Understanding Weather Model Basics


This is a basic weather forecasting guide only. It should serve as a helpful introduction to weather modelling. It can all seem a little daunting at first but stick with it and you will soon be doing simple forecasts. A comprehensive understanding of the weather forecasting models isn't something you can achieve via a simple article like this. To compliment this article, I suggest you visit http://www.weatherzone.com.au/ and become a member there. Many weather lovers - professional and amateur alike - belong to the Weatherzone. You will find plenty of folk willing to help with any questions you may have about this subject and many others, not to mention the many weather products available, including weather models, to feast your eyes upon. But to get started on the subject, I recommend you read the following:

Clicking on the "Detailed Forecast Material" link on my main page will take you to various forecasting links including, "10 Day AVN/MRF Forecasts". This model link is fairly straight forward in its rendering of some basic weather forecasting details as it relates to cold weather. Sometimes this link may change or close down but don't worry too much if it does. Just head to the Weatherzone link above or my other forecasting model links to access an array of weather models. The things you will learn here can be applied in a general way to a number of the computer weather models. As a start, perhaps you can leave this window open whilst viewing my 'Detailed Forecasting Material' link on the main page.

10 Day AVN/MRF Forecasts

This is a nine panel chart that gives a nice and simple rendering of prospective weather patterns. It is especially good for those new to the weather models. The sea-level pressure (black lines) indicate where prospective weather patterns/fronts etc may occur (high pressure circulates anti-clockwise and low pressure circulates clockwise in the southern hemisphere). The colours on the graphic indicate the coldness of the surrounding air. Blue and purple generally represents air cold enough for snow in our area, if indeed there is enough moisture, see Snow Watch below for more about this. These colours actually give an indication of the 500hPa Heights or the vertical distance in metres from sea-level to the 500mb pressure level in the atmosphere. When this distance is less than 5400 metres thick (ie:shaded in blue/purple for this model graphic) from sea level to the 500mb level it is indicative that the air is generally cold enough to produce snow. Remember, you must also have enough available moisture for snow to fall. This model forecast is simply one interpretation of how the weather might unfold and not how things will definitely turn out. Still, it can be an interesting guide. It's when say, three or more models indicate a similar weather pattern (especially when it gets close to the event) that you can start to get excited about some cold weather coming your way.

Snow Watch

This is an excellent chart(s) courtesy of Laurier Williams' site, Australian Weather News. As mentioned under 10 Day AVN/MRF Forecasts, moisture is another important ingredient for producing snow in our area. For obviously without available moisture, it can't snow, no matter how cold it is. There are good notes explaining the charts on the page, so I wont go into detail here. Simple go to the Detailed Forecasting link on my main page to find this link.

GFS (AVN) Model (Medium Range Forecast)

This particular model rendering isn't available from a direct link on my site but because it is quite an easy model to derive simple forecasting information from, I will give you a link to locate this model in the next paragraph. However, you can now also view AVN in a slightly different format directly from my site here. Scroll down to the bottom of that page and you will find links and help notes there. I'd still recommend you read this document first as it will explain a few important things to you.

We'll mostly concentrate on observing forecasted "thickness" and 850hpa temps as they apply to potential cold air for this lesson but there are many other things to use the models for. To access the simplest form of the GFS (AVN) model, head to the the Weatherzone http://www.weatherzone.com.au/ and go to the computer models link there - you have to register to access the models link. So, once you are at Weatherzone, scroll down the left hand margin (if the page format has changed just look for the correct heading) and go to the heading "Weather Maps". Click the "computer models" link here. In the left hand margin you will see a list of computer models. Go to the heading 'GFS' and click on the link. Near the top of the screen you will see terms like "Latest","24hr", "48hr", "72hr" etc. These are simply telling you of the time periods the model is projecting out to. Also at the the top of the graphic (or bottom?) you will see something like "24hr AVN Valid: 00z Wednesday 26th Dec, 2001" etc. This is an example text and of course the date will be different on your graphic but it tells you that this is the AVN model forecast for Wednesday morning at 10am. 00z stands for Universal Time or Greenwich Time, so you just add 10 hours to that to get local time in Sydney (add 11 hours during daylight saving). Are you still with me? Don't worry too much if it takes time for you to understand all of this. Its the same for everyone. :)

On this model chart you will see thin black lines. These are the lines indicating thickness. Thickness in this context demonstrates how cold the air will potentially become and is the distance from the 1000mb level (roughly sea level) to the 500mb level in the atmosphere. Note thickness is slightly different to observing the 500hPa Heights, as mentioned earlier, as the thickness measurement starts exactly from the 1000mb level, not sea level itself. The 540 thickness line (an abbreviated rendering of the 1000 - 500mb thickness measurement of 5400 metres) is the light blue line. This is an indication of quite cold air and air colder than this, say a 536 line or less, is cold indeed for the Australian region. Thickness lines below 540 are usually seen in our winter but can appear in summer too, especially in Tasmania. The darker toned lines are the low pressure system indicators and the high pressure system indicators. So if you are looking at the chance of cold weather or snow in the Blue Mountains area, look for the blue 540 line or less over our region and then also check the rainfall amounts. Rainfall amounts (in mm) are typically indicated near the top right hand side of the rainfall graphic and are typically in the 2 to 15mm range for our winter forecasts.

Its also important to check on the 850 hPa temperatures too, to get an even better idea of just how cold it will be closer to the surface. This information is also under the GFS model link at Weatherzone and rendered as, "850 hPa Temp + Winds" at time of writing. The 850hPa temperature is the forecasted temperature at a level around 1500 metres above sea-level. This altitude is usually the closest distance to the earth's surface that isn't affected by surface heating etc, so its relatively easy to forecast. Of course, the 850hPa temperature (hPa is the air pressure) isn't always 1500 metres above the surface, it usually falls within the range of 1200 to 1600 metres above the surface but explaining the reasons for that are a little more detailed. You should be able to see the actual 850hPa heights via the Weatherzone link I just described above these days but if not go to here if you want to know how to establish the actual height of the 850hPa temperature, as knowing this can make your snow forecasts more accurate. So, for air cold enough for snow to fall in Blackheath (1080 metres ASL), the 850 hPa temperature typically needs to be at least zero degrees or less, preferably -1C or -2C. So, obviously the colder the 850 hPa temperature is for our town/region, the better our chances are for snow. If its forecast to be around -2C at the 850hPa (very roughly 1500 metres ASL) then Blackheath should see temps of around 0.5C at our altitude of 1080 metres, as moist/wet air usually cools at a rate of around 0.6 degrees per 100 metres. The May 2000 outbreak for example had 850hPa temps well below zero which converted to -2C in our town! Try to compare various models (they are listed on the Weatherzone site along with a few on my site) as you learn more about them. This way you can see if they are roughly matching up with each other, which can indicate more certainty in a forecasted weather event. Personally, I enjoy plotting my own forecasts but just do what works for you.

Well there you have it, some basic winter forecasting guides from the weather models. This is a good way to watch for prospective weather conditions but satellite imagery, AWS stations, atmospheric soundings, radar and watching the sky/learning about cloud formations are other important aspects.