As the relatively calm days of summer are behind us, it is that time of year once again that sharper frontal systems begin to march their way southward across the U.S. to deliver crisp and refreshing Fall-like weather across much of the country. This gradual shift in the day-to-day weather pattern is also a catalyst in the return of organized convective systems across most of the central and eastern parts of the country. As cooler air is reintroduced equatorward, the sharpening temperature gradient favors an increasing possibility for strong to severe storms to impact areas closest to this boundary. At times, meteorologists will discuss the topic of “convective modes” and how some modes can favor certain types of severe weather under the right atmospheric conditions. So what exactly are these convective modes that are brought up on occasion?
It’s important to first understand the significance of the classification of certain convective modes prior to diving into some of the specifics. A convective mode is simply just the set-up of storm cells in any particular environment. There are many different factors that govern exactly what a favored convective mode will be leading up to a severe weather event. In short, the primary ingredients that are needed to distinguish different convective modes are moisture, instability (such as convective available potential energy, or CAPE), and lift, the primary ingredients for development and sustenance of organized severe convection.
Discrete convection is the type of convection in where cells develop in an isolated environment with no immediate interference between other storms in close proximity. This is considered to be somewhat more dangerous in the sense that discrete convection tends to lead to the development of more robust supercells. A paper by Thompson et al. (2003) showed that roughly 90% of reports for 2” or greater hail diameter are from supercellular storms which most often are discrete in nature. Moreover, Thompson and Mead (2006) relate the greater probabilities of significant tornado development to discrete supercells in that significant tornadoes are about four times as likely to form as a result of a discrete storm as opposed to any other convective mode. Most of the time, although not always, discrete storms exhibit more of a surface-based nature. That is, parcels of air begin to be buoyant closer to the surface as opposed to elevated off the ground. This in turn favors more robust updrafts and a longer CAPE profile and, coupled with favorable shear parameters, increase the likelihood of discrete and more severe convection. Examples include the 2011 April 27 tornado outbreak across MS and AL, where most storms were discrete in nature and many were capable of producing long-track, violent tornadoes that ravaged both states.
On the other hand, one must consider the development of a multi-cellular, or multi-modal, convective mode. In this particular mode, the atmosphere favors the conglomeration of individual storms under a moderate to strong shear profile confined to the lowest 3 km of the atmosphere. In addition, a stronger cold pool and surface convergence of the air also aid in the development of multi-modal convective mode. The end result is a long line of storms containing many of the severe weather hazards, although flash flooding and high winds become the primary risks as opposed to significant tornadoes. A prime example of multi-mode convection was during the “high risk” severe weather day which led to the flooding that was seen across portions of north and central Oklahoma on May 20th of this year (2019). Discrete cells from earlier in the day moved northward across the Oklahoma City metro area and merged with storms to the north of a warm front to become one “organized” complex of storms, which some meteorologists would argue as being a “messy” system. Several cities and towns in both the Oklahoma City and Tulsa metro areas received north of 4” in just that one event alone with locally higher totals as well, highlighting the significant flash flooding risk that multi-modal storm complexes pose.
Convective modes can be a tricky forecasting aspect for meteorologists and researchers, especially given the pressures of trying to forecast them in a realtime setting. However, field campaigns and more sophisticated modeling techniques have allowed for a more comprehensive insight into the unpredictability that lies within convective mode forecasting. All hazards are certainly possible with any convective mode, but the ability to forecast them will always mean the difference between positive and negative outcomes for both life and property.
Here are the paper references from this article in case there are interests to read further into this topic:
Thompson, R. L. and C. M. Mead, 2006: Tornado failure modes in the central and southern Great Plains. Preprints, 23rd Conference on SLS, St. Louis, MO, AMS, 59, #3.2.
Thompson, R.L., R. Edwards, J.A. Hart, K. L. Elmore, and P.M. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.
Photo Credit: National Severe Storms Laboratory
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© 2019 Meteorologist Brian Matilla