Producing your own synoptic chart
Without a prepared forecast synoptic chart, any attempt at DIY forecasting for more than a few hours ahead needs to begin with the preparation of your own forecast synopsis. Obviously this is unlikely to be as accurate as one prepared by a professional meteorologist with high-tech resources at his disposal, but it can still be a valuable guide.
Ideally, you need to be able to visualise the swirling mass of the atmosphere – it may help if you can imagine the ripples and eddies created if you stir a basin of water – but there are a few general rules of thumb which can help.
Depressions, around us and Europe, usually…
- Move in an easterly or north-easterly direction, parallel to the line of the isobars in the warm sector.
- If a depression has been moving in a particular direction for 12 hours, it is likely to go doing so for another 12 hours unless it meets land or a large anticyclone.
- Lows slow down and fill up when they meet land, or when their fronts occlude.
- If the barometer is rising more quickly behind a depression then it fell when the depression approached, the depression is probably filling and slowing down (and vice versa.)
- When a large depression occludes, a smaller ‘secondary’ depression often develops somewhere along the trailing cold front.
- If two similar lows are close to each other they may merge to form a complex low, with two centres that rotate anticlockwise around each other.
- Each section of a front moves at right angles to the line reprenting the front itself.
- Each section of the cold front moves at about the speed of the geostrophic wind behind it. Warm fronts typically move about 30% slower.