Winds relative to boundary/front

I too have been watching Saturday with some interest. Looks like a warm front set up. Low pressure near Norfolk with a warm front extending SE into SW Iowa. This would not be such a bad set up if upper level winds were not perpendicular to the front. IMO it looks like an elevated wind and hail event for eastern Nebraska and western Iowa.

One of the most important reasons for me joining this forum was to learn from others, so I'm going to ask a reasonable question. Why does the fact that the upper level winds are perpendicular to the front make a big difference? Is it due to the fact that a more oblique wind angle to the approaching front would more efficiently produce a spinning motion in the ambient air mass?

Really wanted to ask about this.


I moved this post out of the FCST thread since it didn't really fit there very well. In addition, it's a very fair question that affects storm mode, so it's in the interested of most chasers.

At any rate, I don't have much time to explain, so I'll be short. It's not so much the direction of the winds relative to a boundary (front, dryline, etc), but it's the vertical wind SHEAR (difference in wind with height) direction relative to a bounday that is important. The 0-6km shear vector (difference between the 6km wind and the surface winds, or the difference between the mean 5.5km-6.5km wind and the 0-500m mean wind) orientation relative to the forcing boundary can aid in determining the storm mode (quasi-linear, discrete, etc). When the angle between the shear vector and the boundary is small (i.e. the two are nearly parallel), quasi-linear convection tends to evolve; when the deeplayer shear vector and boundary are nearly normal (perpendicular), discrete activity tends to be favored. Again, short on time.

Just as a note, remember that a cyclonic supercell tends to move to the right of the deep-layer wind shear, not necessarily the right of the deep-layer mean wind! Depending upon the vertical wind profile, a storm may be moving well to the right of the mid-level or mean winds, yet not to the right of the deep-layer shear vector. Cold-pool / outflow dynamics and shear-induced perturbation pressure gradients (associated with the rotation of the updraft/mesocyclone) initiate and sustain deviant motion. The coriolis force is usually ignored since the time and spatial scale for convection is usually pretty small. For an extremely long-lived supercells (like the 5-state beast from 3-12), however, I'd imagine that the Cor force would affect its motion.
There are three kinematic considerations, perhaps more actually when you want to consider boundary geometry and its influences in storm type.

1. The angle between the boundary orientation and shear vector from near sfc to halfway up the storm (e.g., 0-6 km shear). Jeff pointed that one out nicely.
2. The The angle between the boundary orientation shear vector from near sfc up to anvil level. You could say that this could be the evolution from storm-relative anvile-layer flow, to boundary-relative anvil layer flow. Why would this be important? It could be that even if the 0-6 km shear vector was oriented ahead of the boundary, you could still get linear forcing by having the anvils fall back behind the line of forcing. Given enough storms forming, the combined anvil falling back behind the line can promote a stronger cold pool and a rear inflow jet, that is a boost to the outflow strength as a jet of wind accelerates underneath the anvil and helps drive the cold pool crazy.
3. Boundary relative storm motion. Let's say that you've got nice 0-6 km shear and 0-anvil layer shear such that everything looks great for isolated long-lived supercells. But now the storms are forming on a front that's moving quite fast such that the front is moving faster than regular or supercell storm motion. The storms will find themselves in the post frontal air and whatever instability it contains. If it's no instability, the storms will depend on elevated lifting air. What if there's none of that? Then the storms quickly weaken. Now if the boundary-relative storm motion is zero, then you've got a storm that feeds off the lifting zone of the front for long periods of time. That could be good or bad. It would be good if you've got a strong cap where a storm needs all the help it can get. It could be bad, however, if the cap is weak, frontal lifting is strong. Then any TCU forming on the front will reside on the front a long time and you've got a good case for too many storms.

All of what I said is going to vary depending on upper-level forcing, the strength of the low-level forcing, gradients in CIN and CAPE and of course your regular shear vectors. Cold fronts zipping down a dryline that's just spawned a separated set of supercells will immediately cause a squall line (aka Saturday evening in E. KS). If the boundary geometry's right, perhaps that line could've separated but it would've been tougher than with the dryline (same orientation) earlier. There's just that much more convergence with that zipper. BTW, drylines are very poor at initiation convection - good for isolated supercells.

Here's a link to a draft presentation I'm planning on publishing more formally later. See the second set of links on boundary coordinates

Watch the menu items on the left, some don't have anything in them.
I finally found the paper I was looking for... This is more of an observational study than anything in theory or modeling, but it attempts to find the relationship between mean wind and shear vectors relative to boundaries in regards to storm mode (linear, discrete, and mixed-mode):

Dial, G.L., and J.P. Racy, 2004: Forecasting Short Term Convective Mode and Evolution for Severe Storms Initiated along Synoptic Boundaries. Preprints, 22nd Conf. Severe Local Storms, Hyannis MA. [104K PDF]

From that paper:
<div class='quotetop'>QUOTE("Dial and Racy 2004")</div>
Preliminary results suggest that the orientation of the 2-6 km or 2-8 km mean wind with respect to the initiating boundary and the component of 2-6 km or 2-8 km shear normal to the initiating boundary are good discriminators between those environments where storms remain discrete within the first few hours after developing versus those where storms evolve into lines for storms initiated along a synoptic boundary. It would appear that when the mean 2-6 km or 2-8 km flow and boundary are nearly parallel, the precipitation cascades and associated outflows of neighboring storms may merge and consolidate more quickly. Results also suggest that when the component of middle level shear normal to the initiating boundary is weak, upstream development of a cold pool may occur more readily with a faster transition to lines than when this component of shear is strong. The amount of low level convergence appears to discriminate, but to a much lesser degree. Stronger low level convergence would likely lead to the development of more storms which would in turn increase the chances for storm interactions. Other than geometry and convergence, the type of initiating boundary also appears to play a role. It appears that storms initiated along cold fronts generally have a greater tendency to evolve into lines more quickly than those initiated along pre-frontal troughs or drylines.[/b]