1. 500hPa geopotential height and vorticity. All COLA at
9.30am NT time today
2. 700hPa vertical velocity
3. Precipitable water and totals-totals instability index
4. 850hPa temperature, humidity and winds.
rain in western and central NT
Record February rains fell across western and central parts of the Northern
Territory today. Wave Hill Station, 550km south of Darwin, recorded 145.8mm
in the 24 hours to 9am, its highest February one-day fall in 28 years of computerised
records. This comes on top of the 76.8mm recorded there yesterday morning.
Nearly half the total -- 71mm -- fell in the three hours to 6 this morning
in a thunderstorm.
Not to be outdone, Tennant Creek Airport broke its all-time one-day rainfall
record in the 24 hours to 9am Monday 17 February with 156.8mm in the gauge.
The previous highest 24-hour total was 138.4mm. 100mm of the total catch fell
in 6 hours between 9 this evening and 3am Monday.
The intensification of the monsoon trough, and its southern movement down
to the base of the NT Top End, has given general falls of 100 to 300mm over
the past few days. This particular episode has been generated by the movement
of a Low in the trough to the NE Kimberley, just west of the WA/NT
border. The Low extends well up in the atmosphere and slopes to the SE, as
the first chart (right) at the 500hPa level, or about 6km. The blue
area shows a significant area of negative vorticity, or spin, in the upper
atmosphere, while the negative and green dashed lines in chart 2 show that
there's considerable uplift occurring at 700hPa or about 3km over central and
southern parts of the Territory.
Chart 3 shows
a huge buff area, representing high levels of instability covering the entire
NT, while a tongue of >65mm precipitable water is being injected by the low
from the Joseph Bonaparte Gulf SE into the Wave Hill area. Precipitable water
liquid water that would result if all water vapor contained in a vertical column
air could be "wrung out", leaving the air completely dry. It indicates
the total humidity of the air above a location, and is a good indicator of
the amount of moisture potentially available to supply rainfall. 65 is a
very high level.
The final panel shows the situation at 850hPa, or about 1.5km above
the surface, a useful level as it shows conditions close to the ground
but not affected by surface irregularities. A huge area of dark green represents
relative humidity levels above 90%, while the wind barbs show a gentle
of around 10 knots around the Kimberley low sweeping moisture down from
the north. A tongue of relatively cool air (16°) overlies the Wave
Hill area. The combined result of the cooler upper air, upper vorticity
produced significant instability (uplift), and with huge amounts of precipitable
available, heavy rain and areas of thunderstorms were the natural result.
Details of other heavy 24 hour and short duration falls are given below.