Laurier, I see what you mean about the lack of moisture at 700mb, relying on 850mb moisture is not as good, but at least some is better than non. With the added cold uppers some can still get pushed a little higher with the uplift.
00z GFS has taken the lead from 18z with the colder southerlies on Mon/Tue but as expected with a much more subdued version of events, not really allowing the colder upper temps to make much headway over the tablelands by keeping them more to the SE and out to sea. Still room for improvement though because we are talking 6 days anyway, see what EC says a bit latter.

Blizz/Laurier regarding that note about the snow spots around the CT's there is one more well known spot for snow along the Dark Corner road starting from Sunny Corner for about 4km or so northwards. In a classic WSW cold change wind gets funnelled between Mount Ovens and the range to the north over the gap of Winburndale Dam and produces very cold gale force winds here. It never ceased to amaze me because the stretch of road is only 1170m asl yet has often been one of the last places on the CT's where you could find a snow drift after most snow has melted. Often drifts have lasted here 2-3 weeks after a snow event, but it has to be the classic type. However over the last 10 years classic events that include 850 temps of at least -2 with moisture from that direction have been very few.

A couple of times when I drove to Mount Bindo in the early morning, I was surprised that snow cover was not as good as I expected and found that there were stronger showers forming in the orographic wind in the Yetholme/Sunny Corner ranges when I returned towards the end of a snowfall. I guess the low open country over the Bathurst basin also allows fresh uplift to the east when conditions are right for orographic precipitation.

Yes, the 00z charts with the newest sonde data tend to bring a clearer picture, for now, it seems. Still a long way off this system, so I don't have a firm opinion as yet.

Great info there Laurier re the lumpy 850hPa RH and minor troughs. That is fascinating. Will reflect on that some more. It is fun watching the process of those minor troughts out at Shooters. One season, not long ago, I was lucky enough to watch about half a dozen such troughs move over high hills in the area whilst Shooters remained clear. Very interesting to watch this occuring.

Peter, that is interesting re that heavier snow out there in certain conditions. I had a look on Google Maps. Sounds like some orographic effects and maybe Venturi Effect as the winds are crammed into that area? I can imagine the latter would also deposit a fair amount of drifting snow in your area during heavier falls from the SW. Sounds like a prime setup.

I did not know that was the name for it, but yes that is it...
http://www.islandnet.com/~see/weather/elements/gapwind.htm
Only problem in recent years has been the lack of cold winds from that direction, in the 80's it was common to get the big snow drifts there.
This is a picture of a snowdrift at Meadow Flat in 1990, I am having trouble finding one from Dark Corner Road.
29-06-90 Meadow Flat



Yes, 00z EC not very helpful, still looks quite likely to me that this will just be some cool weather.

Another variation of the venturi effect is a vertical type of gap wind where a strong windstream is forced to lift suddenly, as by a cliff or escarpment. This phenomenon was dramatically demonstrated by the horrendous wind damage caused last year by a night of hurricane-force winds along the exposed Blue Mountains ridge line between Blackheath and Medlow Bath. Blizz can probably put a date to it - I can't pull up a reference at the moment, and I'm sure there would have been a thread on this forum about it.

A strong westerly low-level jet developed and lingered over the area for about 12 hours with Mt Boyce recording many gusts around 150km/h. Along the N-S running ridge line, which is just behind a continuous flank of 100-200m cliffs, damage was phenomenal with many trees that had stood for a century uprooted. Wind gusts there were probably in the order of 200km/h. The greatest damage from even higher winds occurred east of the ridge line down two valleys running west to east. These seemed to have suffered the combined effects of two venturis. The low-level jet was squeezed into a vertical venturi as it was forced abruptly over the ridge line above the cliffs, then the already strengthened windflow further accelerated as it was constricted into a horizontal venturi (or gap wind) down these two valleys. Friends of mine living in Wills Street along one of these valleys found, at dawn, that every plant, shrub and tree - including some huge gums - had been stripped bare and/or unearthed. Their magnificent old garden has had to be completely replaced. Fortunately, their house was farther up the side of the valley near the top, and damage to it and its surrounding vegetation was relatively minor.

GFS 06z run has further eased back in the strength of the cool change while the ACCESS 06 run is slightly less bullish than it was, though both still push a substantial cool change up the Divide from Sunday through to Tuesday. EC 00z run is even less bullish.

However the NAIFS chart for 00z today [here] still shows dark blue across southern TAS, southern VIC, eastern NSW and SE QLD for the week beginning Wednesday 16 May. The dark blue means that 90% of the ensemble of 40 different numerical model forecasts it uses are forecasting temperatures in the lowest 33% of all observations for this time of year over the past 30 years. This would suggest that, even if this change is not particularly snow-producing, it will move well north and will usher in a particularly long-lasting cool spell (for this time of year). The long-range models have also been suggesting a second cold spell beginning early the week after next, which would help prolong the cold conditions along the Divide during the NAIFS period.

That NAIFS chart is interesting. Lower 850hPa temps at the end of ACCESSR seem to suggest that too. This system is still a long way off so anything could happen although it seems it may not be snow bearing beyond the alps.

Scroll to the July 5/6 link below for a report on the damaging winds in the Blue Mountains last year:

http://www.blackheathweather.com/2011.html

Head to Laurier's link here for more comprehensive charts:

http://australianweathernews.com/news/2011/110705.SHTML

Our dog was born on that day, basic phone video, runs around like the wind since:

http://www.youtube.com/watch?v=HT9o8gMgIU4&list=UU-4yuleIWoT-nwt3YabRrXg&index=13&feature=plcp

Thanks for those links, Blizz. For those interested in blow-by-blow (excuse the pun) descriptions on this forum at the time, go [here].

00z runs of ACCESS and GFS still borderline for CT snow, but timing varies. GFS has overnight Sunday/Monday with the coldest temps (850s down to -2 even -3 around Orange and the OP) with patchy but significant 850 moisture. It then warms 850 temps well into positive territory during Monday. ACCESS has some -2 temps up into the southern CT on Sunday/Monday, then brief warming, followed by a second surge of -2 as far north as the NT overnight Monday/Tuesday. ACCESS 850 moisture both nights is patchy but with significant blobs greater than 90%. Both models have negligible 700 moisture. I haven't had time to work out why the two models now differ so much, nor have I looked at other models.

Still a waiting game, then.

Both GFS and EC look very borderline, however I have to agree with Laurier that there is just a slither of a chance for a few snowflakes on the CT's between Sunday and Monday morning from that low level moisture. Still if I was to bet or make a forecast for the CT's my thoughts are that there will be only a cold dry wind with patchy low level cloud, any speck that falls out of this cloud will be barely noticeable.
One thing that is clear now is that there will be no cold pool at 500mb flung to the CT's and wind flow will remain between West and South not swinging to the SE yet. This make this a more standard cold change for the Snowies, so clearly Snowies will do ok for some light dustings.

As potential snow events loom, we spend some time on the forum discussing relatively fine variations between models and model runs. The difference between an 850 temp of -2 or zero over the CT can be a cause for joy or despair.

There is one other variation we need to take into account.

First, look at these two images showing thickness at 10am Monday, and look in SE Australia. Both are from this morning's 00z ACCESS-G run and are for 10am Monday. The first chart is from Weatherzone with the 540 thickness line in thick blue. The second is from the Bureau's own site where the 540 thickness line is the thin dotted line south of the thicker dotted line labelled "544".




Weatherzone's maps (Australia or NSW or VIC - they all show the same) place the 540 thickness line well out to sea from the eastern Victorian coastline. The Bureau's own chart has it enclosing most of central and eastern VIC and all SE NSW. Quite a difference for a rule-of-thumb line that we use often.

Next compare the 850 temp isotherms in SE AUS for 4am Monday. To improve clarity I've used the Weatherzone map for Victoria but the isotherms are located in the same places on both the Australian and NSW versions of the chart.




Again, the Weatherzone interpretation has the zero and -2 isotherms in quite different positions to those on the Bureau chart. Both the zero and -2 isotherms make huge incursions into NSW on the Bureau chart, yet on the WZ one the zero isotherm merely clips far SE NSW and the -2 isotherm only just reaches Wilsons Prom. On the Bureau chart, the -2 isotherm covers most of the CT.

Why the differences? Different software producing the charts is one reason. Another may be that Weatherzone, which has to download very large GRIB files of gridded data from which to construct the charts, is only getting a coarser grid to reduce file size. In this case, the Bureau's version, using the full data available, should be the best.

The why probably doesn't concern us. What is important is to recognise that minor variations on the charts aren't always what they seem to be. In addition to being aware of differences between models, differences between runs of models and the differences between the 3- and 6-hourly snapshots of weather that the models give, we must also allow for the differences in the ways that different website interpret supposedly the same data for the same time for the same run for the same model.

Got all that?

On current indications, I agree with you on all counts.

That makes me feel better about all the uncertainty with the models Laurier. The angst that the chopping & changing of the different models creates can be surprising at times. Taking a step backwards (which can help when viewing Xrays) or viewing the charts with a bit of a squint might be the best way. Local & micro factors also play a role - complicated but also very interesting. The local effect stuff that Blizz & PD have posted recently has been fascinating!

Laurier, thanks for that interesting post about the 850mb temperatures, there is certainly some play in the values. For some reason I have generally found that GFS is a little better than EC for 850mb temps. I think EC tends to smooth out data a bit more while GFS picks up finer detail with better accuracy. Access I believe is more based on EC, but why then WZ and BOM's Access are a little different is interesting too.

Further downgrade at 12z so does not look too good now. At least the weekend will be cool and breazy with a very autumal feel.

Yeah, an old science mate of mine had a good and simple saying, 'What is really happening here?' Basic question but complex is the answer at times and some effort is needed to stay with that sentiment. It reminds me of a tennis player focussing purely on the point at hand or the Thomas Merten, Zen notion of being in the moment. Its hard not letting our wishes and desires (Blizz wants winter to start 15/5 and finish 15/9) get in the way of what is actually happening at this moment, as far as we can determine.

I would love to be in a helicopter hovering over the Sunny Corner area in a classic SW change to watch the effects of the terrain out there. A buffeting trip to be sure but videoing and photographing it would be fascinating.

Let it snow.

Laurier - that example of model variation was very eye opening. I don't delve into the depths of the charts, I just use a few main indicators then as aslaws says I look at the surrounds to see how it all fits. Watching that latest low pass almost exactly west to east has me thinking their is less likliehood of a substantial northward push but then sometimes there's a flick in a tail too.

Agree the diff between a pass and fail for us this time will be very small re placement. Worth watching - winter is approaching though for sure.

As an aside - how crazy is GFS in about 10 days time - if THAT came off some pulses would be racing. Of course Access and EC not out that far yet so can't compare. Suspect that model will come back a wee bit.

If we're talking about the WZ representations, this is at least in part due to different resolutions or grid spacing. Open these up in separate windows: ACCESS [here] , GFS [here] and EC [here] . Now count the number of wind arrows along any line from the left to right sides of the image. I get ACCESS 15, GFS 19 and EC 7. The wind arrows are placed on the grid points, so EC's resolution as portrayed by WZ is at least twice as coarse as that of GFS and a lot of detail washes out as a result.

We're into fairly deep waters here, because it's also not as simple as grid-point resolutions. Some models are spectral (i.e. represent the variation of weather variables over space and time as a series of waves) while others have variable grid-point spacing (wider over homogeneous areas like oceans, tighter over areas like mountains), and all this has to be converted from the native output to grid point data to be transmitted as GRIB files for users like WZ to decode into charts. If you want a (relatively) brief "lesson" on all this, go to these notes [here] from Colorado State Uni - they're a bit old (Autumn 2003) but give a good intro to the problems of weather modelling and the different types of models.

Yes, I commented on that yesterday, so GFS has persisted with it through at least 3 runs. While it looks dramatic, the upper temps over SE AUS aren't anything to write home about yet, but it suggests a cut-off situation looming and they can be interesting. It has started to appear at the end of the ACCESS run at 10pm Wednesday 16 May when the trough is just SW of WA and has an embedded cold pool of -41 at 500, which should get the thermodynamics working.

All the models have eased off the Sunday/Monday situation, I think because they apex the cold upper air (~-33 at 500) just SE of TAS on Saturday afternoon as the upper trough moves through. Consequently, the surface low stays south under the cold upper air in a broadly zonal flow.

Yes, the mslp charts for that strong system across the SE of Oz via extended GFS look pretty nice (although really, that is a standard, good system for late May) but I had a look at the upper temps earlier and also lower and mid level RH, nothing that exciting there, yet. It fragments pretty fast as it moves beyond the alps.

The mets from Weatherzone reminded us some years ago (2007, I think?) that a cursory awareness of the models can lead to pitfalls, as they are complex. I think the public availability of myriad charts and progs do lend to 'the models got it wrong again...' exclamations quite often. I have to remind myself to study the details more, before I make a bold prediction as its not the models erring in a forecast, its me...

I've saved those notes from Colorado State Uni, will gradually work through that.

I'm also watching developments SW of WA where cold pool temps are progged to be impressively low. That 'may' lead to fufilling outcomes for snow lubbers.

Current satpic animation [here] shows how zonal the movement of the major low and cold air is. Low centre should pass very close to the southern TAS coast in 9 hours or so. The Whole Bight visible images show some nice detail [here] and the greater enlargement of the eastern Bight [here] may show snow accumulation across TAS over the next few days during daylight hours.

I was hopeful, too, but both GFS, EC and ACCESS are very zonal at the end of their runs. The 500hPa long wave chart at 192 hours (end of normal GFS run) is as flat as a pancake under Australia [here].