Still Dry – Why?

La Nina is here - so where is the rain? Here's the explainer :)

A quick post today to answer an important question. I’ve been asked a few times this week why the weather is still drier than average despite the change to La Nina. It all comes down to a combination of factors needing to come together before we get rain. You need moisture, and you need something (a trigger) to turn that moisture into rain. That’s the roll of the dice…our dice right now is weighted longer term towards wetter – but like all things weather it’s still a roll of the dice and down to more than a little luck!

So we’ll start with the good news – there is a La Nina in place across the Pacific. This means colder than average close to the South America, and warmer than average close to us:

Good to have that in place…but what we also need is the Indian Ocean to be on side. Again, good news – look at the image above and you’ll see warm water close to NW Aus. This is what has helped bring about the big inland rains over recent weeks. Moisture streams across Aus from the NW, and a trigger turns that moisture into rain. So good news there too…

So why no rain for us? There is a lot that comes into play, but in simple terms we also need the Southern Annular Mode (SAM) to be on our side. And right now it isn’t. We need positive values to increase the chances of rain…and it’s stuck in the negative. Here’s how it looks right now:

A deadset deep dive into the negative. We really need that to turn positive again to see the rains coming in. A few days ago when I posted it was looking like it was about to swing across into the positive, but models are now not looking as positive in the short term. The good news – the SAM works in fairly short cycles, but the La Nina tends to hang around a lot longer…so sooner or later we should see the SAM turn positive and some (likely BIG) rains move in. Until then we’ll likely see increased fire danger when changes move through. The increased risk today is due to a classic cold pool moving across beneath us. This tends to increase NW winds ahead of the change (what we’ve got today) before blasting some colder winds across as it moves through (what we’ll see tomorrow). Here’s how that change looks like in an animation:

Certainly dramatic! And here’s how the humidity is looking right now – you can see the big drop through the day, which, combined with stronger winds and warmer temperatures, is bringing about our increased fire danger ratings today:

While we’re waiting for a change to wetter conditions…if you want to skill up about our ocean drivers then check out this post for heaps of information, videos and more 🙂

Images: Thanks for tropicaltidbits, NOAA, Bellingen Weather

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