Monday, 11 October 2010

how big is the current big wet?

Certainly this October we've had a lot of rain, but apart from relying on the courier mail to just tell us, just what is it like in the perspective of history?

While it is true that this has been a big rainfall for October its not yet a record rainfall. According to statistics obtained from the BoM the graph below shows the monthly records for Octobers in Southport since 1881 ...

To the time of writing, we've had 321mm of rain. The biggest rainfall that I know of in this area was in 1972 when the had 515mm in October. So with October already giving us 321mm here in Southport we are well above average and only 1/3 the way into the month well on the way to it being a record breaker month.

Did you notice the very big spike, thats 1972, but let me get back to that. So we've had a lot or rain and its been causing lots of flooding. Flooding doesn't come from yearly average or monthly rainfall (although monthly is a better indicator) it comes from serious rainfall events.

As it happens I don't have any historical daily data, but monthly indicators are telling stuff. Looking into that graph above we see most Octobers aren't above 100mm and we've already had more than 300mm 11 days into this month, so its no surprise that there has been some flooding.

This leads me to ask do we have that sort of rainfall in other months?

Well below is a chart representing summary data for the last hundred and twenty or so years (since 1881) :

Looking at the highest on record data row we see that January, February, March, April, May and June have all had levels higher than this the highest October.

In fact the "wettest October on record" does not really mean much in comparison to "wettest month on record". This means that its likely that in months other than October we may have bigger wets than this one.

The wettest October on record was in 1972 that wasn't the wettest year on record, that honor still belongs to 1974.

Perhaps no one living here now was here during the floods of 74. Hardly surprising when you look at the growth trends of the region. So with most of the region having no idea as to the climate they've moved into (probably form Sydney or Melbourne) lets have a quick look at what nature has in store for you :-)

Local Weather Trends

Firstly Rainfall is strongly seasonal here, we have a distinct wet season. But as people tend to think in years not seasons we often miss this data when we break it up into chunks starting in January and ending in December. That system may make sense to the Romans but they don't dictate our weather. So I re-arranged the data we record to construct the "wet season view" of things. Here is the view of rainfall from the wet seasons perspective from 1931 till about 2005 (I compiled this during my masters thesis)

It's a messy looking thing, but what it shows is the monthly rainfalls (as height) by season, but starting at August (when its typically dry) and going through to July (when its also typically dry). You can see that the mass of rainfall mainly clusters around December - May, with some individual peaks in strange spots. These are the statistical "outliers" almost like a very wet October is.

Anyway, I've marked two of the curves in that lot for discussion, they are 1972 and 1974. As you can see, in 1972 we indeed had a very wet October (the record one I mentioned above), but it was dry again until Feb when we had actually an ordinary monthly fall and a little less rainfall for the wet season overall.

Actually if you look at all the seasons where the month of October is high we can see far less confusing curve with less data (because October is not normally wet).

It would seem that if it peaks early then its almost as if given all its got too soon and looses out in sheer volume to the seasons which peak later. So while this may indicate good news for those who are getting flooded now (mind you I'm not a forecaster, I look at the past here) it does raise a more significant question:
what happens if we get the sort of rain we're having now in the strengths we've had in our peak wet seasons?

Well folks I think that the answer to that is "stock up on sand bags if you live in a low lying area" and "don't drive your car across flooded roads".

You see, to bring a little local knowledge to bear on all this data, back in 1974 we had a doozie of a flood. My father was rescuing people off their roof tops all over Sorrento and Isle of Capri in our boat.

Since that time we've had greater and greater development with populations growing by more than 6 times in the region (around 74 thousand in 1971 to around 492 thousand now) contributing greater amounts of rain runoff from a much larger impermeable surface (roofs and roads) contributing to greater peak flood events.

So despite the media beat-up on the wild weather, we may be in the clear this month. But, if the past isn't anything to go by and if we get rain for the rest of the month (and continue into other months) then there will be plenty of unhappy people living in what were previously flood prone areas (but have been rezoned as OK during the drought).

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