Weather for Canyoners

WARNING: THIS IS A LONG POST AND IT IS QUITE POSSIBLY BORING!

 Disclaimer: Make your own decisions when you’re deciding whether to head out on a trip. There are so many more factors than just these to consider!

Sydney has been wet recently. I mean really wet. I mean crazy thunderstorms dumping 30-90mm of rain in an hour wet. With all of this weather the facebook group OzCanyons has been buzzing with discussions about whether it is safe to canyon and what canyons are best. While I can’t help with that info – as a few people like Tom mentioned it really is a decision for the group to make based on experience – I thought I would share a few thoughts on weather resources to help make decisions about whether or not to head out. Like all meteorological discussions remember that ‘the forecast is always correct, just a day or month out’ so take everything with a grain of salt and remember there is no substitute for knowledge or experience.

In summary my process for making a decisions would be the dot points below – the rest of the post will explain some of the resources I use in consideration.

  • Check the forecast –if it is crazy I’ll call it.
  • Check the radar – because I like the image and it is useful (but only at the last minute I guess)
  • Check recent weather data – work out what has been happening recently in the area I am going to – if there has been a lot of rain or storms recently I’ll make a call.
  • Check individual rain gauge data – because the popular weather stations aren’t always close by.
  • Check the forecast maps and forecasts again with the background knowledge of what conditions are likely to be at the moment.
  • If I still haven’t made a decision – check with an expert (like the OzCanyons Crew)

 Part 1: The Forecasts

Everyone knows forecasts can be a little fickle and not particularly reliable. We’ve probably all cancelled trips based on what the TV person has told us and then kicked ourselves. Luckily for us weather forecasts are now a whole lot more accurate and user friendly. The Bureau of Meteorology (BOM) has refined its forecasts to included the percentage chance of rain and amount of rain which tend to be surprisingly accurate now, particularly within 2 to 3 days.

Source: WeatherZone

Source: WeatherZone

These forecasts are based on refined models and super computers and take a cross reference of several weather models. Like any computer model the accuracy decreases the further out in time the forecast predicts so is more susceptible to change. The reason for this is that all models use tens of thousands of data points taken at a fixed point in time and then extrapolate what might happen. Each model behaves slightly differently so while for the first day or two all models might show the same trough over the east coast (a common precursor of persistent rain and storms) and the same series of cold fronts moving from west to east (storms and cold changes) by the end of a week these weather features can be days out of sync in different models so uncertainty can be quite high. Still if you are checking forecasts on Friday night for the rest of the weekend your decisions can be based on a forecast with relatively high confidence.

Seven day forecast for Katoomba. The first 3 days are likely to be highly accurate, but accuracy generally decreases throughout the week. Source Weatherzone.

Seven day forecast for Katoomba. The first 3 days are likely to be highly accurate, but accuracy generally decreases throughout the week.
Source Weatherzone.

Part 2: The Radar

We all love the weather radar right? I mean who doesn’t love watching the patterns rain guessing whether you’re going to get wet 5 minutes after you leave work?

BOM Radar

BOM Radar Source: Bureau of Meteorology

There are two main radar viewing platforms for Australia, all based on the same data collected by the BOM. The BOMs radar uses a relatively traditional colour scheme and shows quite defined areas of the country. Weatherzone (or the Weather Company) use a fancier display and a greater range of colours in their presentation of the radar data. As a preference I use the Weatherzone radar because it is easier to tell how heavy the precipitation is (light green is obviously different to yellow, red and purple).

The Weatherzone radar has two additional advantages to the BOM service in my mind. Firstly when zoomed out to a state level Weatherzone displays a composite radar image from multiple radars, this is great as it can give you an idea when a radar imagine is faulty because you’ll see the straight green line across the map which is a nice reminder that technology is not fool-proof. Secondly when viewed at a state level the Weatherzone radar shows lightning which is a really important feature when planning a canyon. The lightning lets you know if the rain is continuous over a wide area or restricted to small storm cells. Knowing this is really important when considering recent weather data in the next stage.

Weatherzone radar showing lightning. Source: Weatherzone

Weatherzone radar showing lightning.
Source: Weatherzone

Part 3: Recent Weather Data

Knowing what is supposed to happen over the next few days, and what is happening now is a great start to making a decision about heading out into a canyon. Just as important however is knowing what the conditions are in the catchment you are going to. If it hasn’t rained for months and the weather has been hot a little light rain is just going to soak into the soil and most likely doesn’t pose a risk to safety. If it has rained ever second day for the past month the ground is probably saturated and 5mm of rain is likely to flow directly into the canyon and raise water levels (which are most likely up already if it has been raining fairly consistently.

Source: Weatherzone

Source: Weatherzone

The BOM lets you check recent weather data via a clickable map, and once you’ve brought up the page you’ll find everything you need with rainfall amounts, temperatures, wind, and if you’re lucky evaporation. If you open up the same data on a Weatherzone page (slightly harder to find) you can compare the current conditions to climatology easily (it often even colour codes monthly extremes) so you can check whether this month is wet/drier/hotter/cold than average.

Weatherzone Climatology. Source Weatherzone

Weatherzone Climatology.
Source Weatherzone

This is where compulsively checking the radar becomes a handy habit for weather nerds. Storm cells dump a lot of rain but often only cover a small geographical area and it is oh so very easy for the storm to miss the major weather stations. It is not uncommon for a storm to pass over the mountains dropping 30mm of rain, but for Katoomba or Lithgow to record 5mm or even less. With some storms the margins are even greater with storm fronts having dropped 100mm in a narrow band and it having been bright and sunny a few kilometres away. Keeping an eye on radars for a few days before a trip is a great way to get an idea of how relevant the recent records at major weather stations are. If there have been lots of small storm cells it is often worth moving onto Part 4.

Part 4: Rain Gauge and Flood Data

While there are only so many locations around that operate full automatic weather stations which give you the nice data we’ve talked about so far, there are a lot more that provide simple records of rainfall. There is also a massive network of flood gauges that report river conditions across the country that can tell you whether river levels are rising, falling or stable. Straight up I’ll say the river gauges aren’t often useful for canyons as they are typically a long way down stream, but if the Colo Heights flood gauge is showing flooding that water has to have come from somewhere right?

So on the Rainfall and River Conditions page you can view river conditions, 24hr Rainfall (which is the most recent 9am to 9am period), Since 9am (cumulative rainfall since the most recent 9am) and Last Hour (no prizes for guessing). The map will often show quite clearly the path of the storms, while clicking on each data point gives you a specific rainfall amount which is very very informative.

24Hr Rain Gauge Data Source: BOM

24Hr Rain Gauge Data
Source: BOM

Rainfall Since 9am Source: BOM

Rainfall Since 9am. Note that the bands of green neatly match the paths of storms cells across the mountains.
Source: BOM

Rain Since 9am Source: BOM

River Levels and Flood Levels around Sydney
Source: BOM

5: Review

By this stage you have probably taken in a whole lot of data which should give you an idea of just how wet the area you’re planning on heading is. Combine this with your experience of the area and I would hazard that you can make a pretty informed guess about just how wet you’re likely to get. If the forecast is for rain then it is generally a fairly safe bet that the rain will fall over a large area and slowly, while showers and storms mean you are likely to get brief periods of heavier rain. Knowing the catchment is the best way to judge how risky each scenario is.

6: Check with friends, other canyoners or just play it safe!

If at this point you are still unsure about whether to go ahead if with your trip you can always have a chat to more experienced canyoners you know or ask canyon forums. My advice though is play it safe. If it looks wet or has been raining a lot recently go for a nice bushwalk to the exit of the canyon and check just how much the water level has risen based on the sand and algae levels. Remember though that even that can be potentially dangerous if the entrance or exit tracks require scrambling etc. Worst case go to your favourite venue, buy yourself a coffee or a beer, and congratulate yourself in making a decision that will keep you safe for that day.

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