Early days so hard to tell for sure yet, but you can start with weatherzone -
http://www.weatherzone.com.au/You can get a "chance of rain" field for all Tas major centres for up to a week in advance.
Then there's the BOM site
http://www.bom.gov.au/weather/tas/Or I made a page with all my favourite BOM pages on it -
http://www.tasadam.com/weatherIf you start with this
http://www.bom.gov.au/products/IDE00902.loop.shtmlThen have a look at this
http://www.bom.gov.au/cgi-bin/nmoc/late ... 3&number=4 to see where it's all coming from, it might give you a picture of what's to come.
Once you get used to these two pages, you can see how long it takes for the weather to come across, like the deep lows of Antarctica really get things ripping through quickly, whereas large highs between Australia and Africa can really get held up sometimes.
As I study it at the moment, forgetting about the forecasts which are all based on computer modelling, I can see there is a good sized high just off WA. There's a cyclone moving south fairly quickly toward this high, so the high could theoretically get pushed further south. However, there are some deep lows that look fairly aggressive hanging about further south, so the high might get squashed and weaken somewhat. So, in short, it could do anything and I haven't really got a clue...
Seriously though, I'd be bringing a good japara and plastic pants. I would be melting as much snowseal into your boots as they will take. I'd be packing a few extra garbage bags so it's easy to waterproof things in your pack to MAKE SURE you keep a dry set of clothes at all times, and to keep the sleeping bag dry. And I'd, most importantly at this stage of the trip, be following the manufacturers recommendations re your japara to make it as waterproof as possible - possibly meaning, depending on make, washing and drying, possibly ironing (that one surprised me), and adding one of the products to help the water bead rather than soak in.
With any luck, it won't be necessary, but preparation can make your trip that much more enjoyable. The Overland track can still be done in wet weather - the nature of where it is suggests many people do in fact do it in wet weather.
You can also talk to the forecasters at the weather bureau in Hobart on 6221 2000 - a handy number to keep with you when doing extended walks in Tassie if you carry a phone.
Good luck & have fun!