Article written for TWO by Michael "doctormog"

An Introduction, some abbreviations and acronyms

Wetterzentrale (or WZ or Weathercentral for the non-German speakers) is the most common source of the weather charts discussed on TWO. For a newcomer to the world of weather watching the terminology and discussion relating to these charts can be a bit overwhelming at times. Hopefully this thread will help address some of the more baffling aspects of the model output discussion.

Meteociel.com (or Meteociel.fr is a French based web site) which like WZ provides wide range of weather charts from all the main weather models.

BOM: BOM refers to the Australian NWP (model) which is based on UK Met Office data but at a lower resolution than the UKMO/MetO model.

CAPE - Convectively Available Potential Energy. Usually discussed in the context of thunderstorm forecasting. The CAPE is essentially a measure of the energy which will be released when convection starts.

dam – decametres (1 dam = 10 metres). Referred to when discussing heights or thicknesses usually in the 500hPa/SLP charts and more specifically in discussion of the prospects of snow when referring to 1000-500hPa values (e.g. sub 528dam air).

ECMWF - European Centre for Medium range Weather Forecasting forecast model. (UK based)

ENS – Ensembles. These are a set of model runs (c. 12 for the GFS and up to 50 for the ECMWF model) which have the same start time but each run or ensemble member has a slightly altered set of initial conditions compared with the control run.

FI (or GFS FI) – “Fantasy Island” an expression often used on these forums to refer to any weather chart beyond a certain time point. FI originally referred to the charts on the lower resolution GFS operational runs (beyond +180hrs) but is now associated with any model run beyond what is viewed as the reliable time period by the poster using the term

GEM – The Canadian Weather Model (Global Environmental Multiscale).

GFS – Global Forecast System one of the most commonly discussed models.

GME – Also known as the DWD (Deutscher Wetterdienst). A German weather model.

hPa – Hectopascal – measure of barometric pressure. 1 hPa = 1 millibar.

JMA – Japan Meteorological Agency. Japanese weather model.

MRF – An old name for the GFS model, (no longer used). MRF can also be used to refer to medium range weather forecasts.

NGP – See NOGAPS

NOGAPS – The US Navy Operational Global Atmospheric Prediction System.

NWP – Numerical Weather Prediction and is essentially just another term for the output of the various models.

PMSL – Not what you may think, it refers to pressure at mean sea level

UKMO – The UK’s Met Office model (sometimes referred to as the MetO).

There are many other abbreviations and acronyms used in weather discussion but the above are the most frequently used in the discussion of the model output.

Let's get started, here's a chart, but what does it mean?

So here’s a chart, lots of nice colours, lines and numbers but what on earth does it all mean?

I’m not claiming to be able to explain everything you’ll ever come across in the Model Output thread but hopefully after reading this you’ll be able to look at the wetterzentrale charts and have a reasonable idea of what is being shown.

The easiest way to explain a chart is to actually post one which is why I have done it! The chart in question is from the GFS model which can be found at: http://www.wetterzentrale.de/topkarten/fsavneur.html

To start with the writing at the top:

“Init Sun 10Oct2010 06z” refers to the start time of the run i.e. T0 hrs when the data was collected. The time and date on the top right, the Valid time as the name suggests shows the weather for the time displayed, in this case the chart shows the setup on Tuesday 12^th October 2010 at 00:00 GMT/UTC.

As you can see from the writing below the initial time this is a 500hPa chart (with geopotential also shown). A what? Well rather than reinvent the wheel in trying to explain what these charts are and why we use them I have quoted a nice description from the following site:http://www.franksingleton.clara.net/

“Surface pressure is about 1000 hPa (1000 mb), this is the total force (weight) of the atmosphere.  The height at which the pressure is 500 hPa roughly divides in half the atmosphere vertically, half the mass of air being above and half below that height.  In terms of height, 500 hPa is about 5,500 metres (18,000 feet) above the ground. The top of that part of the atmosphere in which our weather is formed is known as the tropopause and is at about 11,000 metres (35,000 feet).  The 500 hPa level is, thus, effectively half way up the atmosphere as we know it.

The thickness, or relative topography, is the difference in heights between the level at which the pressure is 1000 hPa and that at which it is 500 hPa. This difference in height is a measure of the volume of the air between those two levels. Because volume varies with temperature, the thickness of the layer is a good indicator of the average temperature of the layer.”

Bearing the above in mind, especially the last sentence, do not be mistaken that lower thicknesses (indicated by blue and sometimes even purple colours) always indicate cold weather this is not necessarily the case. In many cases the blue colours indicate areas of low pressure indicated by T on these charts (Tiefdruckgebiet is German for a low pressure system) and the red/orange colours indicate areas of high pressure (represented by the H symbols). Again though this is not always the case – confusing eh?

The easiest way to get a fuller picture of how cold it is likely to be is twofold. The first is the use of the much discussed 1000-500hPa thicknesses – where the fabled 528dam rears its head. I’ll go into a little more detail on how to calculate this shortly. The other consideration is the 850hpA temperature (850hPA is usually the equivalent of about 5000ft above sea level – so a bit higher than anything in the UK).

Please see the next part for information on 1000-500hPa thicknesses and for t850hPa charts and what they mean.