Special Update Tropical Cyclone Ilsa Rainband & Front April 14th-15th 2023
Special Update Tropical Cyclone Ilsa rainband & front April 14th-15th 2023
G’Day everyone, I was hopeful that I would be able to provide this update with a front linking up to a Tropical Cyclone and fortunately, this is now the case with a handy rainband lined up to help really moisten things up before we head into a broadly drier pattern in the weeks and months ahead. Originally, I anticipated that late March to at best early April would be the wetter period where we get that final flush out of the tropics however this has not only been delayed but also extended with the large amounts of tropical moisture remaining to the NW of the mainland. Now that we have had a strong MJO pulse move through the region, we have finally seen a proper powerful Category 5 Tropical Cyclone develop and (this is the key point) cross the coast in our moisture hookup window. The bonus now is the strong upper trough and cold front that is capturing it all and I will discuss this below and what rainfall we can expect from this system.
Currently on early Friday morning we have seen the Severe Tropical Cyclone Ilsa cross the coast near Pardoo with mean sustained speeds recorded at 218km/hr at Bedout Island just offshore which is an Australian record surpassing Severe Tropical Cyclone George in 2007 which was 194km/hr at the time. Bedout Island also reported a gust of 289km/hr before the station stopped recording – no doubt swamped by the storm surge which would have totally submerged the AWS computer below the anemometer mast. But nevertheless, most impressive. I do however think that the strongest winds would have been in the 300-320km/hr bracket on the front right quadrant (Bedout copped the left/western side of the eyewall).
Ilsa is now moving inland and has been captured by a broad upper-level trough approaching WA from the SW with the jetstream rapidly accelerating the core of the system across the Great Sandy desert. Remarkably it is still a category 3 but will eventually decay to a 1 just before the NT border. The main interest to us down south however is the moisture being pulled down off of this system by the upper trough (see Figure 1). The upper-level divergence and lift is substantial and with the approach of a cold front due across the Central Districts by mid to late evening, we will see the current areas of rain over the W and NW of the state, thicken up with the NW cloudband and extend right across the agricultural areas for a good soaking rainband overnight. The cloudband will lose some juice east of the ranges so expect a tempering of totals across anywhere east of the Mt.Lofty and Flinders Ranges and also the SE Districts but still some useful falls for this time of the year, especially considering the isolated good falls from the last couple of events.
By sunrise on the 15th, we should see the rainband and associated cold front contract eastwards and weaken, mostly clearing the eastern border by mid-afternoon. There will be a temporary lull from the back of the rainband for a few hours where the sun will poke through with only isolated showers being present, perhaps scattered in the Adelaide Hills. However, to the west a sharp mid to upper-level trough (See Figure 2) will stand right up and ‘break’ into the southern agricultural areas specifically from late afternoon and evening and contain some cold air aloft. The bulk of the activity will reach the Lower Eyre Peninsula by about 1:30-2pm and into the Adelaide region by approx. 4:30-5pm. The strong lift from this trough will generate squally showers and isolated cold air thunderstorms, with some heavy to very heavy showers at times. This activity will be a little less widespread north of a line from Cleve to Jamestown and also as mentioned anywhere east of the ranges as the rain shadow affect will shred some of the showers. If you are planning on attending the Port V Dogs game, I suggest to take all the rain protection you can get. There is a slight chance of some small hail in the most intense cells as well, especially anywhere from Adelaide and south of. Showers and winds will moderate overnight as a new ridge moves in, but onshore shower activity will continue in a more isolated manner on Sunday 16th.
Figure 2: 500hPa chart (18,500ft) for 15/04/23 @6:30pmCST showing sharp upper trough moving over the south of the state - source Meteologix
Rainfall:
Now the all-important rainfall and certainly there will be some good falls across the state, in particular the NW and W portions where moisture is the most plentiful but also the Mt.Lofty Ranges where orographic lift of both the rainband and aggressive convective showers, some in streams will result in isolated heavy falls.
The following is from the start of the event to about 9am Sunday 16th
NW Pastoral:
Thought I would add this area in given how much rain this area will get. General 30-40mm across much of the district with potential for 60-70mm totals in heavy rain areas from broadscale convergence and anywhere with isolated thunderstorm activity, especially towards the Southern portions of the district and also the very far NW Corner closer to the cyclone passage.
NE Pastoral:
General 20-30mm in the west decreasing to 5-15mm further east of the Flinders. Isolated 40mm totals still likely in the far N and W of the district.
Eyre Peninsula:
General 20-30mm with scattered totals to 40mm in the far NW. Totals decrease about far NE EP down to around 8-15mm.
Upper North:
General 15-25mm with scattered falls to 35mm. Isolated falls to 40mm about higher peaks and known wet spots. Totals decreasing to the far N and E off the back of the ranges down to 8-15mm.
Lower North:
General 20-30mm falls, with scattered 35mm totals. Isolated heavier falls to 40mm about higher terrains and known wet spots. Totals decreasing off backs of ranges down to around 10-20mm.
Yorke Peninsula:
General 25-35mm falls, with scattered 40mm totals. Isolated heavier falls to 40mm possible if any heavier cores occur. Totals decreasing towards eastern coast around 20mm or so.
Kangaroo Island:
General 15-25mm falls. Isolated heavier falls to 30mm possible about the western end. Totals decreasing over far east down to around 10-15mm or so.
Adelaide Plains:
General 25-35mm falls, with scattered 40mm totals. Isolated heavier falls to 50mm near foothills and anywhere with repeat heavy showers. Totals decreasing over far southern and western suburbs down to around 18-25mm or so.
Mt.Lofty Ranges:
General 35-50mm falls, with scattered 50-60mm totals. Isolated heavier falls to 75mm about known wet spots and higher peaks and anywhere with heavy cores. Totals decreasing over far southern/SE ranges down to around 25-30mm or so. There may be some isolated rapid rises in creeks and streams so take care.
Riverland:
General 7-12mm. Isolated falls to 15mm in the south. Totals decreasing towards the N Riverland around 5-8mm.
Mallee:
General 10-15mm. Isolated falls to 20mm in the south/SE. Totals decreasing towards the Riverland around 7-10mm and also in extreme lee to the SE of the ranges near the Lakes down to 5-10mm also.
Upper SE:
General 8-15mm. Isolated falls to 20mm near the coast and also the north.
Lower SE:
General 8-15mm. Isolated falls to 25mm in the north and about the coasts.
Western VIC:
General falls of 7-15mm, isolated totals to 20mm about higher peaks and the coast but decreasing as low as 5-7mm potentially in parts of the Mallee.
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Happy Farming and Cheers from The Weatherman
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