given this configuration, the same area is often experience differential positive vorticity advection (DPVA) as the trough axis nears, which also causes upward motion. Thirdly, the same area is often in low-level warm air advection (evidence by the veering wind profile which favors supercells and tornadoes), which is yet another source for upward motion. Fourthly, this area is often between the trough axis (to the west) and the ridge axis (to the east), which is an area of ageostrophic curvature divergence aloft (yup, you guessed it -- another source for upward motion!).
Of course, don't get this confused with the term "coupled jets" either! LOL, yes this can get confusing keeping straight all the different terms straight... Coupled jets occur most commonly when the left-exit region of a jet streak lies near the right-entrance region of another jet streak. Since both of these are areas of divergence aloft (per transverse circulation -- don't worry, just think upward motion), the "coupled jet" area often experiences more intense upward motion, which can lead to surface convergence, pressure falls, etc. Couplet jets are often seen with very intense winter-time cyclones. It's common for the more intense "Nor'easter" events to be associated with strongly coupled jets.
CLICK HERE for the 300mb analysis from the evening of the 5-4-03 tornado outbreak. Note how the winds really "fan out" over the area from Iowa to Arkansas (and farther west into southeastern NE/eastern KS/ eastern OK). That's very nice difluence.
