Its all about timing - when the forecasts are issued versus when new data is available.
Most model guidance is run twice a day, giving us new information at roughly 5am and 5pm, and meteorologists get rather excited in the lead up to the new model "run". BoM need to issue their forecasts at 5:05am and 4:20pm, so the new shift takes the guidance that is available and spends many hours deciphering what is the most likely weather scenario. Then a computer uses official terms to describe that situation.
I'm lucky enough to have a different deadline - my 7am and 8am weather crosses on 3aw, and my 6pm news on Channel 7. This means that I have access to the next set of data, which I quickly analyse to bring you the latest information.
But, there is one more caveat. BoM's 5:05am forecast will make changes to the details for today, but leaves the outlook the same until the 4:20pm issue.
So, what you hear from me on 3aw can be very different to what is on the official forecast - as that is based on information that was issued yesterday morning (more than 24 hours old), whereas mine is based on data from this morning.
Today was the perfect example. Yesterday's 5pm run, and this morning's 5am run shows a low pressure system taking a path that produces rain over eastern Victoria and Melbourne on Friday into Saturday. I hinted at the possibility in my Channel 7 6pm news last night, then upgraded the forecast to rain for my 3aw crosses this morning. BoM should do the same in their 4:20pm issue today.
BoM Melbourne forecast this morning:
My forecast from 3aw this morning:
If you are after the very latest guidance, take a look at 'Latest Guidance", which changes to the weighted average of all the weather models as soon as they are issued. Otherwise, I'll have the latest information for you in 7 News tonight :)