Snow in Western Australia
Even when snow is likely it is often not mentioned in official weather forecasts, so the keen snow seeker must work out for himself when it is likely. Fortunately, with the internet, this is something any keen weather watcher can attempt.
Snow is delivered to the Stirling Range in the cold, moist and unstable southerly airstreams which follow the passage of significant cold fronts in winter and spring.
Many cold fronts cross the region each winter, spawned by the low pressure systems which move in an easterly direction to the south of the south coast. Most, however, do not put any snow on the ground - they are simply not cold enough at the altitudes required. The following ingredients are required following the passage of a cold front if snow is to be found on the Stirling Range, or anywhere else in WA:
- Southerly wind. The northwest and westerly winds that precede a cold front may be wet and stormy, but they are not cold enough to deposit snow in Western Australia. A wind from the south is required (typically between SSW and SE), originating from as far south as possible.
- Moist unstable air. The southerly airstream must have a high moisture content, and enough instability to cause it to fall as snow.
- Low temperature. The air must be significantly colder than that which follows the average winter cold front.
Keep an eye on weather forecasts for significant cold fronts passing through the state's south west with a southerly wind following. TV or radio reports are sufficient for this. If a series of fronts are expected, the last front in the series will be of most interest (between fronts the wind usually stays between SW and NW). A high pressure system moving in behind a departing low is what generates the required southerly airflow.
Most of the time, just keeping an eye on the TV weather reports is sufficient to rule out any likelihood of snow. On the odd occasions when a cold front of interest is identified, its potential for snow can be investigated on the internet by checking the following:
Mean Sea Level Pressure Charts
These are the charts commonly shown in TV weather segments. Check the forecast charts for msl pressure on the internet (often abbreviated MSLP) to see what cold fronts are approaching, and to get an idea of the expected timing. Check when a cold front that looks promising is expected to pass the Stirling Range area. The Bureau of Meteorology (BOM) 4 Day Forecast MSLP Prognoses show the forecast charts for the next four days in a single small image.
What to look for - the last front in a series, with a high pressure system following it which extends (or will extend) far to the south. The further south the southerly winds blow from, the colder they are likely to be. In the example shown, snow fell on the Stirling Ranges in the air behind the second front.
Various Computer-Modelled Charts
A variety of computer-modelled forecast charts are available. The excellent WeatherZone website has a good selection, and BOM's Numerical Weather Prediction page has more. The US National Center for Environmental Prediction produces some very useful charts as part of its Medium Range Forecasts for Australia and NZ. To become familiar with these sort of charts, it is advisable to read the Guide to the Forecasts and Analyses, which provides explanations of what the charts show, and their significance.
These charts make use of a variety of computer models to produce their forecasts, and opinions differ as to which is best - its a good idea to compare them. Whether different sources are in close agreement, or show significant discrepancies, can give an indication of how much confidence should be placed in them. For predicting snow, the following three chart types are most useful:
850mb Temperature, Humidity, Wind, Height
This type of chart shows predicted conditions at the 850mb level - the altitude at which the air pressure is 850mb. This varies from around 1200m to over 1500m above sea level; above the influence of ground topography, and nearer to the height of the Stirling Range summits than surface forecasts. These useful types of information may be shown:
- Wind - the small "L" marks show the forecast wind speed and direction.
- Humidity - the green colouring (on the NCEP charts) shows the forecast humidity, which is an indication of the amount of moisture available to fall as snow.
- Temperature - the isotherms (black lines on the NCEP charts, coloured shading on the GASP and GFS charts) show the forecast temperature at the 850mb level.
- Height - the height lines on the GASP and GFS charts show the altitude at which the air pressure is predicted to be 850mb. This indicates which altitude the forecasted temperatures apply to.
What to look for - Wind needs to be approximately southerly (SSW to SE). Humidity - the higher, the better. The 850mb temperature should ideally be below -2 for freezing conditions to be possible on Bluff Knoll's summit. With sufficient humidity snow can fall in air with a temperature of +2, so the zero isotherm could still allow prospects of snow falling, although reduced prospects of it remaining on the ground.
Vertical Velocity and Precipitation
Links: NCEP 6 day set
This shows the forecasted vertical velocity of the air at the 700mb level. Large negative values indicate rising air, leading to rain or snow if the moisture is available. The green shading shows forecasted precipitation (millimetres), whether it be rain or snow.
Mean Sea Level Pressure, 1000-500mb Thickness, Precipitation
These charts show forecasted air pressure at sea level, which can be compared with the Bureau of Meteorology's 4-day forecast charts. The 1000-500mb thickness is the vertical distance, measured in tens of metres, between the 1000mb level (near the ground) and the level at which the ir pressure is 500mb. This total thickness is a measure of the average temperature of the air between those two levels - lower values indicate colder (denser) air. The 540 line (corresponding to a vertical distance of 5400m) is commonly used as a rough dividing line between rain and snow at low levels.
Check the BOM Latest Australian Region Satellite Image. Look at the region behind where the front of interest is currently located - a white speckled appearance indicates cold moist unstable air, which is desirable. Check the Satellite Photo Loop to get an idea of its direction of travel, and development - confirm if it is approaching, and not degenerating.
Also check the text forecasts for the region for the next few days, such as these BOM forecasts:
4 Day Forecast South West Land Division - synoptic summary and description of future developments
WA District Forecasts (South West Land Division) - forecasts for towns, including temperatures
A forecast maximum temperature for Albany (Southern Coastal district) of 12 or below, in a southerly wind, is a sign that the airstream may be cold enough for snow on Bluff Knoll. Its not a direct correlation - remember Bluff Knoll is 90km away and other factors are involved. However it's a useful clue, as most instances of snow in WA are accompanied with temperatures failing to exceed 12 in Albany.
Forecasts which include a sheep weather warning and the chance of thunderstorms or hail in the Southern Coastal or Great Southern districts often accompany weather that is suitable for snow. Again, they don't necessarily correlate with snow, but they are good indications of cold, moist and unstable air. The absence of any mention of snow in a forecast is not important.
Don't forget that what actually happens may be quite different from what was forecasted! When snow-producing conditions appear likely, its good to monitor the forecasts and charts daily to see if an approaching system is strengthening or decaying, and to see if the unfolding weather resembles what was forecasted a few days earlier.
Closer to the event, the actual conditions in the region can be checked online to see if it really is turning out to be as cold as it needs to be. Relevant links are:
- Current Weather for the South Coastal and Wheatbelt - The nearest sites for which data is available are Albany Airport, Jacup, Katanning, and Lake Grace (the Stirling Range area is in the middle of a triangle bounded by Albany, Katanning and Jacup). Hint to BOM staff: an automatic weather station on top of Bluff Knoll would be useful!
- 256km Albany Radar Loop - shows where rain (snow?) is falling in the region.
- Albany Airport Weather - a WeatherZone page showing current weather, chart, radar, sunrise and sunset times, and the Albany forecast.
Local knowledge can also be valuable. If you are in the area, the opinions of people who live there and have experience with the winter conditions may be sought.
I like to keep it simple and have found that these key factors are the best indications of snow potential for the Stirling Range:
- The final cold front in a series, with a deep southerly airstream behind it
- An 850mb forecast temperature of -2 or below
- A forecast maximum temperature of 12 or less for Albany
- Forecasts of thunderstorms and hail in the south coast or great southern districts (indicating moisture and instability)
- Some overlap between the precipitation associated with the front and the area of cold air behind it
Of these, the 850mb temperature of -2 or below is most important - without that, it probably won't be cold enough for snow to settle on the Stirlings peaks, regardless of other factors being favourable.
It's easy to get bogged down in detail when trying to analyse all the other variables, but not necessarily any more productive - because there is such a delicate balance between snow and no snow. One degree either way can make the difference, and none of the computer models can be relied upon for that degree of accuracy. Timing is also hard to forecast, but can be critical to the outcome. I've found that the cold temperatures that would support snow often don't arrive until after most of the moisture has moved away. An hour's difference either way in this timing - too little to forecast accurately - can make the difference between bare ground or a blanket of snow.
A forecast may indicate good probabilities for snow on the Stirling Range, but the only way to be certain is to go there and have a look. That's what I've been doing the last few years on the few occasions when conditions have looked right, in the hope of becoming better at predicting this rare phenomena.