bolt.gif (4942 bytes)

Volume 6 Issue 1, May 1999

Produced by the Bureau of Meteorology, NSW Severe Weather Section

 

Hi everyone and welcome to the latest edition of the NSW Lightning Bolt. We’d like to take this opportunity to thank everyone for their contributions throughout the season. Don’t worry if you didn’t get to report any severe thunderstorms. It’s just lucky that you didn’t have to go through the dangers these storms can pose.

In this issue we have included a small crossword for you to have a go at, as well as some summaries of some of the larger storms in NSW over the past year or so.

Please remember to only report Severe thunderstorms (see our description on page 2) by telephone and by card, apart from those areas that in the past two newsletters have been nominated to report hail of all sizes.

At the most recent Severe Storms Conference in Brisbane, the value and importance of the Spotter network was discussed. It was unanimously agreed that the network is an invaluable resource for the Severe Storm warning and verification services offered by the Bureau.

The traditional severe weather season (October to March) was a relatively quiet one with around 80 severe thunderstorm events being reported on 41 days. Some of the more significant events during the season were:

  • Northeast NSW storm of 18/12/98
  • Sydney flash flooding of 24/1/99
  • Flooding on the South Coast on 28/1/99

Again we saw that severe storms aren’t just confined to our severe weather season with a couple of major events occurring, namely :

  • Wollongong flash flood of 17/8/98
  • Sydney hailstorm of 14/4/99

The Season in Review

The most number of severe thunderstorm days reported occurred in the Northern Rivers with 8 days, followed by the Hunter with 7, Mid-North Coast and North West Slopes with 6 days. Of the total number of severe thunderstorm events reported during the season, 46% produced large hail, 48% severe wind gusts and 38% very heavy rain generally giving rise to flash flooding. No tornadoes were reported this season although several significant wind events produced considerable damage.

A graph below showing the distribution of severe thunderstorms for each of our weather districts, summarizes the frequency of storms this season.

graph.gif (12394 bytes)

Seasonal Statistics

80 severe thunderstorm events on 41 days

Hailstorms

During the season the most frequently reported severe hail size was 3cm diameter. This size was reported on 32% of occasions. This was followed by reports of 2cm hail on 30% of occasions and 4cm diameter on 27% of occasions. The largest hail size measured was 8 cm in the Ballina area on 18/12/98. This event damaged many homes and property in the Ballina and Yamba areas.

Outside of the season, on 14th April 1999, a severe thunderstorm produced hailstones which were measured at 9cm.

Strong Wind Gusts

The strongest measured wind gust was 139 kph (75 kt) at Taree on 4/3/99. The strongest estimated wind gust was approximately 130 km/h (70 kt) at Boggabri on 30/1/99.

Heavy Rainfall & Flash Flooding

We had a number of heavy rain and flash flood events around the State with one of the heaviest being 70 mm falling in 20 minutes at Loomberah on the North West Slopes on 8/12/98. On 24/1/99, 88mm in 60minutes was recorded at Little Bay, near Sydney Airport.

Tornadoes

A tornado occurred in the Willow Tree area on 27 July 1998. Apart from a few funnel clouds being reported, no tornado events were logged during the season. However, this does not mean that none occurred.

Crossword

Why not have a go at our May crossword. You’ll find many of the answers within the pages of your Storm Spotters Handbook. For those who want to have a peek, the solution can be found at the bottom of this page.

xword.gif (7564 bytes)

 

 

 

 

 

 

 

 

 

 

 

Across

1. The coldest season

3. The Bureau’s primary provider of severe storm reports

6. A short burst of wind from the base of a storm

8. Damage scale used to estimate the severity of tornadoes

9. Rounded pouches sometimes protruding from the base of a thunderstorm’s anvil

10. Central core of a tropical cyclone characterised by light winds

11. Tornadoes, gustnadoes, dust devils and waterspouts are all examples of these

13. Precipitation characterised by its steady, continuous nature

15. The nursery rhyme character who fell in puddle right up to his middle

16. These can sometimes be seen extending out from a wall cloud

19. The wall cloud is the organised, circular ……. of the cloud base beneath a storm

21. The change in wind through different levels in the atmosphere

22. Muggy

23. The most common hail size reported- Golf …..

24. The part of the storm where moisture in the updraft condenses to produce precipitation

Down

2. This is often seen in the cloud base of supercell thunderstorms

3. The time of day when the sun drops below the horizon

4. Weather phenomenon associated with cumulonimbus clouds

5. December-February

7. This should be the primary concern of severe thunderstorm spotters

11. Rain falling from the base of a cloud which evaporates before reaching the ground

12. The warm, moist air moving into a storm

14. Water drops deposited overnight on leaves following condensation of moist air

16. The …….. updraft severe thunderstorm is characterised by its short lifetime

17. Visible indicator that the atmosphere has reached saturation at a particular level

18. Large …. is one of the criteria used to define a severe thunderstorm in NSW

20. The type of warning issued if the wind speed is expected to to exceed 33 knots but not reach 48 knots.

 

whatto.jpg (4133 bytes)

Spotters are asked to report to the NSW Bureau of Meteorology if they observe:

Hail 2 cm diameter ($2.00 coin size) or larger

Damaging winds (90 km/h) or greater (eg. trees snapped, uprooted, large branches down; roofing tiles / iron lifted; structural damage to well constructed buildings)

Tornadoes (rotating funnel clouds)

Very heavy rainfall with "flash" flooding of low lying areas (around 25mm in 20 min or 40mm in one hour)

As soon as it is safe, phone your report to our "freecall" number.

This greatly assists the Bureau in monitoring severe storms and passing on warnings to other communities.

Please post your spotter report card to us even if you do call. This ensures that our record of your event is correct. The information you provide helps us build a better picture of severe thunderstorms across the State.

Significant Events During the Season

18 December 1998 - Severe Thunderstorms in Northern NSW

Severe thunderstorms affected the Mid-North Coast and Northern Rivers weather districts of NSW on the 18th December 1998. Large hail, destructive winds and very heavy rainfall was reported during this event. The storms formed over the ranges of NE NSW in a convergent zone ahead of a cold front advancing northwards on the NSW coast. The low-levels of the atmosphere were very moist and unstable with further destabilisation provided by an approaching upper level trough and jet streak.

24 January 1999 - Heavy rain & flash flooding in Sydney

The Greater Sydney area experienced severe storms during the early morning of the 24th January. The storms developed in the vicinity of a surface trough in the moist easterlies combined with an upper level low pressure system. Very heavy rain was reported over eastern parts of the Metropolitan area between approximately 7:30 am and 10:00am. The worst affected area was the Randwick local council area. Another storm produced torrential rainfall over some northern suburbs with properties in Berowra damaged and the main northern highway cut at one stage.

Some rainfall totals included:

  •     Berowra Heights 141mm 24hrs to 9am Sunday
  •     Randwick 74mm in 24hrs
  •     Sydney Airport 60mm [55mm between 7am and 9am]
  •     Little Bay reported 117mm between 7am and 10am (88mm between 8:00 and 9:00am)

13th December 1998 - Severe Thunderstorms in the Wagga Wagga area

Thunderstorms developed in southwest NSW mid to late afternoon along an inland trough. Damage was mainly wind related with many trees and power poles down although some local flooding was reported due to heavy rain. At the height of the storm, a light plane broke free of its mooring at Wagga Wagga airport and was blown 100m across the ground. Large areas of Wagga Wagga were without power during the night. The maximum wind gust reported at Wagga Wagga Airport Automatic Weather Station (AWS) was 106 km/h.

28 January 1999 - Heavy rain and flash flooding NSW South Coast and Illawarra

Very slow moving thunderstorms formed in moist conditions during Thursday 29 January 1999. An approaching upper trough further enhanced the meteorological setting. The first reports of flash flooding came at approximately 12:35pm with 6 homes underwater in the St Georges Basin area near Jervis Bay. The worst affected area seems to have been the area between Batemans Bay and Narooma with some very heavy rainfall reported. Storms were also evident over parts of the Central Tablelands and the Sydney Metropolitan area.

hail.jpg (51475 bytes)

14 April 1999 - Giant hail in Sydney     A large thunderstorm moved over the eastern suburbs of Sydney during the evening of 14th April 1999. This supercell thunderstorm dropped hail measured at 9cm on the city, equalling the largest confirmed hail report in the NSW Severe Thunderstorm Database. The fact it occurred in April makes the storm all the more unusual. Our records suggest the Sydney Metropolitan area has only had one other large hail event in April, this being 2cm hail reported in 1994. To the left is a photo taken by one of the Bureau’s staff members at Randwick on April 14.

Hailstones (left) collected at Randwick on 14 April 1999 (Photo courtesy Mike De Salis)

Training Tip

The voltage in lightning strikes ranges from ten million to thirty million volts. By contrast, a domestic power supply circuit carries 240 volts.

Each year in Australia lightning claims, on average, 10 lives and causes over 100 injuries. Emergency Management Australia state that, generally, more lives are lost by lightning strikes than by bushfires, cyclones, earthquakes or floods.

People often ask us how to work out how far away a storm is away. A simple test is to count the number of seconds between seeing the lightning and hearing the thunder. The sound of thunder travels at about 1 kilometre every 3 seconds. Therefore a 30 second gap means that the storm is about 10 km away and you should get out of its path. A 15 second gap tells you it’s 5 km away and a gap of less than 10 seconds means that it is moving overhead. Up to 80 of lightning strike injuries happen when people use fixed phones during thunderstorms. They receive electric shocks, hearing damage, or burns when lightning strikes telephone wires in their area. Please do not phone in a severe thunderstorm report until the storm has passed. We will not phone you when we know there is a storm in your area.

(This article is courtesy of Storm Watcher, the newsletter produced by the Victorian Regional Office of the Bureau for the Victorian Spotter Network. Many thanks guys).

xwordsoln.gif (4186 bytes)Crossword Solution

 

Thankyou once more to all Spotters for taking part in the Network .

footer.gif (8894 bytes)