QLD, NSW, VIC, TAS: Wild weather hits eastern states

See also the BoM's report

Between 2 and 5 February, an exceptional meteorological situation brought widespread storms, large hail, hurricane-force winds, torrential rainfall, flooding, duststorms, unseasonable snow and intense heat and cold to eastern states. About 100 rainfall and low temperature records were broken across 5 states, many for locations with over a century of observations.

Wind and flood damage in TAS was put at about $5m, possibly the state's most expensive weather-related damage bill. In VIC, Melbourne City chronicled its heaviest one-day rainfall in 150 years while rainfall records fell across much of central and NE VIC. Across eastern AUS, records for both heat and cold fell in droves -- Balranald NSW shaved nearly 5° off its previous record for a cold February day in nearly a century of observations.

10 to 15cm of snow carpeted the Snowy Mountains as temperatures fell to -5 and wind gusted to 154km/h. Thunderstorms dumped hail up to 6cm diameter across eastern NSW, and severe dust storms raked northern NSW and southern QLD.

Why it happened

A polar outbreak more typical of winter pushed into SE AUS displacing a very dry airmass nearly 30° hotter. The cold airmass -- one of the coldest of the past century to strike the continent during a February -- arrived at the same time as a tongue of very humid, unstable air fed southwards down the NSW coast.

The images at right show the mechanics of this extraordinary event. Click on them for animations or enlargements that show how the event unfolded.

shows the situation mid-afternoon on Wednesday 2 FEB. VIC and southern NSW are buried beneath the cloudmass of a developing low as a cold front sweeps east, whipping up murky brown duststorms several hundred kilometres across in southern QLD and northern NSW, and a line of violent hailstorms in central eastern NSW.

The surface situation is shown in . A heat trough arcing from northern WA through central QLD into inland NSW is a regular summer phenomenon. This was the situation in the first chart in the animation at 10am EST on Tuesday 1 FEB, with a broad 1002hPa low in the trough in southern QLD. Six hours later, pressures over the NSW Riverina had fallen 8hPa as the trough low relocated southwards ahead of the colder airmass just about to arrive on the SA coast.

Overnight, the trough and low continued to move east about 200km ahead of the surface cold front. During the late morning of Wednesday 2 FEB, a low level circulation to the west of Canberra became evident on satellite images and shows as a kink in the cold front on the 10am chart. By the 4pm chart (where the animation pauses), a separate low had been analysed just NW of Canberra while the original low crossed the coast near Newcastle. The Canberra low tracked south across East Gippsland during the evening, its central pressure dropping to 988hPa as it crossed the coast.

By the early morning hours of Thursday 3 FEB the low was approaching Melbourne where it hovered for a day and a half before slowly moving away to the southeast. The low was at its most intense during the early morning hours of Thursday, reaching a central pressure of 987hPa near Wilsons Promontory at 5am EDT. This coincided with the heaviest of the rain in and around Melbourne.

Chart shows the situation at 850hPa or about 1.4km above sea level where temperature, humidity and wind are free from surface influences. Three elements stand out in the static chart at right, drawn for 11pm EDT on Tuesday 1 FEB when the surface cold front lay from the West Coast of TAS to Port Augusta in SA. The first is the huge temperature contrast across the front -- it is 26° over Hillston in SW NSW but only 2° over Adelaide 600km to the WSW. The mean 850hPa temperature over Adelaide in early February is 12°.

The second feature is the depth and uniformity of the cold airmass streaming up from the SSW -- between the SA coast and 55°S, a distance of nearly 3,000km, the airmass varies only between +2 and -4° indicating the strength of penetration of the polar surge.

The third is the stream of high relative humidity ducting down the NSW coast. Humidities are between 75 and 90% in temperatures generally around 20°, representing large amounts of water vapour in the atmosphere at low levels. The high humidity in the cold airmass is misleading, as air at these temperatures carries low amounts of water vapour even when fully saturated.

The animation shows that by the evening of Wednesday 2 FEB most of SE AUS was covered by 850hPa temperatures of around 2 to 4°, a situation that continued until Friday morning after which temperatures slowly rose as the system moved away to the SE. The exception was the tongue of warm very humid air wrapping around the developing low. On the 11pm EDT (12z) chart for Wednesday 2 FEB, this can be seen extending temperatures of around 14° into eastern Bass Strait, punching warmth, moisture and energy into the middle of the cold, unstable airmass -- right over Melbourne. On Thursday 3 FEB this train of warm, humid air moved south over TAS, but left a large legacy of moisture over VIC in its wake.

Chart gives considerably more information on the levels and movements of moisture and instability. The lines show precipitable water available -- i.e. the total depth, in millimetres, of liquid water that would result if all water vapor contained in a vertical column of air could be wrung out, leaving the air completely dry. It is a better indicator of the moisture available for potential rainfall than relative humidity which, as the chart above shows, is dependent on temperature. The yellow/brown shading indicates instability using the Convective Available Potential Energy (CAPE) formula.

The static chart at right shows the situation at 11am EDT Wednesday 2 Feb, when moderate to heavy rain had already commenced through central VIC and thunderstorms were occurring in the vicinity of the two lows in NSW, around the Southern Tablelands and in the Hunter. High moisture levels of 40 to 50mm extend down the NSW coast and to the east of TAS, while instability levels are moderate to high across the eastern half of NSW. The cold air pushing into western VIC is much drier -- 10 to 15mm -- but shows mild levels of instability common in post-frontal cold airmasses.

The animation shows that by 12z (11pm EDT) Wednesday 2 FEB, the moisture train had begun to wrap around the intensifying low in Gippsland, bringing levels as high as 50mm into eastern Bass Strait. Between this and the next chart, the moisture train moved over Melbourne giving some of the heaviest rain of the event around dawn. By 00z (11am EDT) Thursday 3 FEB, the moisture had wrapped farther around the low in winds that were onshore to eastern TAS and the western VIC coast. While moisture levels continued to diminish during Thursday and Friday, the train completed a full circle around the low, feeding a rain mechanism that produced heavy rain to the north of the low where winds moved upslope over the VIC Alps.

Chart shows the situation at 500hPa, and is representative of conditions in the middle atmosphere. The lines show the height of the 500hPa pressure surface and can be compared with the isobars on a standard surface chart. The static chart at right shows a deep upper cut-off low over SE NSW at the same time as the surface low was moving into Gippsland, to the south of the upper low. Broadscale steering over VIC was cyclonic (clockwise) around this upper low, forcing the surface low to move west into Bass Strait, then continue northwest into the Melbourne area.

The animation shows the development of this cut-off low during Tuesday and Wednesday. By 00z (11am EDT) Thursday, the upper low itself has backslid to the Melbourne area and aligned vertically above the surface low. The entire system, now perfectly stacked and strongly independent, became stationary as external steering forces ceased. It was not until late on Friday 4 FEB that the system began a slow southeastward movement in response to its own weakening and to the arrival of the next upstream trough in the westerlies.

Chart shows that CAPE (instability) was not significant after 12z Wednesday, indicating that the main rain-producing mechanisms were not instability but uplift. The animation for chart shows an enormous area of deep blue surrounding the upper low for much of the period. This represents strong vorticity, or the amount of spin around the low, which produced strong broadscale uplift through the atmosphere. This was enough to produce widespread heavy rain. Very heavy rain occurred where there was additional local uplift caused by the roughness of the surface around coastlines or rising ground in mountain areas.

Chart shows up and down motion in the lower atmosphere at 700hPa (about 3km). The dotted blue lines represent differing strengths of upmotion. The static chart at right for 12z (11pm EDT) Wednesday 2 FEB shows a bullseye of very high uplift already in place over Melbourne, with a band of equal or greater uplift across Bass Strait. This band aligns with the moisture train in . By 00z Thursday 3 FEB, uplift has moderated, but it seems likely that the enhanced rainfall over Melbourne around dawn was assisted by a continuation or increase in low level uplift.

By 00z Thursday 3 FEB, an arc of moderate uplift covers TAS and western VIC, and bullseyes over NE VIC, all coinciding with areas of heavy rain.

The final chart underlines the strength of the vertical structure of the system and gives clues to its origin. This chart shows windstreams at 200hPa or about 12km in an area where jetstreams roam.

The first frame in the animation shows a major kink in the jetstream had developed by 00z (11am EDT) Tuesday 1 FEB. Windspeeds in the core of the jet along the length of the kink are over 70m/s (250km/h). More importantly, the axis of the jet is nearly due southerly from around 60°S, sweeping cold air swiftly northwards from near the Antarctic coastline. A feature of most severe polar outbreaks in Australia is the development a southerly jet with a long fetch. While everything in the atmosphere is interrelated in a way that makes chickens and eggs seem simple, the kink generated in the jetstream by dynamics on a hemispheric scale could be said to have created this event.

Over succeeding frames, the circulation at 12km can be seen cutting off and aligning above the surface low, producing a broad spinning column of air reaching up to the tropopause.