The National Severe Weather Warning Service is an impact based system based on the likelihood of low, medium or high impacts from weather. In other words its a risk assessment that takes place. That means that actual weather expected, antecedent conditions (ground states/state of rivers), time of day, time of week and time of year are all taken into consideration when deciding whether a warning is or is likely to become necessary. For example the windy conditions of last weekend would have been unlikely to warrant a warning in November - simply because those winds are entirely normal in late Autumn and trees are out of leaf. Consider gusts 40-50mph in August when folk are on holiday - camping, towing caravans putting up bouncy castles etc etc and trees topple easily its a completely different scenario.
When it comes to risk assessment the colour of a warning is derived from the risk assessment of impact against the likelihood of those impacts occurring. So forecasters will decide how much rain is likely in an hour from thunderstorms (which itself depends on speed of movement, CAPE and precipitable water) and decide the most likely impacts - remembering also that ground/river conditions are in the mix. If low impacts are expected, then the likelihood will most likely be assessed using things like frequency of storms and locality to be affected - IE highly populated-v-sparsely populated.
Bear in mind also that being a risk assessment a medium likelihood - (say a 40% chance) means that some places will, by definition not see a storm or impacts from a storm(s) within the warning area. This is often covered in the text of the warning with the phrase, 'while some will see few if any storms' other places may see a torrential downpour causing the risk of surface water flooding and some lightning damage'.
Surely nobody realistically expects exactly the same weather in all places even on a non-warning day so why should anyone expect to see warnable weather at every spot within a warning zone - that's clearly not going to happen.
However its worth pointing out that research undertaken with the public has revealed that the public wish to know about impactful weather that may affect their journeys - so just because its Ok in your driveway or back garden - doesn't mean its going to be Ok 5 to 10 miles down the road.
Warnings currently consist of a map showing the area affected, a headline which states the main weather and impact, a set of 'what to expect bullets' determined by the risk assessment and for the weather anoraks among us - a more details text explaining rainfall amounts. Basically something for everyone including those who just want a glance and those who like a little more detail.
The current system is in place as the result of about 10 years of intense research and questioning of the two main users of the Service - the public and responders/blue lights who come to the public's assistance when necessary.
Nobody wants to issue warnings when they're not necessary - but sometimes when you get a long spell of the same type of weather it can seem like there are too many warnings.
Finally, in the month of May - only one yellow warning was issued for the whole of the UK in 31 days. Just goes to show - its swings and roundabouts.....
John.
The orange County of Armagh.