Jetstream settling?

[morls]

Hi,
Looking at the various Melbourne forecasts, it looks like the jetstream will be slowing down over the next few days. Hopefully we'll get some steady nights (if the clouds hold off :whistle:)
Stephen

[Wavytone]

Your "jet stream" is lower than you realise. More likely the surface wind.

First clue - weather map. Bookmark http://www.bom.gov.au/australia/char...ptic_col.shtml because seeing is usually best when there is a big, fat high pressure system - labelled "H" - with the centre smack over your location.

The weather systems move west to east and a high-pressure cell west of Perth will take 2-3 days to reach the east coast.

Second clue: Isobars (lines on this map) close together = lots of wind = poor seeing. Isobars far apart = calm, and (maybe) good seeing. A cold front (curve with pointy bits) means seeing will be utter crap.

And guess what - the isobars are quite tight AND there's a front coming. The next high-pressure system for you is at least 2-3 days away (its west of Perth).

Lastly, I don't suppose you have noticed this yet, but there is a seasonal pattern to the latitude of high-pressure cells - in winter the centres tend to pass over Sydney's latitude (well north), while in summer they tend to pass further south (Melbourne/Hobart).

[morls]

Hi Nick,

This is the animation I was referring to:

http://www.bom.gov.au/australia/charts/viewer/index.shtml?type=windbarb&level=200hPa&tz=AEDT&area=Au&model=CG&chartSubmit=Refresh+View

This shows the jeststream at 200hPa level (about 12km)

Cheers

Stephen

[morls]

There are also some figures here showing a slowing trend in jetstream speeds:
https://www.meteoblue.com/en/weather/forecast/seeing/melbourne_australia_2158177

It's interesting to see these sites, as I'm trying to work out how much use they are. According to Meteoblue Sunday night looks promising, have to wait and see...

There's a cold front hitting Melbourne this weekend so maybe (fingers crossed) Sunday night will be cold and clear...

[Wavytone]

I would be paying more attention to the BOM site as it’s the source of data. The others are just mathematical models.

If you want the wind strength at altitude the BOM has aerological diagrams based on the data from weather balloons released daily.

However most of the seeing results from the airmass in the first 1000m above your site - low down. At 12km up the density is so much lower I’m fairly skeptical that it has much effect. And at that altitude it never stops, either.