Forecasting: How to Differentiate between Linear and Discrete?

[Drew.Gardonia]

When you are forecasting, how can you tell whether a weather event will become linear in nature or remain as discrete supercells?

what are some key components/ingredients that would indicate storms will become linear? and how do you identify them?

 

[Skip Talbot]

You can have discrete storms that are linear, or embedded supercells that are not discrete.

When forecasting discreteness you want to look at how strong the forcing/lift is, the orientation of the initiating boundaries against the upper level flow, and how much of a cap is in place. If you have no cap, a fast moving, sharp cold front, and a powerful jetstream parallel to the boundary, expect a solid line of storms to go up and for them to train into each other. If you have a modest cap that burns off just enough, and just enough convergence at the triple point with modest flow aloft, expect a few isolated storms to go up.

For forecasting whether your storms will rotate, look at the vertical wind profiles. Do the winds veer with height (shift directions clockwise from the ground up)? Check out the storm relative helicity. If you've got at least 200-300 3km SRH expect some supercells, especially if you're forecasting for discrete storms. Also keep in mind that your directional shear will be enhanced if your storm moves against the mean flow, or interacts with a boundary like a warm front or outflow boundary.

 

[Chip Redmond]

What Skip said. Definitely winds winds winds! Look at hodographs. Although limited by many other things as well, curved hodographs show the turning with height suggest supercell structures, while straightline hodographs, or lack of any definitive feature from the origin will suggest linear unidirectional shear or pulse storms that move with descended cold pools. Keep in mind also the synoptic/mesoscale features that can change these factors into either more linear (a cold front, etc) or more supportive of supercells with increased directional shear (warm front, outflow boundary, etc) within the area you are analyzing. These move with time and you can often easily overlook them.

 

[John Peters]

Another important factor that likely differentiates sustained discrete supercell events from supercells evolving quickly into linear structures/MCS is the degree of deep layer sheer.

An environment with very strong low-level sheer and helicity - and only modest deep layer sheer, along with strong instability may produce strong supercells/mesocyclones when there are weak/sparing large-scale forcing/triggering; however, if there is substantial/strong large scale forcing/triggering, storms will often become very outflow dominant, in many cases evolving upscale into a cold pool driven MCS.

Crank up the deep layer sheer ensures sufficient separation between downdraft and updraft... so strong deep layer sheer, instability, and low level sheer/helicity, strong large scale forcing/triggering becomes less of an issue - you can have individual storm cells go up everywhere and still remain discrete.

I tend to think of things this way (of course there are often exceptions to these rules):


Low Level Helicity | Deep Layer Sheer | Instability | Forcing/trigger | Mode
________________________________________________________________________
moderate/strong | strong | moderate/strong | any | Classic/torn
weak/moderate | mod/strong | mod/strong | local in warm sect | Classic
moderate/strong | weak/moderate | strong | local in warm sect | HP
moderate | moderate | moderate/strong | front | squall
moderate/strong | weak/moderate | moderate/strong | LLJ | MCC/Derecho
weak | weak | any | any | pulse


When i say "local in warm sect", I'm referring to discrete (not widespread) triggering mechanisms off a linear feature, like a frontal boundary. When storm motion vectors turn parallel to a triggering boundary, such as a front, things tend to become more linear. (with bowing segments with stronger sheer). High LCLs and dry air aloft, along with strong deep layer sheer, modest low level sheer sometimes favors LP supercells.

/end rant

 

[Jeff Snyder]

I'll include links to two papers (one conference paper and one published article) that are available to download and read:

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.

--> 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.

The relationship between the type of initiating boundary (e.g. cold front vs. dry line) and convective mode isn't terribly surprising. In the case of a good cold front, the thermal wind relation tells us that we should expect that the vertical shear of the geostrophic wind (i.e. the thermal wind) will largely be oriented along the frontal boundary (with cold air to the left). With drylines, there tends to be a lesser density gradient involved, which means that the thermal wind tends to be weaker, and there's a reduced tendency for along-boundary flow aloft. In other words, for many cold front situations, a significant component of the flow aloft tends to be aligned along the front, which tends to support anvil seeding and the congealing of convection (caveat: there are a lot of variables involved, as we all know). Of course, if a strong trough is digging in or other processes are at play, there can well be very strong ageostrophic motions that modify this very general idea...

And a formal publication:

Storms that develop within a zone of strong, deep, linear forcing can experience rapid upscale growth into lines regardless of the orientation of the cloud-layer wind and shear vectors. Very rapid upscale linear growth is often observed within the zone of enhanced convergence and mesoscale ascent resulting from a cold front merging with a dryline. A small boundary-relative normal component of the mean cloud-layer wind can promote storms remaining within the zone of linear forcing, which can accelerate upscale linear growth. Conversely, a large boundary-relative normal component of the mean cloud-layer wind directed toward the unstable warm sector can promote storms moving away from the initiation zone shortly after they develop. This scenario would tend to favor persistent discrete modes, but storm evolution would likely also depend on the nature and strength of the convective outflows and the capping inversion in the warm sector.

--> Dial, G.L., J.P. Racy and R.L. Thompson, 2010: Short-term Convective Mode Evolution along Synoptic Boundaries. Wea. Forecasting, 25, 1430-1446.