(multicell system south of Darwin)
Storm observation in Darwin can be so easy it's not funny. The storms either come from the southeast and head northwest or they come from the Gulf of Carpentaria east to west across Darwin. There's plenty of sealed roads with wide open spaces to stop and wait for them to come to you, but you may run out of road on some chases simply due to vast areas of flat plains! If you want to actually 'chase' seriously then you've got a bit more homework to do.
Really you have three choices; you can either wait to hear the rumbles and then head out. You can watch the radar all day and see which way they're coming or you can educate yourself with some meteorology and get an advantage to save yourself chasing after rain!
That's the easy way out, but I've found that educating oneself really does get results and if you learn about the atmosphere and what to look for you may be rewarded a little more. So on this page I'll include some technical tools to assist in determining 'if' and 'where' storms are likely. One important rule though is this: THE ATMOSPHERE IS CONSTANTLY CHANGING THROUGHOUT THE DAY.
Atmospheric sounding charts or SKEW-T plot charts:
This bit can be very long winded and technical but I'm going to put it in layman's terms!
Weather observation sites and the bureau send up weather balloons twice daily, around 9am and 9:30pm (NT) time (or 00Z and 12Z) These times are Universal Time Cordinates which are similar to GMT Central Standard time = UTC + 9:30hrs and all flights, worldwide are supposed to be taken at these times - the 00Z flight is the most important. These balloons gather information of air temperatures, dew point, wind speed at all levels and pressure. One important thing they measure from all the indices is CAPE - which stands for Convective Available Potential Energy. This is measured from near the surface, but is dependent on atmospheric instability as to how close to the surface and how high above ground-level the measurements taken. These balloons take a while to complete their ascent naturally but the aim is to have the data reflect the atmosphere at 00Z and 12Z.
The easiest way to determine CAPE during the day is to work out the dew point factor during the day and alter the days expected maximum temperature from the surface to around 1000mb. Dew points have a significant affect on the instability and if you replot the sounding chart you will see the changes that can tell you whether your air will become more unstable or turn stable. What might seem like a good start on viewing the morning sounding will always change, so take the time to learn a bit about the charts and you'll find out as I have that there's more than meets the eye!
From the chart above how many indices on the right hand side there are to observe. The wind barbs are next to them and they indicate where the wind is coming FROM and at what speed. A half line at a 45 degree angle represents winds of less that 5 knots. Half lines represent 5 knots in speed, long barbs represent 10 knots and triangles are 50 knots. The numbers on the bottom from left to right are temperature ranges.
You can also see (hopefully) the CAPE value over 1727 j/kg and LI figure at -3.4C. The Australian version online does not have most of the abbreviations from the actual sounding which is fine, because you'll be looking at around 50 different things! It must be noted that all these figures can change during the day and the chart should be used as a guide, but those figures are good indicators of convection and unstable air. You can also see the wind not changing in direction but does have some shear in the lower half.
The numbers running up the left hand side are your millibar pressure areas or height. The figures along the bottom are temperature margins. Typically between 850 and 500mb is used to look at storm convectiion potential and steering winds. The sounding above shows a huge increase in -LI values and CAPE values have increased significantly. Of special note should be the wind directions and speed indicating shear in the lowers. Certainly strong storms with this sounding and one in which any chaser would be happy to see.
Storms are usually directed by the winds at these levels so as a guide you can see that the wind at 500 is from the south to southeast at 30knots so if I was chasing I'd compare that to radar images to see if the storms were actually traveling into Darwin from that direction. The majority of the time they do just that but on occasion winds at other levels can dominate and depending on where the convection is, some storms will move in other directions.
Just a a bit more on CAPE: It is used to understand what the potential might be for severe weather (and for storms in general) and how strong the storms may be. CAPE is calculated mathematically and I won't do that here, but basically it is a measurement of energy represented as Joules Per Kilogram. High CAPE values are sometimes termed as high instability and chasers look for values of 2500 J/kg's or better, although you can have low CAPE for storms and it really depends on the conditions at the time wherever you are.
Typically in Darwin our CAPE values range about 1400 to 2800 during the wet season, but with the right conditions such as we had on March 1, 2007 when we had a deepening tropical low pass over us (which ended up being Severe Tropical Cyclone George) our CAPE values for that day skyrocketed to 4000!!! That's just insane for Darwin and as a result we had a series of severe storms that produced 2000 lightning strikes in 3 hours AND created a supercell which spawned an EF2 tornado in Kakadu National Park 300km away that same day!
The F scale and the new EF scale is now in use in the USA as from 1 February 2007 and on the tornado page there's info on it.
(Here's a pic of the supercell that created the tornado)
(notice overshooting top from the anvil centre)
As far as convective potential goes here's a list below of values for CAPE or as chasers put it, 'storm potential'
CAPE value = convective potential (negative values)
0 = Stable
0 to -1000 = Marginally unstable
-1000 to -2500 = Moderately unstable
-2500 to -3500 = Very unstable
-3500+ = Extremely unstable
There's another thing to look at and that's the lifted index figure. LI numbers represent a measurement of atmospheric instability. Its value is obtained by working out the temperature of the air near the ground would have if it were lifted to a higher level above 15,000 feet, then comparing that temperature to the higher level's temperature. Negative values represent instability, the more negative the value (say -4 to -10) then the more unstable the air is and the stronger updraughts with any storms. If the number is positive ( + numbers) then the air is stable hence no storms!
There's a lot you should have to look for, but for simplicity look for high dew points, strong CAPE, good negative LI numbers and some sort of wind shear. If all else fails, just watch the skies!
In a nutshell all you need to remember is CAPE determines updraught strength and what type of storm it produces and along with directional shear or speed shear, these two determine the severity and assists in the characteristics of storms. Good shear mixes the water droplets, air and everything else in the updraught like a tumble dryer- the faster it goes, the more intense things get! Storms prefer moist air in the lower levels, dry air in the middle and moist again in the upper levels.
When observing radar images be mindful that what you see is delayed by a few minutes, even when watching it 'live'. So base the storm that you see to the next closest town within a few kilometres to be safe. Watch for any dissipation or weakening as the storm moves along also. If there's too much rain in front and it shows up as a faint white outline ahead of the storm it may mean the storm is cold pooling or blunty put, cold air from the downdraught ahead of the storm is killing it off so look elsewhere or go outside and have a look!
Satellite images are a handy tool also but you can only get them an 'hour or three' prior to what you are looking at on the radar, but it may be useful to see how the storms are growing or heading regardless during the day in any event, so you can monitor the cloud images on satellite and see what's happening.
There will be times when you've driven out and gotten 'busts' - that means the storm is done, gone, weakened. Every chaser gets them, it's part of chasing and you'll soon be able to recognize the storms that are worth chasing and the ones aren't and do peater out. I've chased what I thought were good storms with lots of lightning only to turn up and they've just gone dead! You'll save a lot of petrol and driving if you take your time in observing the storms. So long as there's convection happening as you watch it, strong updraughts and maturity of the storm then they're the ones to watch.
The key is to make sure you're prepared before you go out - sometimes you can get lucky and the storms are close by so you don't need all the tools, but to initiate the good storms early and get the jump on them once you get to your destination, then that will add even further satisfaction knowing that you've done the homework and been rewarded.
Some tools you can take with you are compass, maps, laptop with wireless internet, torch, your phone - in case you want to report anything to the bureau regarding severe weather - and not forgetting your camera, tripod etc and spare batteries!
Good luck and always have some commonsense and safety when you're out there on the road, not everyone is chasing storms and if you act professional then you'll gain the respect and won't become a nuisance!