A "jet streak" refers to a portion of the overall jet stream where winds along the jet core flow are stronger than in other areas along the jet stream. Entrance and exit regions of jet streaks are very important in terms of vertical motion, surface pressure systems, and organized precipitation given sufficient low-level moisture. Exit regions are where air parcels "exit" out of a jet streak and decelerate downstream from the jet core (Figs. 1&2). Entrance regions are where parcels "enter" into a jet streak and accelerate upstream from the jet core (Figs. 1&2).

Fig. 1:Idealized example of the entrance and exit regions of a straight jet streak. Highest winds are within the streak along the line/arrow labeled "jet." Divergence (div) usually occurs within the left exit and right entrance regions, while convergence (conv) normally occurs in the right exit and left entrance regions.	Fig. 2:Example of the entrance and exit regions of a straight jet streak. Dashed lines are lines of equal wind speed (isotachs); solid lines are height lines along which the total wind blows. The small arrows denote a component of the ageostrophic wind due to jet streaks that results in divergence and convergence in exit and entrance regions.

Within the exit and entrance regions of jet streaks, air parcels moving at different speeds become out of balance with the existing thermal (temperature) gradient in these regions. Thus, the atmosphere attempts to restore (thermal wind) balance through vertical motion. The vertical motion is attained through ageostrophic winds. Thus, vertical motion is required within entrance and exit regions of jets.

In general, the more the "along-stream variation of the total wind" within an exit and entrance region (i.e., the faster the winds are accelerating within an entrance region or decelerating in an exit region per unit area along the flow (Fig. 2), the greater the vertical motion must be to restore thermal wind balance. The "cross-stream variation" (i.e., how quickly wind speeds change in a plane perpendicular to the jet axis) also is important in promoting vertical motion fields.

Within jet entrance and exit regions, the cross-stream component of the inertial advective part of the ageostrophic wind (i.e., the small arrows in Fig. 2) dictates the amount of divergence/convergence (and subsequent vertical motion) due to jet streak dynamics. Upper-level divergence (convergence) often is associated with upward (downward) vertical motion in the atmosphere.

However, even without the presence of a jet streak, curvature in the flow (i.e., upper-level troughs and ridges) results in divergence/convergence (and subsequent vertical motion) due to the along-stream component of the inertial advective part of the ageostrophic wind (Fig. 3). The stronger the curvature (i.e., the more amplified the jet pattern) and the shorter the wavelength between a trough and ridge axis aloft, the greater the upper-level divergence pattern will be due to the along-stream component of the ageostrophic wind.

Fig. 3 Example of an upper-level flow pattern showing trough and ridge axes. Solid lines are constant height lines. Arrows are a component of the ageostrophic wind due to troughs and ridges that results in convergence (con) and divergence (div) aloft.

Thus, upper-level divergence (of the ageostrophic wind) is caused by 1) jet streak entrance and exit regions, and 2) curvature and wavelength of the overall flow (troughs and ridges). It is very important to consider both these phenomena. It can be advantageous to look at the two components of the upper-level ageostrophic wind individually to assess which phenomena is most important to the production of vertical motion. These two components also explain 1) why 4-cell divergence patterns associated with straight jet streaks become 2-cell patterns for curved jets, and 2) why divergence values for curved jet streaks usually are stronger than that for straight jets (see below).

For STRAIGHT jet streaks, a 4-cell pattern of divergence aloft and vertical motion usually occurs (Fig. 4).

Fig. 4 Four-cell pattern of divergence/vertical motion associated with a straight jet streak. In the dashed left exit (i.e, upper right portion of image) and right entrance jet regions (lower left portion of image), divergence aloft and upward motion usually occur. Convergence and downward motion usually prevail in the solid line right exit (lower right) and left entrance (upper left) regions of a straight jet streak.	Fig. 5 Two-cell pattern of divergence/upward motion (dashed; jet exit region, i.e., right half of image) and convergence/downward motion (solid; jet entrance region, i.e., left half of image) associated with a cyclonically-curved jet streak. Values often are greater than those with a straight streak.	Fig. 6 Same as Fig. 5 except for an anticyclonically- curved jet streak. Divergence aloft and upward motion occur in the jet right entrance region (left half of image) with convergence and descent in the jet right exit region (right half of image).

Within entrance regions, a thermally direct secondary circulation (Fig. 7) occurs associated with the ageostrophic wind. Warm air usually rises within the right entrance (right rear) region while cold air sinks in the left entrance region. To complete the circulation, horizontal ageostrophic winds often flow from warm-to-cold air at upper levels, and from cold-to-warm air at low levels. The circulation is on the order of approximately 400-600 km in horizontal extent.

Within exit regions, a thermally indirect secondary circulation (Fig. 8) occurs. Cold air rises in the left exit (left front) region and warm air sinks in the right exit region. The horizontal ageostrophic components include flow from warm-to-cold air at low levels and from cold-to-warm air at upper levels.

Fig. 7 (far left): Idealized "box" direct thermal circulation in the entrance region of jet streaks. Warm air rises in the right entrance region; cold air sinks in the left entrance region. Horizontal ageostrophic flow occurs from colder to warmer air in low levels. Fig. 8 (near left): Idealized indirect thermal circulation in the exit region of jet streaks. The circulation is opposite that shown in Fig. 7.

However, circulations associated with jet streaks are not "boxes" as shown in the examples above. Typically, vertical components are sloped along isentropic surfaces (Fig. 9). Thus, jet streak circulations and isentropic surfaces are not independent. In other words, in jet entrance and exit regions, enhanced upper-level divergence may lead to enhanced flow and vertical motion along isentropic surfaces.

Fig. 9 Cross-section of an east-west jet streak exit region. The core of jet is directed into the page so that the right (left) side of the image is the right (left) exit region. A more realistic sloped ascent (bold arrow) roughly along isentropic surfaces (sloped thin solid lines) occurs toward the level of maximum upper-level divergence.

For CURVED jet streaks, the "classic" 4-cell (Fig. 8a) vertical motion pattern can be more complicated, and usually becomes a 2-cell vertical motion pattern.

For a cyclonically-curved jet (Fig. 8b), maximum upper divergence values and subsequent ascent usually are found along and to the left of the core of the exit region, with descent along and to the left of the entrance region.

For an anticyclonically-curved jet (Fig. 8c), upper divergence and ascent are strongest along and to the right of the entrance region, with descent along and to the right of the exit region.

Ascent/descent values usually are greatest for cyclonically-curved jet streaks, second greatest for anticyclonically-curved jet streaks, and relatively weakest (but still significant) for straight jets assuming adequate along-stream variation in the wind (Fig. 8).

If varying temperature patterns (isotherms) are superimposed on jet streaks, different thermal advection pattern will result aloft. This can cause the location of maximum divergence and convergence to shift slightly with respect to the jet core.

Occasionally, the ascending branches of two separate jet streaks may be coincident over one location. This merger (coupling) is associated with the ascending branches of the direct circulation in the entrance region of one jet and the indirect circulation in the exit region of a second jet. This interaction maximizes upper-level divergence and vertical motion.

TOP

A "jet streak" refers to a portion of the overall jet stream where winds along the jet core flow are stronger than in other areas along the jet stream. The entrance (exit) region of a jet streak is where winds are accelerating into the back/upstream (decelerating out of the front/downstream) side of the streak. Within the entrance region of a jet streak, divergence (of the ageostrophic wind) usually occurs along and to the right of the jet core (i.e., right entrance region). The upper-level divergence causes pressure/height falls at the surface and/or lower-to-middle-levels underneath the upper divergence maximum. However, the upper divergence usually DOES NOT create a localized ascent maximum concentrated directly under the upper-level divergence field (i.e., the so-called "box" circulation is invalid; Fig. 1).  Instead, Fig. 2 shows a plan view and Fig. 3 a vertical cross-section of more realistic processes that can happen in the atmosphere, as described below.

Fig. 1: Idealized "box" direct thermal circulation in the entrance region of jet streaks. Warm air rises in the right entrance region; cold air sinks in the left entrance region. Horizontal ageostrophic flow occurs from colder to warmer air in low levels. However, air typically does not flow in a "box" as shown at left.

Fig. 2: Example plan view of a low-level/upper-level jet configuration along with areas of upper-level divergence/low-level pressure falls and ascent. The image at right identifies key elements of the schematic. Explanation of the diagram is given in the accompanying text.

Fig. 3: Example vertical cross-section of a jet streak (solid lines are equal wind speed lines/isotachs in kts) and the divergence (bold dashed circle; DIV) and low-level jet (LLJ) isentropic ascent (large bold arrow) response to the upper divergence in the right entrance region of the jet. The outer solid line with arrowheads represents a simplified direct thermal circulation within the entrance region. Dashed lines are lines of equal temperature/isotherms.

In a baroclinic system (systems that tilt with height), the lower-level pressure/height falls under the right entrance region/ divergence aloft cause a wind to be directed toward the pressure falls area (i.e., wind moves toward lower pressure). The wind moves toward the falls from all directions, including from the south where it is warmer and from the north where it is colder. This wind is the isallobaric component of the ageostrophic wind (small arrows in Fig. 2). South and/or east of the right entrance region/upper-level divergence/low-level pressure falls area, the southerly ageostrophic flow moving toward the falls area complements the environmental southerly flow already in place (southerly bold arrow labeled t1 in Fig. 2). Thus, an increase/ acceleration in the actual wind occurs near and south and/or east of the jet right entrance region (i.e., right rear quadrant of the jet streak).

This enhanced low-level flow (southerly bold arrow labeled t2 in Fig. 2 and LLJ in Fig. 3) then rises isentropically toward the upper-level divergence zone (large bold arrow in Fig. 3). Therefore, the area of strongest ascent (dashed-dotted circle in Fig. 2) may actually occur south and/or east of the maximum upper-level divergence area in baroclinic systems. In addition, as upper-level divergence increases with time, so will isentropic lift as the actual wind accelerates up the isentropic surface toward the pressure/height falls and enhanced divergence zone. In more "vertically stacked" systems, then the ascent maximum may be located closer/underneath the divergence maximum aloft.

At the same time, underneath or to the left of the jet entrance region, the low-level northerly or northwesterly ageostrophic winds (i.e., the small arrows to the north and west of the jet streak in Fig. 2) and the arrowhead on the outer solid line in the bottom of Fig. 3) directed toward the pressure falls area help to direct colder air toward the region of upward motion farther south and/or east (i.e., the bold t1 arrow to the northwest of the jet streak in Fig. 2 turns and becomes t2). This could help promote the maintenance of or even changeover to frozen or freezing precipitation in borderline rain/snow cases. The northerly ageostrophic wind component also may slightly negate the overall southerly low-level flow resulting in low-level convergence underneath/near the entrance region.

Now, we have an entrance region of a jet streak causing upper-level divergence which results in increased lift as a stronger low-level jet south of the entrance region causes enhanced moisture transport and warm advection up the isentropic surface. At the same time, colder air may be trying to filter in from the north. The result is that we get a tightening of the thermal/temperature gradient between the warm and cold advection zones and due to the convergent winds. The tightening thermal gradient is called frontogenesis, which in itself can elicit an atmospheric response that results in a direct thermal circulation (low-level warm air rising on the warm side of the gradient and cold air sinking farther north on the cold side). The upward motion/vertical circulation associated with frontogenetical forcing appears to enhance or focus the vertical circulation associated with the jet entrance region and isentropic motion.

One more item to consider: In direct thermal circulations (which occur in jet entrance regions and in response to frontogenesis), low-level ascent usually is on the warm side of the jet core and/or frontogenetical zone. On the warm side, if relatively unstable air is present (even near neutral stability air), the vertical motion values will be "pumped up" even more, because forcing for ascent will result in stronger upward motion in relatively unstable versus stable air. The ascent will then tilt northward with height toward cold air.

So, the end result is we have entrance regions of jet streaks, isentropic lift, frontogenesis, and possibly some degree of instability all seemingly working together in a complicated manner to cause strong upward motion. Moisture and system movement/propagation then also must be considered to determine if and where heavy precipitation will occur and how long the precipitation will last in any one area. Given sufficient moisture, these processes can result in heavy snow in the cool season and mesoscale convective system (MCS) development in the warm season, even if there is little or no surface low development. Certainly, these processes do not always work "just right" together to cause extreme weather conditions. But, one should be aware that processes in the atmosphere cannot always be separated from one another; they often are inter-related.

These processes worked together to produce a record snowstorm in Kentucky on January 16-17, 1994. In fact, 1 to 2 feet of snow that fell across parts of north-central Kentucky in less than 12 hours. In this case, there was little surface low development, but strong isentropic lift, a pronounced entrance region of a jet streak which increased in time, and substantial frontogenesis all where present, along with relatively unstable air aloft and upstream that helped intensify upward motion and produce elevated convection.