STORM TRACK: July 31, 1982 (Volume 5 Issue 5)

Charles Vlcek wrote earlier this spring with comments on the Garden City, Kansas storm that ST featured in its January issue with emphasis on the unusual microburst characteris- tics. "Take a good long look at, a good-sized waterfall. The spray will readily show the pattern of 'outflow similar to that in rain and hail-laden downdrafts. The side view illustrates a 'macro (or 'meso', if you will) downburst from a relatively steady-state smooth flowing waterfall or precip core.

Often, the current pulsates, and blobs of water come down in 'Vs' (front view) with resultant, outward bursts of spray.... These 'V's provide your minibursts. If you have a strong updraft next to a strong downdraft, as is often the case (and both are pulsating), two things can happen.

In the first case" (diagram),"turbulent, eddies form in the shear zone (b), collect rain and hail (c), then surge downward under the increased load (d), and fall back into the main updraft (e) before striking ground, so that it picks up more downward momentum.

The second case is of 'pockets' of hail (a) which grow rapidly (b) in an updraft which weakens just as the hail zone was getting too heavy anyway (c) -and falls into a downdraft, while renewed updraft, activity separates fresh hail from" that just, fallen. "I'm not sure if the second example is significant on the mini-scale. anyway, think of mini-bursts as huge (several hundred meters) turbulent eddies full of large hailstones.

I have noticed that precip rates are highly variable along an interface, especially on the SW side, where new cell formation on the flank complicates the picture. If 'nodules' formed instead of whole new cells, that, would be another source of minibursts. Time- lapse movies of the precip shield would be revealing here.

[Editor's note: One characteristic of the Garden City storm which I failed to note was that most of the micro-bursts observed occurred well outside of precip areas. In fact, no heavy precipitation was evident anywhere to the northeast (or anywhere else) during the early half of micro-burst activity.]

With regard to the direction of outflow: basically the flow is in all directions from where the downburst lands, but the observer is also getting environmental and storm movement flow. Thus, the net wind is rarely from the E or SE, at least with sufficient force to grab attention. You mentioned that the cells were moving slowly, possibly the environmental field (at the surface) was weak also. Thus, westward moving burst outflow was not, being cancelled by other airflows.

The last observation brings to mind an ... incident. I was a student pilot at Max Westheimer Field in Norman, Oklahoma, preparing to make my first unsupervised solo in a Cessna 172. As I taxied out to Runway 17-35, I noted clear skies, except for a cluster of CBs 20 to 40 miles to the east (distance indicated by NSSL radar). As I did my final pre-flight warmup, I was suddenly buffeted by strong breezes from the east. I switched to runway 14-32 (not my favorite) and took off, hoping that the wind would subside before I had to land (it did). Upon returning to NSSL, I took a look at the wind trace and noted a sharp spike where the wind had shifted from 180 deg at 5 kts to 090 deg at 29 kts. All I could think of was, "Gee, if I had been landing on one-seven when that squall hit ..."