What to Report
The previous sections in this booklet are intended as background information to familiarise you with thunderstorm features and characteristics. The following section outlines events which warrant reporting, although regional guidelines provided by your local Severe Weather Section should always be followed when filing your report.
Severe Thunderstorm Events
Thunderstorm cells are small (5 to 25 km diameter) compared with the coverage of satellite and radar imagery available to the Bureau's staff, so the spotter's role as an 'early warning system' to verify local severe events is invaluable. Many warnings are issued as a direct result of reported sightings.
There are three aspects of hail that merit reporting:
- size, if 2 cm diameter or larger;
- quantity, if hail covers the ground to a considerable depth; and
- impact, if noticeable damage occurs.
Photo 18. Hailstones from the Sydney hail storm, New South Wales,14 April 1999 compared
with a seven-centimetre diameter cricket ball. Photograph courtesy of Milton Speer.
Weaker thunderstorms may produce large quantities of pea or marble size stones that can cause extensive damage, especially when driven by high winds or when piling up on roofs. On the other hand, very large stones may be widely scattered and require some hunting in the grass to determine the biggest reportable size. Remember that falling hailstones are dangerous and have been known to cause head injuries and kill livestock.
Large hailstones are evidence of very powerful updraughts, and therefore, stronger thunderstorms. When very large hail occurs, you should definitely be watching for other severe events, including tornadoes.
The Australian definition of a severe thunderstorm specifies wind gusts of 90 km/h or more. Most spotters do not have the equipment (an anemometer) to measure wind strength, so will have to estimate maximum wind strength using the modified Beaufort scale shown on page 32-33. Wind gusts of around 90-100 km/h are strong enough to break large branches off trees, blow over some signs and remove roofing tiles, so you will be looking for effects such as these before sending your report. Wind gusts in excess of 100-120 km/h may result in widespread damage with trees uprooted, larger-scale structural damage to buildings, light planes and caravans overturned and moving vehicles pushed off roads.
Damage caused by a tornado can differ from that of regular high winds, although in some cases this will be difficult to distinguish, especially with fast-moving thunderstorms. A straight-line squall causes most of the damage to line up with the wind direction. This is especially noticeable with fallen trees. Also, large trees will be uprooted or blown over rather than snapped off. Tornadoes produce more abrupt and localised damage. Trees are often snapped off at 2 to 5 metres above the ground. Structural damage will be severe in one place, yet almost nonexistent nearby. Heavy objects such as farm equipment, trailers, etc. may topple with high winds but will be rolled, lifted, dropped or turned around by tornadoes. The tornado damage path is often narrow (50 to 200 m) when compared to its length (sometimes many kilometres) and clearly marked. Straight-line wind damage will be more widespread, ranging over several kilometres, but the greatest damage is likely right at the start of the surge.
The combination of torrential rains and local terrain conditions can give rise to flash flooding. The heaviest rains occur in slow-moving thunderstorms when the precipitation core passes directly overhead, or when a succession of thunderstorms move over the same location. The extent of flooding depends on soil type, amount of vegetation cover, land slope, saturation from previous rains, and the rainfall rate. Typically, rates of 25-50 mm in one hour and continuous heavy rain for hours will cause street flooding, overflow of creeks, mudslides and washouts, etc. The exact rain rate required for flash flooding will depend on the location and drainage system. In addition, your local Severe Weather Section may have set certain rainfall criteria for reporting.
Photo 19. Flash flooding in Elizabeth Street, Melbourne CBD, 17 February 1972.
Picture by Neville Bowler, The Age.
Tornado and Funnel Cloud Sightings
Any tornado or funnel cloud should be reported immediately if it is safe to do so. To test your sightings, refer to the 'false funnels' section for examples of look-alikes, but always report your sighting even if you are unsure.
Other Situations to Report
Any weather event that poses a hazard to life or property can be reported, even if not associated with thunderstorms. Examples might be widespread strong wind causing damage, extensive dust with poor visibility which may be hazardous to aviation, or other unusual phenomena. Your State or Territory Severe Weather Section may provide guidelines of other significant weather phenomena that you can report on.
There are several ways that you can file your report. The online storm spotter report form (your State or Territory Severe Weather Section can provide you with the access details) allows you to submit your thunderstorm reports online, with the details sent directly to the forecaster via an alert system and by email. You can also directly email the Severe Weather Section in your State or Territory, or print off report forms from the storm spotter website to mail in to the Bureau. Each State or Territory Severe Weather Section also dispenses local reporting procedures, including free-call telephone numbers and report forms. Apart from this, no special equipment is needed, although a ruler, a watch and maybe a compass will come in handy together with this booklet. A few general points on reporting procedures follow, with safety tips below.
Precise and thorough descriptions and measurements are also important. Whenever possible, give exact figures for damage, for example the diameter of snapped tree limbs or trunks. Measure large hailstones with a ruler or compare them to an object of known size. You may even wish to store these in a sealed plastic bag in your freezer in case a Bureau damage assessment team visits your area. If you do, include a selection of the largest and most representative sizes, and remember to mention this when sending in your report. Heavy rain can be measured from a rain gauge or estimated from the water depth in a can put out in the open.
Even after a severe event has passed, your discoveries of damage while driving around or talking with neighbours are also invaluable. You may find someone who witnessed a tornado but who isn't part of this program, or come across signs of damage in a remote rural area. You might also consider documenting your experience of severe weather events and damage with a camera or video camera. Your information will help the Bureau complete its records and contribute to improving the warning service in the future.
Next: Safety Tips