Jeff's Weather

The weather connects us all. You know, it never ceases to amaze me how much it affects our lives, no matter where we are or what we're doing. So when we notice changes in the weather, we want to know why. Pat, it seems to me that the days are more and more humid. Is this a trend and is it likely to continue? "We have had a few humid days in January, there were a few days early on that were particularly humid but over all January and this summer's been average in terms of humidity."

We seem to have had more hot nights than usual. Is there any increase in the minimum temperatures? "Minimum temperatures have gone up across all of WA. In fact January was the hottest on record in terms of overnight temperatures." For decades, weather watchers have noticed we're getting less rain in winter and it doesn't look like that's going to change.

Pat, why are we seeing so much more summer rain and less winter rain? "One of the main drivers over the weather in southern parts of WA is what's called the sub tropical ridge. Now that's a band of high pressure that extends right around the globe, and we've seen an increasing trend in this sub tropical ridge, so what this means is we get more highs in the bite, and they're stronger than usual. So that forces the cold fronts, where we get all our rainfall, makes them slip to the south."

"We've been noticing since the 1970's a general decline in rainfall, particularly winter rainfall." Associate Professor Grant Wardell Johnson from Curtin University, studies the environment and extreme weather patterns. "Perth, and in fact the south west generally, will become warmer, dryer."

Bryson Bates says "What's become quite clear when you look at the winter half year from May to October, it's the May June July rainfall in particular that's dropped off."

Take a look. This was Perth in 1955 - safely within the blue and purple bands which indicate high rainfall. Now look at the map from 2010. See how the dryer, yellow and red bands have migrated south west. "Over the last five years you'll see a marked drying in this south east region here." Dr Bryson Bates is a climatologist with the CSIRO. "When you actually look at the animation you can see from 1940 to 1960 that the region slowly wets up over time but post 1960 it starts to dry. There's a little bit of a wobble around the 1980's where the drying seems to slow for a little while, then it moves on, continues to dry, but particularly since about 2005 there's a rapid acceleration across the whole region."

Now, does this indicate a permanent change in our weather pattern? Patrick says "The trends we're seeing in terms of decreased rainfall in winter don't show any signs of reversal."

It looks like we're in for some better rainfall soon ... "The next three months we're looking at about a 60-70 per cent chance of above average rainfall", but probably not enough. "There's not going to be much impact on the dams. For the dam levels to increase we really need that heavy winter rainfall, as well as some decent follow up rainfall."

Some long range forecasts are pointing to another dry winter.

Martin Palmer says "Obviously we're moving into the wetter months and towards winter but certainly in the autumn it really is looking a little bit dryer than we would expect."

Pat, we seem to be experiencing a shift in the seasons with summer pushing more into Autumn and winter overlapping with what's traditionally been spring. are you seeing any evidence to back this up? "The decrease in winter rainfall is also evident through autumn as well, so what we're seeing is a later break to the season, so people may interpret that as a shift in the seasons. The seasons that we typically use in Australia have been imported from the northern hemisphere so they're a bit of an artificial stamp on when we traditionally see the hottest period and the coldest period. We use the seasons because they're easy to convey, people understand what months they are. Traditional cultures have identified that there's more seasons than we use, up in the north they identify six seasons over the year."

Some of our weird weather is down to the one of the strongest La Nina's on record. "During a la Nina, we get very cold sea surface temperatures in the central and eastern pacific and very warm sea surface temperatures over further towards the Australian latitudes. So what that means is there's much more potential for evaporation and tropical moisture to get the very heavy rainfall that we've seen over much of Australia."

Cyclone after cyclone. One of them - the biggest in the nation's history. Yasi affected us too ... fanning those horrendous bushfires in the hills. Grant says "Massive cyclones, flooding, big fires in Victoria, these are events that we can expect to become more frequent and more extreme."

Pat says "Very wet conditions over much of Australia, apart from south western WA which missed out on a lot of rainfall. That's unusual, in a very strong La Nina event usually we'd expect down this way at least average rainfall."Well, what does it all mean for the Easter long weekend? Well, Easter's later than it was last year and according to the long range forecasters, that's significant. Martin Palmer says "The long range weather models are starting to come into line, obviously later in the month that in was last year it is a changeable month so as we move into the cooler months there is much better chance that we'll see temperatures a little lower than perhaps last year."

So we can probably expect some pretty good weather around Easter, cool and fine, with temperatures around 18 to 24 degrees. "Rainfall looking fairly similar as well, a fairly average time but perhaps a little bit dryer as we move through the autumn. The good news is, the light at the end of the tunnel, is as we move into the middle of winter there are some indications that we could see some decent rainfall finally."