Longer Term Dry, Shorter Term Wet?

Mixed messages about the weather right now…and that comes from the mix of longer term and shorter term forecasting….so best to start with my perspective of the differences between the two.

Shorter term forecasts use the current state of the atmosphere and ocean and then project forwards with that information to forecast what will happen over the next one to two weeks. They’re getting better and better at this – but the problem is small miscalculations in the short term magnify quickly – so once you get beyond a few days the outlooks can change wildy from run to run.

Longer term forecasts look at the current state of the atmosphere and ocean and use this data to try to work out what that means for the coming season. They don’t forecast so much from day to day, but try to average out what they think that means for the season.

A good way of looking at how they work together is to think of a dice being thrown. If 1,2 or 3 means rain and 4,5 or 6 means dry, the shorter term forecasts will try to project which numbers will come up over the next week or two. The longer term forecasts don’t do that – they instead look at how the dice will be weighted…so for example if weighted towards 4,5 or 6 then they’ll forecast that it will likely be drier than average as we’ll likely see more 4, 5s or 6s than 1, 2 or 3. It does NOT mean that we won’t see a 1, 2 or 3 – just that we’ll likely see less of them. In weather terms that means that the longer term (seasonal) forecasts might indicate drier than normal – but that doesn’t mean we won’t see wet spells. We could be lucky and see a couple of bigger wet spells come up, even though the longer term forecasts were weighted towards drier than average.

Hope that makes sense – because you’ll need that to understand what is forecast right now 🙂

Shorter term forecast:

The current 10 day forecasts across Eastern Australia are finally showing the potential for some rain. The last few changes have been dry, but the next one coming up early next week looks like it could capture some moisture off the Pacific. Here’s the 10 day outlooks from 4 of the top models:

There are some significant differences between them in terms of who gets what – but the key take out from it is that a more unsettled spell is likely for many parts of NSW. Would be great to see some good rains across the tablelands to help with the huge fires up there. The change will be moving through early next week before it then dries up again. The next change might also bring some rain – but a long way off and I’ll post more about that as we get closer to the time.

Longer term forecast:

The outlook is pretty bad. Even if we get some rain over the next couple of weeks we’re unlikely to be out of fire season yet. Here’s the forecast from the EC model for October through January. This shows rainfall anomolies…so how much more of less than normal we will likely see. As we head into summer our average rainfall increases signficantly, so slightly drier than normal can still mean decent falls for us…but right now November would be a significant concern. The outlook is a result of the forecast temperatures and weather patterns in the Indian Ocean (positive forecast until end of the year – much drier than normal across SE Aus), Pacific Ocean (neutral for now – much less of an impact) and Antarctic Oscillation (how far north the westerlies come – and right now this one has the potential to head into record negative territory through October and November – which is not good for us at all).

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