The importance of a surface boundary should never be underestimated when choosing a target area for a chase day. Models often underestimate the enhancement of horizontal vorticity and low-level shear that can occur along and near surface boundaries. This enhanced low-level shear often aids in the processes of low-level mesocyclogenesis and tornadogenesis, and this article is my best attempt to explain why. This article mostly deals with weakening sub-synoptic boundaries that are likely slow moving or even stationary. However, certain concepts and ideas can be applied to all boundaries and frontal zones.

First, let's look at a hypothetical surface map for a potential chase day:



The surface map above is my best attempt to depict a weakening surface boundary that is still evident due to a wind shift, and weak temperature gradient. Boundaries like these are very common, and can be caused by a large number of factors including differential heating gradients, outflow, and old/occluded synoptic boundaries. Note the higher dewpoints along the boundary due to moisture pooling; this is very common and often underforecasted by numerical models. It is clear from looking at the surface map that vertical vorticity exists within the boundary interface. If environmental conditions are correct and updrafts/thunderstorms develop along this boundary, they can produce non-supercellular landspout tornadoes simply by stretching this vertical vorticity. It will now be argued that boundaries like these can aid in the development of supercellular mesocyclone tornadoes as well.

Basic meteorology tells us that horizontal vorticity must be enhanced along, and just north of the boundary due to temperature/density differences, and corresponding baroclinic vorticity generation. Effects from baroclinicity can still be evident in the wind fields near the surface even as a boundary begins to "mix out", as the wind field takes time to respond. For this reason never underestimate the potential horizontal vorticity along and just north of a boundary, even if temperature gradients are weak. The following figure shows a reasonable vertical wind profile above two regions - A: the warm sector B: the northern interface of the boundary.



The above two wind profiles are the same above the lowest four wind observations, as just enough deep-layer shear exists through the column to support mid-level mesocyclogenesis and supercells. The dashed line represents the top of the boundary layer, under which the wind field is strongly influenced by the surface boundary and associated baroclinicity. For this argument let's assume that other necessary environmental variables, like CAPE, are also favorable for supercellular development. It is clear that low-level shear above region B (along/just north of the boundary), is much more favorable for low-level mesocyclogenesis and potential tornado formation than Region A. Although wind speeds are very weak in the low levels on both profiles, the changing of direction with height within the northern part of the boundary interface is far more favorable. This can be seen more easily by comparing hodographs for the two regions. For arguments sake it is assumed that storm motions are equal (270, 25) in the two regions, although the motion in Region B would likely be slightly slower and a little more to the right.



The above hodographs clearly show the increased amount of storm relative helicity that exists near the frontal boundary than in the warm sector. Additionally, almost all of the extra helicity near the frontal boundary is created just above the surface, or in the area that is most important when considering low-level mesocyclogenesis and tornadogenesis, as the additional low-level horizontal vorticity can be tilted and then stretched by a supercell's updraft. The incredible thing is that all of the horizontal vorticity that is created from density and buoyancy gradients across a boundary is perfectly streamwise relative to a storm that rides the boundary. This may be the most important point made in this article, as streamwise vorticity is more readily ingested by a supercell's inflow and updraft. The following diagram is my best attempt to show this visually:



Once again note that the generated horizontal vorticity in the previous image is maximized very close to the surface. Combine that idea with the streamwise nature of the vorticity and it is clear why low-level mesocyclogenesis and tornadogenesis are very much enhanced when a storm can ride a boundary like this, as the generated horizontal vorticity can be ingested perfectly into the lowest levels of the updraft. One could argue that vertical vorticity along the boundary may also be important in low-level mesocyclogenesis. However, it can be shown mathematically that horizontal vorticity along/above such boundaries is usually a full order of magnitude greater than the vertical vorticity (Markowski et al., 1998), and therefore likely far more important when dealing with a long-lived supercellular updraft. Here's a figure from the Markowski et al. paper that shows 3-dimensionally what I have been attempting to explain with 2-D figures:



Part (a) of the above figure shows the baroclinic zone that exists on the "cold" side of a boundary, and the associated horizontal vorticity tubes. Note that air on the cold side of the boundary must be sufficiently buoyant, otherwise a storm's updraft will get "choked" off and either die or become elevated. Part (c) of the figure shows the storm moving over the boundary and becoming elevated, but this may not happen if the storm motion is sufficiently parallel to the boundary, as the storm's updraft will often try to "ride" the boundary due to the increased convergence and upward velocities along the frontal zone. This "riding" of a boundary can also change a storm's motion, which will often enhance storm-relative helicities for altogether different reasons.

It is true that ambient environmental shear may be sufficient for tornadoes, and even strong tornadoes, on certain chase days. On these days you may not have to worry about looking for boundaries to enhance the low-level shear; it's already good enough. This is why I chose hypothetical figures that show an event that appears to require enhancemant from a boundary for tornado formation. It is also true that supercells themselves likely enhance low-level horizontal vorticity through processes like cirrus anvil cooling, and baroclinicity along the forward and rear flank downdrafts. These concepts are key to understand, as a supercell has the ability to create it's own low-level horizontal vorticity which can then be tilted and stretched by the updraft. Ignoring these processes along with any potential enhancement from a boundary, and only looking at environmental shear when forecasting tornado target areas is a very large mistake in my opinion.

That said, it is true that without decent ambient low-level shear in the environment strong tornadoes are unlikely to form, even if you have supercells interacting with boundaries. However, supercellular tornadoes (not landspouts) are certainly possible on days when low-level shear appears modest, if favorable boundary interaction can occur. Favorable boundary interaction can also heighten the threat on a more synoptically favorable chase day as well. Data from the VORTEX experiment show that 70% of significant tornadoes in the project were found to occur near low-level boundaries (Markowski et al., 1998). Also, simulations from Atkins et al. (1999) show a stronger, more persistent mesocyclone when a pre-existing boundary is present, as well as much faster low-level mesocyclogenesis. As a final note, boundaries should be avoided as potential chase targets if storm motions will likely cross the boundary and move into an unfavorable area, or if the cold side of a boundary is very non-buoyant. This is why I often avoid fast-moving cold fronts as chase targets, since the air behind them is usually very non-buoyant, and storms are often likely to get undercut by the front. Each case should be considered seperately though, as there are cases when the cold front is the best place to go, especially if it's the only boundary that will initiate storms.

In conclusion, there are several reasons why you may want to choose a weakening sub-synoptic boundary for a potential chase target. The first, and perhaps most important, deals with storm initiation along a boundary due to increased convergence and upward motion. You can't chase storms if they don't initiate! Secondly, even if supercells are unlikely due to weak deep-layer shear, non-supercellular tornadoes are still possible if adequate vertical vorticity exists along the boundary and it can become collocated with an intense developing updraft. Thirdly, low-level horizontal vorticity can be enhanced along and on the cool side of a boundary, which can be tilted and ingested by a supercell thereby aiding low-level mesocyclone development and tornado formation. This process is enhanced when a storm can "ride" the boundary, since in this case the low-level horizontal vorticity is oriented perfectly streamwise relative to the storm's motion. Most of this article dealt with the generation of horizontal vorticity along and near a boundary, as I wanted to emphasize the potential for supercellular mesocyclone tornadoes even when ambient low-level shear appears modest. Also, when environmental low-level shear appears adequate for strong tornadoes conditions can still be augmented by additional horizontal vorticity from a boundary resulting in a heightened localized risk. Fourthly, the strength of a supercell's updraft can be enhanced by a boundary due to increased convergence and upward motion near the surface, which is likely very important when considering low-level vertical velocities and tornadogenesis. Fifthly, moisture pooling near a boundary can also strengthen the storm's updraft as well as lower the updraft base which also would aid in the development of tornadoes. I could write another entire article explaining the benefits of this increased moisture content and depth for potential tornado formation, but I'll refrain. And finally, the orientation of a boundary may cause a storm's motion to change, which could possibly enhance storm relative helicity values through the entire layer (deep-layer and low-level shear). So there you have it: six quality reasons to pay close attention to boundaries when choosing a chase target. I hope you enjoyed the article.



References:

Atkins, N.T., M.L. Weisman and L.J. Wicker (1999). "The Influence of Preexisting Boundaries on Supercell Evolution. Monthly Weather Review: Vol, 127, No. 12, pp.2910-2927.

Markowski, P, E. Rasmussen, J. Straka (1998). "The Occurrence of Tornadoes in Supercells Interacting with Boundaries during VORTEX-95".
http://ams.allenpress.com/perlserv/?request=get-abstract&issn=1520-0434&volume=013&issue=03&page=0852