Duh, yeah (sone sarcasm just for you). We get them in my line of work too.
But you know what - it's never a surprise? Cold weather in the winter, a heat wave in the summer. I've yet to see one that actually gives us advance warning of something we couldn't have figured for ourselves.
It's like the recent flood alerts and warnings. From the number of drivers we'd already rescued, the incessant heavy rain and from the way the Westshire river was lapping at the edges of the banks had us kind of guessing bad times were on their way.
It is to do with deviation from the average, issuing warnings when the weather is going to be significantly colder than normal in winter or significantly hotter than normal in summer is perfectly reasonable, and is actually what a meteorological organization is expected to do. Having an alert makes things stand out, grabs attention so that people can make preparations if necessary, that may mean just the most vulnerable members of society or it may mean entire communities in the case of flooding or heavy snow, for example.
As for flood alerts, warnings need to be issued BEFORE the heavy rain arrives and the rivers start lapping the edges of their banks to give people time to sandbag their properties and move furniture upstairs, it is no use waiting until the event is unfolding, it is too late by then.