Updated: Sep 10, 2020
Welcome to Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks. Tonight's question is about rainfall reliability.
Did You Know:
"Can we rely on our rain here in North Queensland?"
I'm going to expand your question to encompass all of Northern Australia and take you step by step each three monthly period to answer this one.
Want to know more? well read on
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RELIABILITY OF RAINFALL JANUARY TO MARCH EACH YEAR
These months are characterised by widespread ascending air and large amounts of moisture advection. That means lots of widespread rain areas and thunderstorms. However each year varies depending on the strength and location of the monsoon trough and any depressions and convergence zones that form. The only places that can rely on solid rainfall year in year out is the Northern Cape York Peninsula, a small section of the NE QLD coast between Gordonvale and Cardwell, The Top End north of Katherine and the North Kimberley. Those wet seasons almost never completely fail but nor do they usually receive a ridiculous amount of rain either (with the exception of the NEQ region). For those of us south of those regions we have significant variability of rainfall in these months and we have some huge years (much bigger than any of those locations I just mentioned) but we also have a lot of dud years and especially as we head into SW Queensland, Central Queensland and the Pilbara/Gascoyne, without a monsoonal incursion, a decaying rain depression or a Tropical Cyclone impact in those areas we simply can not rely on wet season rainfall being there for us every year.
RELIABILITY OF RAINFALL APRIL TO JUNE
This is characterised by the end of the Northern Wet Season. Strong trade winds from really strong HIGH pressure systems become established by the end of April. The only areas of Northern Australia that can rely on reasonable rainfalls during this time of year are locations on the North Tropical Coast. Strong South-Easterly winds are uplifted powerfully by those mountain ranges we spoke about last night providing some solid rainfall year in year out at these locations. For everyone else, most of the time this is a dud period, but then occasionally we still have late season Tropical Cyclones to contend with and those Tropical Cyclones or late season rain events can dump record or near record rainfalls making the variability of rain at this time extremely high and the reliability of rainfall extremely low.
RELIABILITY OF RAINFALL JULY TO SEPTEMBER
if you live in the NT or Northern WA there simply isn't enough rainfall over the last 120 years to judge any sort of reliability (which basically means in any one year you get none). For the rest of us it's extremely unreliable. For rainfall to occur during these months we need to bank on a strong cold front perhaps being preceded by some moist easterlies to be able to dump a few mm on us. Perhaps a North-West cloudband might be able to make it this far eastwards? Even the reliability of rainfall in NE Queensland has dropped as the HIGH pressure systems move further north and the trade winds turn more southerly resulting in just isolated showers, however on some stronger La Nina type years, those trade winds remain more south-easterly and are able to pull in more moisture, whereas in some El Nino years we can barely get a drop out of the sky as the trade winds are almost non existent.
RELIABILITY OF RAINFALL OCTOBER TO DECEMBER
For most of this period in time the north relies on convection to give it its rainfall. Ascent of the air is more localised compared with the January to March period. Therefore it's thunderstorms that bring us a large majority of our rainfall during these months. You will hear me say it time and time again thunderstorms are 'hit and miss'. We can show you all the pretty maps and charts but not one weather service or meteorologist will definitively tell you well ahead of time that you WILL get a thunderstorm and if they do they are lying to you. The amount of times you will see folks on our socials even from the same town/city whinging that the storm missed their part of town are too numerous to count and they just become white noise to all of our admins. This is the time of year rain dreams are shattered or boosted but in any one year it is more a matter of luck than anything if you get good rainfall. Places that tend to see the most luck year in year out during this time is the Darwin/Daly district of the NT, The Western Arnhem district of the NT, South-East and Central East Queensland inland of the coastal ranges and the Central parts of Cape York Peninsula. In El Nino years this luck is much harder to come by because there is less moisture availability for storms, in La Nina years more people tend to get lucky (meteorologically speaking) because there's more moisture available for storms but it still requires lots and lots of luck to see good rainfall in these months whether you are in an El Nino or a La nina or a normal year. On the map you can see that we simply can't rely on this rainfall.
SO WHAT DOES ALL THIS MEAN?
Australia is a land of droughts and flooding rains, it always has been and it always will be. We saw two nights ago that the North is getting wetter, but that doesn't mean every year is getting wetter, it just means at the extreme end of the spectrum we are getting much wetter. We saw last night two towns that have over 4000mm of rain, no town in the world needs 4m+ of rain a year so how can we harness that excess and give it to those with nothing on any given year. Unless you live in the most northern parts of this continent you simply can not rely on the 'average' rainfall being dumped on you every year or even most years. Our ability to manage our water in good years and to distribute that excess water to those who have had a bad year needs to be a water management focus for the decades to come. That becomes a political issue and you know how much I love to get involved in politics... (insert sarcastic eye roll here) So that's where we leave tonight's 'Did You Know'
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