Updated 1855AEST - 25th May 2000 - J ONeill

May 2000 Forecast Outlook, Discussion & Report Page

ASWA Victoria

Information

Saturday morning at 7am has better figures with LI's of 0 to -1 and CAPE of above 100jkg in the coastal areas with CAPE above 200jkg in the region down near Apollo Bay - Cape Otway - Port Campbell increasing to 300jkg or so by 10am and LI's down to about 0 to -2. With these sort of figures I would say that cold air tornadoes/waterspouts are possible.

The current satellite picture shows the cold pool and front pushing up from the S and even though it is not expected to peak in its intensity until it is virtually over us it still looks spectacular. I can't wait to see photos from this outbreak of cold weather.   Personally I think we will see thickness fall to somewhere in the vicinity of 526 on Saturday and there should be plenty of low level snow falls to boot. There will also be plenty of cold air CB's around with lots of hail - some possibly reaching severe sizes and there is a chance of cold air    tornadoes/waterspouts as well.

The latest (00z) GASP has a tongue of sub-528 thickness covering most of Victoria and southern NSW by 00z Saturday, moving into the east of both states and weakening to sub-536 by 00z Sunday, and broadening to be sub-540 south of a line Port Augusta to Port Macquarie by 00z Monday. GASP develops a low on Tasmania at 00z Friday, moving it slowly away SE through to Monday with little real development, but significantly places it in a NW/SE trough extending SE from eastern Bass Strait. The GASP 700 humidity pattern is interesting, and differs from the other models in having a pronounced dry slot after the initial frontal rainband, but then brings >90% humidity from the S across Vic and southern NSW by 00z Sunday. 850 temps are sub -4 across most of Vic and S NSW at 00z Saturday, contracting to eastern NSW by Sunday, but GASP keeps the 850 temp below 0 over almost all Vic and NSW through to the end of the model run on Wednesday.

The EC is broadly similar, but about 12 hours later in its timing, and moves a low from just under SE SA east to around Gabo Island on the Saturday evening, whereupon it bombs from 996 to 982hPa in 24 hours while moving only very slowly east. This splits the southerly flow, trending the surface flow southeasterly by Tuesday, and maintaining a NW/SE upper trough back through Victoria and western NSW, all with a thickness of around 536. Unfortunately, I don't have humidity or temp details for the EC.

The latest (00z) MRF develops a low west of Bass Strait at 00z Friday, moving it rapidly to just SE of Tassie by 12z Friday, then bombing it to ~977hPa by 00z Sunday with little further movement. This produces SSW then SWlies over Vic and NSW, but MRF keeps most of NSW and Vic enclosed by the 540 line through to Tuesday, with most of the country from central NSW southwards showing >90% 850hPa humidity, -3 to -6 850hPa temps and decent precipitation, also through to Tuesday! The MRF also has an extraordinarily agile cold pool at 500hPa, which appears to perform a somersault around Tasmania with a central temperature around -36. Starting west of Tas/South of Adelaide at 00z Friday, it crosses Melbourne at 00z Saturday, and covers much of eastern Vic, SE NSW at 12z Saturday before drifting away to the east.

Meantime, a second pool of <-38 forms on NE Tas at 12z Sunday before diminishing and shifting off to the ENE on Monday.  The 00z NOGAPS is rather a dry argument. Its timing is similar to GASP and MRF, with the main tongue of sub-528 air covering most of Victoria at 00z Saturday (sub 522 over Tassie!). The thermal trough moves slowly east, cutting off over the NSW central/Illawarra coast by 00z Sunday. NOGAPS keeps moving everything away to the east or SE, though it still leaves a substantial 500hPa temperature trough (sub -24) lying back E/W through central NSW. NOGAPs, however, has no rain anywhere over land after the initial rainband with the front.

All this detail is nicely tantalising, but my guess is that, on the day(s) in question it will be local-scale developments that will decide where the most interesting weather occurs. Certainly, Victoria from the ranges south would appear to be the place to be, but the tendency of all of the models, in one way or another, to produce a stagnant pool of very cold air over the SE of the continent could result in quite a protracted period of widespread, though probably light, snow.

I think the things to watch for on the weekend will be the movement of the upper cold pool(s), possible surface developments as far north as central NSW, the movement of the major low and the orientation of surface airflow, and the dewpoints -- the feature the models *dis*agree on most is where the humidity will be.

At last this present phase of high pressure control is coming to a close. We can all stand and wave goodbye and welcome potentially what could be a very significant cold outbreak. It looks as if the 300hpa trough will intensify to the west of Victoria on Thursday together with a strengthening high south of west Aust. A very strong temperature contrast should develop across the Australian Bight area. The strengthening middle and upper flow across central and north west Australia is another indication of potential cyclogenesis near to south east Australia on Friday and Saturday. At this stage the bulk of cold air should reach Victoria late Thursday and Friday. Eastern Bass Strait seems most favoured for low pressure development. For Victorian weather nuts, both professional and amateur, at last some significant weather is possible on Friday and Saturday i.e. large cold air cbs over southern Victoria and Bass Strait both Friday and Saturday with hail and local thunder possible, isolated waterspouts, snow over high ground, and possible sleet to low levels. Some moderate to locally heavy rain is possible over southern and mountain areas with a focus on east Victoria especially if the expected low pressure development becomes slow moving.

Many regards from Clyve Herbert and the Geelong Weather Service Office.

Has sped up considerably from yesterday. Now has the coldest air over  Melbourne on Friday night and rapid warming during Saturday.  Thicknesses are still down around 522-523 over Melbourne at 1200 GMT Friday, and sub-520 over most of Tasmania. They have a low developing in the Tasman on Saturday, but much weaker and further east than the other models.

Holds the cold air back a fair bit; on Friday night the sub-528 tongue is lying along the SA coast from Port Lincoln to Mount Gambier.  Reading between the lines it seems to be trying to spin up a small low over SE SA on Friday night, which it then has tracking across Victoria on Saturday and 'bombing' as it hits the Tasman on Saturday night. On Saturday night there are sub-528 pools over central Victoria and over  Tasmania, and the sub-532 zone reaches central NSW. The low deepens further over the Tasman on Sunday, pushing the flow more southerly (as opposed to SW), and there is a further cold pool over SA by Monday (sub-536) moving to north-western NSW by Tuesday.

Low SE of Tasmania on Friday night with the sub-528 tongue covering most of SW Victoria and SE SA. Develops a low east of Gabo Island on Saturday night, but not as intense as ECMWF. Not quite as cold on Saturday as some others, bottoming at 529 in Gippsland.

Only goes out to Friday night. Similar to UKMO at that stage but a  little colder, with thicknesses down below 524 near Mount Gambier. David's done a pretty good job of summing up the likely implications.

The real wildcard, although I think it is a bit of a longshot, is the possibility of a small-scale low moving across Victoria on Saturday accompanied by very cold air. This could bring quite heavy precipitation with it if it happens. The likelihood of a Tasman low and the consequent more southerly flow also makes me more optimistic  for snow in Canberra than I was 24 hours ago. (Canberra often misses out in SW outbreaks because of the rainshadow from the Brindabellas; a southerly, or even south-easterly if it's cold enough, is more favourable).

(for the last of these to happen, the most favourable scenario would be a combination of the ECMWF's potential mesoscale low and the slightly colder air of GASP) The higher-resolution local models (LAPS, MESOLAPS) will start to become available tomorrow, which will add a further element of interest.

As a guide flurries are likely to sea level with thickness values ~526 (provided you have the precipitation happening - which with TOT TOTS of +55 this shouldn't be a problem). Each GPDM very nearly equals 0.5C in average (pressure) weighted temperature for the 1000 to 500 hPa part of the atmosphere, so UK, US, GASP suggest snow around Melbourne to about 150m, 0m, -300m. Of course the models may have it all wrong.... though they first picked up on this cold outbreak late last week, and have only intensified it since, so there is hope they are onto something.

Two noteworthy features about the progged outbreak are,
1 - the best is expected to come through at night,
2 - the flow is progged to be between SW and S when the coldest air passes through.

The flow is particularly important for Melbourne as flow north of SW tends to be quite unfavourable for showers at night over Melbourne - from memory the cold outbreak we had in early June 1994 and Aug 1996 (97?) both had relatively unfavourable W flow, despite thickness values cold enough for snow in the city.

As for snow depths, the progs suggest moderate snowfalls at quite high levels ahead of the Friday front (mostly above ~1500m) - I'ld guess about 5-25cm (with Buller probably getting the least). Behind the front significant snowfalls will probably be confined to the southern slopes - meaning Baw Baw, Lake Mt etc. The real limiter with this system will probably be lack of moisture, as is often the case with cold outbreaks.

It seems to me that the reason the weather looks like it'll be so exceptional on the weekend is that the following high is so far south that it is pushing up air from half way to Antarctica! This suggests to me that during the following week, rather than having severe frosts, the ranges will have cloudy possibly showery weather which will make things very bleak and that we could see some record maximums in WA and SA with the airflow coming so far over the continent.

They have all maintained the intensity of the cold outbreak, but have turned it into a more classical southwesterly outbreak without the closed low near Tasmania, and clear things out more quickly. GASP has a tongue of sub-524 air almost reaching Mount Gambier on   Friday night and sub-528 covering most of southern Victoria from Melbourne westward. On Saturday the sub-528 zone covers eastern Victoria and SE NSW south of Sydney, with 524 locally over Gippsland. By Sunday night, although the flow remains a deep SW with a long fetch, thicknesses are above 536 over the full mainland.

The ECMWF thickness pattern on Friday and Saturday is almost identical to GASP over Victoria, but they have an even colder pool over Tasmania (sub-524 generally, and locally down to 519). They also push things back up over 536 on Sunday, but with a more westerly flow (as a prelude to a cutoff developing over SA on Monday).

UKMO, unusually, is not quite as cold as the other two, with thicknesses bottoming out around 528 in SW Victoria on Friday and 530 over eastern  Victoria on Saturday, although they push sub-532 air as far as the Central Tablelands.

No model is now going for cold pools moving into Queensland.

This is what I think will happen. A strong cold front, from deep in the Southern Ocean will move north over SE Australia on Friday, producing a narrow band of rain. This front in combination with a strong high positioned south of WA will force an extremely cold pool of air northwards producing conditions ripe for snow. The question mark at this stage is the amount of moisture associated with this system. Air dragged from so far south has the habit of been dry.

Snow is not the only feature to look out for in this system, hail showers and thunder are also likely and for storm chasers there is a better chance of seeing a waterspout at this time of year as the very cold air moves over the still relatively warm water.

My Tip = 25-50cm with possibility of more if moisture is good

I will be tracking this beast on my website, SE Australia Satellite Images

http://seaustraliasatpics.virtualave.net/

Friday could be the day to head for the coast, whether it be Port Phillip Bay, Phillip Island, Torquay or Wonthaggi for the chance for waterspouts and accompanying thunderstorms with hail.

ECMWF:

Saturday 1200Z - little change in high. Secondary low develops over NE Tasmania with main centre moving off to SE and very tight pressure gradient to its west. Sub-536 thicknesses over all of SE Australia  south of a line Port Lincoln-Broken Hill-Newcastle with an extensive pool of sub-532 over southern NSW and northern Vic (528 locally  near Canberra).

Sunday 1200Z - low moves well off to east, high edges SE. Deep S/SW  flow over all of eastern Australia. Thicknesses over most of NSW/Vic in 536-540 range.

GASP:

Friday 1200Z - very similar to EC except low is centred over Tasmania. Cold air does not push quite as far north, but still sub-528 thicknesses over SE SA.

Saturday 1200Z - again surface circulation similar to EC, except no secondary low over Tasmania (still the very tight cyclonic gradient  over Bass Srait, though). Sub-532 thicknesses over all of Victoria, with a sub-528 pool over Gippsland and sub-528 tongue still touching coast near Mount Gambier. Precip progs show heavy precip over alps.

Sunday 1200Z - surface again similar to EC. Large sub-532 cold pool over central NSW.

Monday 1200Z - remaining in deep SW flow, becoming increasingly  anticyclonic. Cold pool (down to 534) moves NE to New England/SE Queensland.

UKMO:

Friday 1200Z - low a bit further SE than others and with a more N-S orientation. Sub-528 tongue reaching SE SA (similar to GASP). If these models were to come off as planned, then I would expect the following: - heavy snowfalls over the eastern and north-eastern highlands of Victoria (with 3-day totals likely to approach 1 metre in favoured locations) - falls down to 500 metres, and perhaps lower, on the central highlands, Otways and Grampians in Victoria, most probably on Friday night and

Saturday morning. Possible snow showers to near sea level in SW Vic and SE SA. If the EC is correct then the low-level snowfalls could extend to the Adelaide Hills and Flinders Ranges. - possible snowfalls on the central and northern tablelands of NSW, depending on how much moisture is left in the flow by the time it gets there (GASP being the most promising for this). It is not  impossible that this could reach the Queensland border highlands.

Unless the flow turns S/SE at some point (unlikely) I would not  expect significant snow in Canberra, although the air will probably be cold enough (and I wouldn't be surprised to see a maximum well below 10 there on Sunday) - some whopper frosts throughout inland eastern Australia early in the new week, with many May records possible (especially given the favourable timing at the end of the month)

...this system looks more promising than anything I've seen since August 1997.

They talk of super outbreaks of severe weather in the US. Well - this is our equivalent of a super outbreak except the severe weather will most likely be restricted to large quantities of snow at fairly low levels (and HUGE amounts at higher levels) and the chance of some coldies.

For starters NGP, GASP and MRF have the front pushing through late Thursday. The timing of this is important as to when the first flurries of snow will start to fall on the higher peaks. Early evening through Melbourne would see snowfalls on the higher peaks (down to 1400m) overnight Thursday. Now for the fun part. The models vary when it comes to thickness. Some models have the 536 (yes 536) line over Melbourne from Friday until Monday morning!!!!!! NGP has the thickness bottoming out at 529 which would almost certainly result in snowfalls in suburbs of Melbourne. Even though the models are still 5 or so days out it is unlikely that this system will produce nothing. Even if they were to scale back the system we will still see thickness of about 536 over Melbourne giving us a good chance of hail showers.

Personally I think we'll see some awesome weather over the weekend with snowfalls down to at least 400m (with the chance of snow even lower depending on the strength of the system). The best day for cold air CB's will most likely be Friday with strong unstable SW'lys forecast at the surface and a mammoth upper trough (the kind we dream about in the summer time) giving NW'ly of at least 60knts at 500mb and temperatures likely at that level to drop down below -30C. It will be worth while keeping an eye on the LI's with this system. If they drop even close to negative at the surface it is likely that they will be negative in the middle layers. I haven't got time now to do a proper check of the models but I hope what I've put up already gets the blood pumping and a few cases of the flu beginning to develop (so you can take Friday off work to chase). Keep an eye on this page for the next few days as I'll be doing a daily update each evening up to and including Wednesday and then a final update before I go to QLD (to miss the whole bloody event) on Thursday.

Yes, its the follow up to that really really bad movie which occurred on TDU98 which caused a full week of boredom and SDS for a group of storm hungry chasers. All models are in agreeance (don't be so shocked) about this high pressure system pushing into our part of the world this week. The only real variations are the strengths of the high and they vary from 1032 up to 1037hPa. Unfortunately it seems like this will be the dominating feature of our weather for the coming week. Now we all know that this high cannot stay around forever (thank god) and it has to move away sooner or later. Some of the models are really unkind and have this bloody thing hanging around for TWO WEEKS.....now pick yourself up off the floor and dust yourself off....thankfully, that is only one model and in my opinion its the most unreliable at the moment. Most of the other models are going for some sort of frontal system coming into play by sometime next weekend. NGP is actually going all out and suggesting that we could be in for a half decent cold outbreak by Sunday. The model runs are currently only up to Saturday night but the 540 line is steadily marching NE with the 534 line not too far behind it. The interesting thing with this though is the high doesn't move out to the E it actually moves N and then W and then S meaning it would probably come and sit over us for another week again.....DOH!!!!

For the coming week.....enjoy The Death Ridge from Hell Returns - its supposed to be just as bad as the first one.

Friday night and into Saturday will see patchy rain across the entire state with no really significant falls likely (probably in the 10-15mm range). It should ease in the SW late in the day. Overnight Saturday will see a second front push through the state bringing with it our first really cold air for the winter. The 540 line is expected to take a hike N of Melbourne and the 850 temps are expected to drop below zero in the morning on Sunday. There is a chance of cold air CB's on Sunday morning with showers and possibly small hail. I believe it will start to snow on the higher peaks early in the morning on Sunday and snow showers should continue higher up for most of Sunday. I would expect snow falls of between 5 and 10cm on the resorts and probably a decent dusting for Mt Donna Buang. Minimum level for snow falls should be about 1400m although if this entire system speeds up a little and the front pushes through mountain areas overnight Saturday then we could see snow down to about 1200m. As usual, another ridge will become the dominant feature for the next week. Some fellow forecasters are suggesting another, stronger system will come through next weekend. I am yet to see evidence of this - the long range charts I have seen have a weak system looking likely for next weekend but this is a little too far off to be making judgements.

Enjoy.

Risks: cyclogenesis WSW of Kangaroo Island Friday / Saturday.

MRF 9 Day Panel Analysis: This also has the low pressure system forming but as I'd expect it moves more ESE. They have it digging deep into the cold, air and directing a strong SW'ly airflow over Victoria on Saturday. If this was the case then I think we'd see some rain overnight Friday and then clearing up early Saturday and then Saturday afternoon would probably see cold air Cb's.

GASP Analysis: This is similar to NGP with all the cold air staying further S. We could get some more fairly strong wind from this low pressure system if it is to happen this way with two fairly decent high pressure systems either side and all models taking the low down to at least 1000hPa.  The thing with forecasting these systems is the way the low pressure systems move. Its like trying to track a cyclone (well it basically is). You see it moving SE and then all of a sudden it starts moving ENE and you've got a whole different kettle of fish. The amount of cold air available to this system is largely dependant on the location of the high pressure system in the Indian Ocean. If this sits a little further S than currently forecast then it allows the low to penetrate into the colder air. Current forecasts have a ridge pushing down the SW side of the low which cuts off the cold air coming into the system.

Whatever happens, we are in for some very interesting weather. Same weekend as the cold outbreak last year!!!!

Wildcard: circulation may develop over north central Victoria and remain slow moving.  If this occurs, rainfall may spread further westward and may be more significant.

Thank you Queensland for sending your cloud patch southwards over the last several days!  Who said there is only one way traffic between Victoria and Queensland.  This cloud system brought significant rain from Qld through NSW to Victoria. Remarkably, one can trace this disturbance from the previous week when most of Queensland was affected by extensive rain, which then drifted into the Coral Sea and then some of which returned to the southeast Queensland coast, and eventually headed for Victoria.  Not Bad!!!  A computer modelling prediction of such an event would be very rare indeed!

Risks: possibility of cut off low developing north of Victoria and drifting southwards.  If this occurs there would be more potential for widespread rain throughout the state. It appears that this area of SE Australia extending to the SW of Victoria shows potential for cyclogenesis over the next 2 days.

Meanwhile in the weird and wonderful weather section, a rather unusual low pressure system near New Caledonia has been wallowing about.  To me it has the signature of a mid-latitude cut-off low pressure system, and will probably remain that way moving slowly southwest.  If you think you can generate decent bath waves by moving backwards and forwards in your bath (I'm sure all of you have done that!), the pressure gradient between this low and the high near New Zealand should generate some respectable wave heights over south east Queensland and northeastern NSW.  Another oddity is a disturbance over NE South Australia.  This system is moving NE, while the Coral Sea system is moving SW.  The air over NE SA is somewhat colder (-36C) while the soundings over Brisbane are -28C.  It certainly would be interesting if some of the colder air over SA drifted NE over Queensland.  I'm sure the Queensland weather enthusiasts will be keeping an eye on it.

A wish list forecast we'd all like to see - a super east coast low develop, move southwards along the coast and then moor itself in Port Phillip Bay.  Unfortunately though for Victoria, pressure remains rather high with cold nights and very pleasant days until the arrival of what appears to be a rather feeble frontal system Wednesday night through Thursday night, and then yet another high pushing into the western Bight region for the weekend.

Wildcard: the low pressure system NE of Brisbane moving SW to the interior of Queensland and then shifting S towards Victoria, although steering flows appear to favour a shift towards the SE as it approaches the Qld coast.