Weather watches and warnings are strange things. As meteorologists we are often being accused of not forecasting the intensity correctly or over-egging the pudding (that's a technical phrase) when we forecast severe weather and it fails to materialise.
So, how do we decide what situations warrant a weather watch or warning? This page gives an in depth look into what criteria we look at and hopefully will allow you understand the workings behind the scene of Metcheck.
Preparation
Every morning at Metcheck Towers we have a forecasters discussion. This is where a team of senior and junior forecasters discuss three time periods. Firstly we look at weather in the next 24 hours across the country. The second is the 24 to 72 hour period and finally we then look at 72-192 hours to see whether any potential severe weather is on the horizon.
The tools we use for short term forecasts range from nowcast tools like radar, satellite and observations to allow us to build up a picture of developing severe weather and then allow us to see who will be affected and when.
For events which may occur in the next week or so we look at ensembles and about 8 other weather models to see the agreement between them and where the similarities occur. This then allows us to once again see when and where will be affected.
The When
This is an incredibly important factor when it comes to issuing weather watches and warnings. Lets say for example London will be affected by 60mph winds. This in itself could cause some problems but if it was during February at 3am on some idle Tuesday then this would only be a slight concern.
If this was to occur during daylight hours or especially peak time travel then it becomes more of an event as more people are likely to be affected.
The time of year is also very important. 60mph in February is almost expected, but if this was expected during Summer holidays in July then it takes on a whole new aspect as people will have outdoor plans and travel as well as coastal activities which would mean this would probably warrant a severe weather warning.
The Where
We also look at historical weather for this location. Has it already had a few days of wet and windy weather which would loosen top soil? This would mean trees would be more likely to fall. Are the trees still in full leaf or are they bare? Is the wind coming from a direction which this area is used to? London is primarily, as is much of the UK to deal with prevailing South-westerly winds. Our infrastructure, buildings and town design is geared for this. So 60mph in February but from the North-east would mean exposure to more things like weak fences and weak trees than a South-westerly.
We also look at things like moon phases and sea information to tell us whether the worst of the weather will affect coastal locations and cause problems with coastal defences.
The Result
After a huge amount of information is carefully processed we decide then on how to issue the watch or warnings. We then decide when or if they should be updated and by whom. Senior forecasters then create the watch or warning maps and this is sent by email as well as displaying a banner across the top of the site linking to the latest watch or warning.
Of course the weather always holds the cards. Whilst we base many of the watches and warnings on numerical weather models, our trained meteorologists at Metcheck understand the weaknesses and strengths of each model and adjust watches and warnings accordingly. So, whilst you may see impressive articles in national newspapers of impending doom and gloom, this is usually as a result of unqualified meteorologists calling newspaper reporters based on numerical weather output but not understanding the complexities of weather models.
At Metcheck we do pride ourselves on providing an honest and robust watch and warning system and we are forever working to ensure we get the balance of warnings right. After all, too many warnings and eventually the public become immune to the severity of them, too little and the public won't trust us to get the situation correct.