Weather Forecasting Tools
Weather forecasts are generated by complex mathematical models. The mathematical formulas used in the models are based on historical weather trends and are fed with real time observations from all over the world. The numerical output is presented graphically in the form of forecast charts that are available for viewing on the web.
There are several different mathematical models used to generate forecast charts. The most commonly available weather model forecasts are Global Analysis and Prediction (GASP), Global Forecast System (GFS), National Operational Global Atmospheric Prediction System (NOGAPS) and European Centre for Medium Range Weather Forecasts (ECMWF).
The Australian Bureau of Meteorology relies primarily on the GASP model. As its name suggests GASP is a global weather forecast model. It provides a forecast for a period of 14 days from the time the model is run. The GASP model has a resolution of approximately 85 km. That means that there is a model input and output for every 85 km x 85 km square on the earth’s surface. Given the relatively large scale of the GASP model it is unable to provide accurate localised output. For example: it would not be possible to accurately account for the effects of topography on weather in the Australian Alps as topography is likely to vary significantly over an 85km grid square. In order to provide more accurate localised forecasts the BOM run two smaller scale model for areas of Australia. The Limited Area Prediction System (LAPS) model provides forecast for Australia over a 37km grid for the period extending 72hrs from the time the model is run. The Meso-LAPS model provides forecasts for Australia over a 12km grid for the period extending up to 48 hrs from the time the model is run. Both LAPS and Meso-LAPS rely on the forecast data generated from the GASP model to formulate their boundary conditions (ie. the conditions along the edges of the modelled area, in this case the areas away from Australia). The GASP model is run every 12 hours and the outputs are published at around 6am and 6pm each day. The LAPS and Meso-Laps models are also run twice a day and the outputs are available at around 4am and 4pm each day.
The Global Forecast System is run by the US National Oceanic and Atmospheric Administration (NOAA) and the NOGAPS model is run by the US Navy. Both are global models that operate on a 50 km grid and provide a forecast for up to 14 days. The output from the GFS model is more readily available on the web and is provided at a more user friendly size than the NOGAPS forecast data. The GFS and NOGAPs models are run every 12 hours and the outputs are published at around 3am and 3pm each day.
The ECMWF model is a global model that operates on a 40km grid. Like the NOGAPS model the output available to the public is not as user friendly as the GFS and GASP models.
As stated above the weather models are typically run every 12 hours. The time the model was run and the time to which the forecast output applies is usually visible somewhere on the forecast chart. The first chart provided in any model output is at Time = 0 hrs, and is referred to as the analysis. The analysis chart is based on real data from the weather observation network across Ausralia and the rest of the world. The data presented in subsequent charts is what has been generated by the mathematical model. The model outputs are published around 6 hours after the analysis (T = 0hrs).
The most commonly utilised weather forecast charts for predicting snowfall show sea level pressure, precipitation and atmospheric thickness. I’ve already discussed how to interpret weather based on air pressure. The other parameters provided on these charts particularly useful for forecasting snow in the mountains.
The atmospheric thickness is the distance between air pressure of 1000 hPa and 500 hPa. Why is this useful? Cold air is more dense than warm air so the smaller the distance between 1000 hPa and 500 hPa the more dense (and colder) the air is above that location. In general, a thickness reading of approximately 5400 m corresponds with a freeze level at approximately 1400 m above sea level. A thickness reading of 5340 m corresponds with a freeze level of approximately 1000 m above sea level. A thickness of 5280m corresponds with a freeze level of approximately 600 m above sea level. Basically as the thickness decreases and increases so does the freeze level. Whilst there are many other factors that affect the freeze level and the level to which snow will fall these charts are probably the best way for an amateur to interpret the weather forecast.
The precipitation is represented by areas of colour shading, each colour representing various ranges of precipitation. The precipitation shown is relative to the time that the model was run (ie. Time = 0 hrs). So the precipitation shown on the Time = +24 hrs forecast chart is that which has fallen during the previous 24 hr period and the precipitation shown on the Time = +48hr forecast chart is that which is predicted to fall between 24 and 48 hrs since the model was run.
So for it to snow across the alps you typically require moisture and the 1000 hPa – 500 hPa thickness to be less than 5400 m. The amount of snow that actually falls will be dependant on several factors which I’ve discussed on the weather basics page but simplistically you want to see lots of colour shading (representing precipitation) underneath the 5400 m thickness contour.
The most commonly available forecast charts show forecast air pressures at sea level. (ie. the isobars show the forecast air pressure at a location assuming it is at sea level). This is the only way to provide a meaning full representation of air pressure across varied terrain. This is air pressure is much more susceptible to change based on altitude rather than lateral movement on the earths surface.
In order to predict the weather in a 3-dimensional atmosphere the weather models calculate conditions over several layers within the atmosphere. The models provide output for layers of equal air pressure, not altitude. For example, a common model output is for a layer where the air pressure is 850 hPa. This layer is typically at a height of approximately 1500m above sea level but the exact altitude of this layer varies depending on other factors. Wind, temperature and humidity data is widely available for the 850 hPa, 700 hPa and 500 hPa forecast layers of most models. These are useful in assessing conditions in the upper atmosphere, which generally drive conditions lower down.
Given that we’re not all forecasters and don’t have all day to go through every forecast output available its probably best to regularly scan the sea level pressure, precipitation and atmospheric thickness charts of the various models. If the weather looks promising you can get a better ideal of what might come off by studying the upper level temperatures and the more detailed models like LAPS and Meso-LAPS.
I've provided links to some of my favourite weather pages here: