I have a question regarding how one should interpret the 28 day rainfall forecast. At the moment it seems to indicate that it will rain on nearly every one of the next 28 days East of a line from about Bairnsdale (Vic) to Rockhampton. For example, it suggests rain for Sydney on 19 of the first 24 days of January (taking 'High' = 7/8 probability of rain, etc).
This would be quite extraordinary (not to say depressing). The forecast for the rest of the NSW coast looks similar. I note that the 12 month rainfall forecast for Sydney indicates slightly above average rainfall for January (6th decile), while the BOM and US models for early January don't look particularly wet.
What does the 28 day rainfall forecast mean? Does it give the probability of some measureable rain somewhere in the district on each day? If this were the case, then I would expect larger areas to have more 'wet' days. Or does it indicate the average probability of measureable rain at any given location in the district on the day indicated? Does it refer to the probability of any amount of measurable rain (e.g. 0.2mm)? I am curious.

I didn't keep a copy of the forecast when started this thread. From memory it said a couple of days around New Year would be dry, with about 80% of days to January 24 having a 'high' probability of rain (i.e. about 7/8) with one or two low and a couple 'medium'. The 28 day forecast still looks crazy - rain expected virtually every day for the next four weeks on the coast & tablelands from SE Qld to the Victorian border, with East Gippsland having a few more dry days.

The record in Sydney for the first half of the period projected when I wrote this (Dec 28 - Jan 10) has been:

No rain: 10 days / 14
< 1mm: 3
>= 1mm: 1

Not a great result.

I remember last summer the 28 day forecast looked similar. It wasn't particularly wet in Sydney, but there were widespread floods in Qld, NSW & Vic. There was therefore a large quantity or rain but it didn't rain nearly every day.

Right now for Sydney it says 1 dry (Jan 11), 2 low (Jan 12-13), 2 Med (Jan 16, Feb 2) and 23 high. Counting 'nil' as a 1/8 probability of rain, low = 3/8. medium = 5/8 and high = 7/8, the forecast is apparently expecting rain on 22 of the next 28 days. We have had periods like this, but it would be quite remarkable. I really question the forecast.

During the period Dec 28 to Jan 24 Sydney (Obs Hill) had rain on 12 of the 28 days:

[*]Nil: 19 days
[*]< 1mm: 4
[*]1 - 10mm: 1
[*]> 10mm: 4

Sydney's rainfall for the period in terms of total amount and number of days was close to average.

Assuming that we get measurable rain in Sydney by 9:00 tomorrow (inevitable I would think with a brisk S-SE'er blowing), the number of rain days at Sydney (Observatory Hill) in the 28 days since my January 10 post will be:

Dry ...........11 days
< 1mm ......... 4
1 - 9.9mm ..... 7
10 mm+ ........ 6

So the result is that 17 of the 28 days had measurable rain, as against the 22 implied by the forecast on January 10. Still, it has been cloudy much of the time and the forecast for Sydney included the words 'Showers' or 'Rain' on nearly every one of those days. There was one hot day in the period and that was very humid, a reminder of the heatwave this time last year but only one day, not seven.

Here is a post made on 16/1/2012 in the NSW Day to Day Weather thread that seems to me to give a useful way to interpret the forecast: