Cheech
18-07-2008, 09:38 AM
There seems to be a bit of discussion lately about Seabreeze being unreliable. I also noticed in another thread that others may be interested in understanding synoptic charts.
Now I am a long way from an expert, but I thought I would share my very laymans understanding of what I see when looking at the charts, and hopefully others that know more will add in their comments. This should help explain how they come up with the predictions, and why sometimes the predictions dramatically change.
Recently I have started to compare Seabreeze with BOM 4 day charts to try to understand how Seabreeze get their predictions from. Did you know that BOM is one of the places Seabreeze get their information for their model?
I printed these from BOM and Seabreeze yesterday. The first 2 days of charts from BOM show a big high right over Brisbane. As there is no change in barometric pressure, there is very little wind. You will see that there is a cold front heading our way. It is over Adelaide at the moment. This is indicated by the thick line with triangles on it. The other thing to note with the cold front is that the thin lines with the numbers on them (1012, 1016, 1020 etc) that state the barometric pressure are really close together. The closer together they are the faster the air is going to travel from one pressure area to the next. This means windy.
Then going into Friday, it is a similar story with the edge of the high still over Brisbane. You will see that the cold front that was over South Australia has now moved to the east coast, but has slipped by under Brisbane. Will mean that it is windy today on the NSW coast, but Qld will still have light winds.
http://i344.photobucket.com/albums/p351/craiggrendon/thursdayfridayforecast.jpg
Now if we look at Saturday and Sunday, Saturday is still looking good due to the high (and very wide gaps between the barometric lines). You will see that there is a second cold front moving through South Australia again, so will be windy there on Saturday.
Moving into Sunday, you see that the cold front that was over SA has now moved to the NSW coast. Thoug different to the earlier front on Friday, this time the front is coming through a bit higher. The outer edge of the close together pressure lines will go over Brisbane on Sunday morning. That means we will have a sharp increase in wind strength Sunday.
As BOM has only 4 days of chart predictions, if you have a look at what is coming our way, you will see that there is a real lot of wind on Sunday near teh WA/SA border, and that the tight lines extend up pretty high towards teh Northern Territory. As all the weather moves from left to right, you can expect this wind is going to eventually get to the east coast. If it was lower, it could slip below QLD, but because it is sitting high, the indication is that it will probably go through southern QLD and be windy for a few days into next week.
http://i344.photobucket.com/albums/p351/craiggrendon/saturdaysundayforecast.jpg
Now compare this with Seabreeze. Pretty close really.
http://i344.photobucket.com/albums/p351/craiggrendon/seabreezeforecast.jpg
You can also work out the expected wind direction because the highs rotate anti clockwise.
If others can add or correct any of the above, please do. As I said, I only have a basic understanding of it all, but I thought this would help some people in making there own decisions about the forecast rather than only working off one site.
I think we need to cut sites like Seabreeze a bit of slack, because (as an example) all it could take in the above projections is a weakening of the cold front, and the windy weather that is expected on Monday, Tuesday could easily slip below the Qld border, and be better than projected.
There is probably a lot that can be explained about wind directions and the impact of the highs across teh top of Australia. Hopefully someone can comment on that.
Cheech
Now I am a long way from an expert, but I thought I would share my very laymans understanding of what I see when looking at the charts, and hopefully others that know more will add in their comments. This should help explain how they come up with the predictions, and why sometimes the predictions dramatically change.
Recently I have started to compare Seabreeze with BOM 4 day charts to try to understand how Seabreeze get their predictions from. Did you know that BOM is one of the places Seabreeze get their information for their model?
I printed these from BOM and Seabreeze yesterday. The first 2 days of charts from BOM show a big high right over Brisbane. As there is no change in barometric pressure, there is very little wind. You will see that there is a cold front heading our way. It is over Adelaide at the moment. This is indicated by the thick line with triangles on it. The other thing to note with the cold front is that the thin lines with the numbers on them (1012, 1016, 1020 etc) that state the barometric pressure are really close together. The closer together they are the faster the air is going to travel from one pressure area to the next. This means windy.
Then going into Friday, it is a similar story with the edge of the high still over Brisbane. You will see that the cold front that was over South Australia has now moved to the east coast, but has slipped by under Brisbane. Will mean that it is windy today on the NSW coast, but Qld will still have light winds.
http://i344.photobucket.com/albums/p351/craiggrendon/thursdayfridayforecast.jpg
Now if we look at Saturday and Sunday, Saturday is still looking good due to the high (and very wide gaps between the barometric lines). You will see that there is a second cold front moving through South Australia again, so will be windy there on Saturday.
Moving into Sunday, you see that the cold front that was over SA has now moved to the NSW coast. Thoug different to the earlier front on Friday, this time the front is coming through a bit higher. The outer edge of the close together pressure lines will go over Brisbane on Sunday morning. That means we will have a sharp increase in wind strength Sunday.
As BOM has only 4 days of chart predictions, if you have a look at what is coming our way, you will see that there is a real lot of wind on Sunday near teh WA/SA border, and that the tight lines extend up pretty high towards teh Northern Territory. As all the weather moves from left to right, you can expect this wind is going to eventually get to the east coast. If it was lower, it could slip below QLD, but because it is sitting high, the indication is that it will probably go through southern QLD and be windy for a few days into next week.
http://i344.photobucket.com/albums/p351/craiggrendon/saturdaysundayforecast.jpg
Now compare this with Seabreeze. Pretty close really.
http://i344.photobucket.com/albums/p351/craiggrendon/seabreezeforecast.jpg
You can also work out the expected wind direction because the highs rotate anti clockwise.
If others can add or correct any of the above, please do. As I said, I only have a basic understanding of it all, but I thought this would help some people in making there own decisions about the forecast rather than only working off one site.
I think we need to cut sites like Seabreeze a bit of slack, because (as an example) all it could take in the above projections is a weakening of the cold front, and the windy weather that is expected on Monday, Tuesday could easily slip below the Qld border, and be better than projected.
There is probably a lot that can be explained about wind directions and the impact of the highs across teh top of Australia. Hopefully someone can comment on that.
Cheech