Hailstorms

SEVERE THUNDERSTORM OUTBREAK OF 9TH SEPTEMBER 2002

During the late afternoon and early evening hours of Monday the 9th, ahead of a cold frontal change, scattered thundershowers flared up over areas north of the Vaal river, promising welcome spring rain relief from the dry, hot and gusty conditions which prevailed for much of the day. This was, however, a mixed blessing as the thunderstorms generated a widespread outbreak of severe deep convection extending across 3 provinces. A host of occurrences of storm damage were noted, including large hail, damaging wind gusts as well as at least two confirmed tornadoes.

The main mechanism for the initial development of the thundershowers was diurnal heating of a region (including much of the Highveld) that was located within a tongue of locally high energy air at the surface; promoting airmass thundershowers. This pattern, however, was set to change markedly later in the day. Winds aloft were reasonably strong, with 35-40kt NW'ly at 700hPa (10 000' above mean sea level) overlying much of the area north of the Vaal river, with a sharp upper air trough to the west, over the central interior of South Africa. Surface winds were also fresh to strong (and very gusty), with a NW'ly on the Highveld and NNE'ly to N'ly over Limpopo. Directional shearing in the lower few thousand feet was thus 'better' over Limpopo (than Gauteng) for long-lived severe storms (aka "supercells").

The image below is a satellite image in the infra red (IR) band, showing cloud development over Southern Africa on 9/9/2002 at 16h14UTC (18h14SAST). The cloud band associated with the thundershowers is visible over the Eastern Highveld of Mpumalanga as well as over southern Limpopo.

SATELLITE IMAGE (IR) at 18h14SAST"

The approaching cold front, which entered the Highveld region from the south-west towards sunset, is inferred to have been the catalyst for initiation of the severe weather outbreak, as the events correlate well in space and time with the advance of the cold front. Airmass boundaries at the surface (such as cold fronts and outflow boundaries) are an acknowledged focussing feature in the local enhancement of conditions favouring severe thunderstorms. The dramatic clearing of thunderstorm conditions over Gauteng was quite noticeable, once colder, dry and stable air moved in, accompanied by fresh south-westerlies. Many members of the public commented on the pronounced deep blue hue of the sky at this time (indicative of very dry air aloft).

Radar-based SMS warnings to Disaster Management and Civil Defence were issued as early as 15h45SAST, followed by 3 subsequent warnings, the final one being at 18h45SAST. Lastmentioned report specifically targeted Mamelodi as a region particularly at threat from "strong surface winds and large hail in the following 30-40 minutes. Reports of hen's egg sized hail were noted at Mamelodi. The storm progressed eastwards, roughly parallel to the N4 highway, passing Raeton and Cullinan where downburst type wind damage occurred, then further east through Bronkhorstspruit (trees uprooted, west to east,due to gust front), finally dying out in the Witbank/Middelburg area. Golf ball size (albeit soft and 'mushy') hail occurred at Witbank and also at the Middelburg Ultra City, where minor damage was inflicted on vehicles.

Other significant hail reports included Rooiberg and Mabula Game reserve (west of Warmbaths) where dove egg size occurred, as well as the Pretoria suburb of Waverley at 17h00SAST (golf ball size). This storm was first identified in the Beestekraal area, NW of Pretoria and was the subject of the second SMS warning at 16h15SAST. Unfortunately this message was unable to be tranmitted due to communication to the cellular service provider being interrupted. The storm in question also moved over Mabopane en route to Pretoria, causing hail damage at Mabopane.
Had the SMS warning been transmitted, Waverley would have had a full 45 minute lead time ahead of the approaching storm.

Strong wind gusts (exceeding 50kts),in association with thunderstorms, were registered by SAWS Automatic Weather Station (AWS) installations at Witbank (54kt at 19h45SAST) as well as at Ermelo (51kt at 19h00SAST). Trees were also uprooted in the Ermelo area, while damaging hail (of indeterminate size) smashed many windows near Arnot station. Lastmentioned anecdote gleefully passed on by a local glazier.

An F1 tornado was also documented as having occurred at about 8pm on a farm within sight of Arnot power station (east of Middelburg, Mpumalanga). Large trees were snapped off at the top, a farmhouse had half its roof removed, while (curiously) a standard windmill less than 100m distant was left completely untouched and was churning away asit normally would. The path width was about 100 to 150m, with a total length of about 4km. The photo below shows the damage to the roof of the farmhouse. Note the cracked masonry at the top of the wall.

"F1 TORNADO DAMAGE TO FARMHOUSE NEAR MIDDELBURG"

A tornado devastated the village surrounding the Hans Masebe school (near Mokopane, formerly Potgietersrus) in Limpopo, shortly after 21h30SAST. Extensive damage was inflicted on houses in the area and the well built brick classrooms were almost totally destroyed in some cases, while others nearby escaped with roofs either missing or partially peeled back (see photos below).

"F3 TORNADO DAMAGE AT HANS MASEBE SCHOOL, MOKOPANE, LIMPOPO"

Sadly, a 5 year old boy, William Mavalela was killed when a wall of his home caved in on him. As many as 50 people were treated by hospital staff for an assortment of injuries, including cuts and lacerations from flying sheets of corrugated iron.Note in the one image how a green water tower was left untouched amidst the devastation. An F3 on the Fujita scale would be consistent with the type of damage documented at this locality. The tornado then moved on to the Anglo American open-cast Platinum Mine, leaving an intermittant damage swathe of 200 to 250m, skipping over the boundary fence and railway line and inflicting a wide range of damage to mine facilities. Roofs were peeled back or selectively removed. Small to Medium size trees were either twisted off or uprooted, to be deposited elsewhere (one still upright!).

Indisputable evidence of 'twisting' type damage was noted at a few localities on the mine. Sand and gravel deposits adjoining mine heaps were also scoured and striated in arcs, while mine heaps of some 40 to 60m high were "mudblasted" up to a height of 30m. A closely parked line of tipper trucks each had their right hand side window popped outward and smashed. It is miraculous that no mine employees were killed or injured, with the tornado coming through at about 10pm at the time of a shift change. Two mine employees in a bakkie report that their vehicle was either "moved slightly" or temporarily "levitated", but that they escaped unscathed, with no damage to the bakkie. The photo below (courtesy of Ms C. Pretorius, PPM/Anglo American) shows damage to the structure of the boiler room at the mine, with large steel beams and roofing crumpled and tossed some distance.

: "DAMAGE TO BOILER ROOM OF PLATINUM MINE"