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Changing accuracy of forecasts

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Firstly, I do not mean this thread to be a dig in any way whatsoever, I am genuinely interested.


Second, I apologise if it is in the wrong place.


What I am wondering is; what makes weather forecasts go through periods of accuracy and huge errors. I understand that storms and snow are difficult to forecast, but that aside, I have noticed many sites going through phases of continually getting it wrong. For example at the moment the BBC has been accurate only 20%* of the time in the last month, compared to being pretty much spot on the previous month. That's not in regards to unusual weather but things such as rain, temperature etc.


I know that many sites use the same models and such to get their forecasts, so my question is what causes phases of inaccuracy in forecasts, even with 'normal' weather?


* I measured that in terms of if the weather was doing what it said at that time range (give or take an hour) and if the temperature was what it said to within 4-5C, for example.

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I believe the hardest to forecast would be the times when we are subject to the Atlantic weather, subject to ridges troughs and lows which despite the best intentions of the forecasters very often have their own agenda as to their speed, direction and location and this makes the weather harder to predict.


For example after seeing the forecast yesterday evening I was expected a mostly cloudy and wet day here for Watford but at the moment we have pleasant sunshine with about 3/8 cu with a base of about 4000 feet and about 1/8 of alto stratus at about 10,000 feet with little in the way of rainfall and the temp in the low 20's with a moderate westerly breeze. There is still time for the cu to get a little puffed up - whilst most are not showing very much vertical growth, one or two do have this, so there is the possibility of a shower. As we all know, showers are hit and miss and very often the local topography can have a bearing on this, so under these condition predicting exactly where and when rain will fall is very difficult.


The advance of fronts is a little easier because they follow a more recognisable pattern, which in the case of a warm front the progress can be seen from the ground by the advance of the cirrus stratus, often detected by a halo round the sun or the moon, alto stratus the nimbostratus, so it is a safe bet to say that a period of continuous slight or moderate precipitation is on the way, usually rain for us.


Cold fronts are different and very often consist of cu nimb at the leading edge with the higher cloud trailing behind and usually give rise to heavier precipitation but of shorter duration but often followed by showery weather. In their most dramatic state they form a squall line which is easily detectable as a line of cloud approaching, a drop in temperature, a change in wind direction and speed, becoming quite gusty for a time with a fairly short mostly heavy period of precipitation.


Occluded fronts and a mixture of both warm and cold fronts and their nature can vary depending on the way they combine.


As I have been typing a layer of stratocumulus is now starting to develop.


In anticyclonic conditions forecasting is easier because these are more stable conditions and do not tend to move as much. In the summer it can reasonably be said that they will give rise to settled summer weather which will last for days on end and the chances are that one day will be pretty much the same as the previous one. It can be a little different in the winter and here a lot depends on the origin of the anticyclone - from the continent it will usually be dry, sunny and cold, whilst from the sea it is often the case that cloud forms in the inversion layer making for a gloomy day - apart from drizzle, or snizzle if it is cold there are not normally any great amounts of precipitation. Also a high starting off as continental can sometimes pick up moisture from the seas surrounding us which can form into a miserable layer of stratus and with the elevation of the sun being low, there is little heat to burn it off.


The problems of forecasting were once explained to me by imagining you are putting a saucepan of water onto the stove to boil and the knack is trying to predict where the first bubble will form. Despite the best efforts of many weather forecasting is still an inexact science, though improvements are being made with time and advances in research. To really improve, I expect we would have to be able enter the details of each cubic metre of air over the whole of the globe but even then it would still break down as the chaos theory kicks in.

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Thanks Mike, that's helpful.

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