Under forecasting wind speeds on the mountains of Scotland
There seems to be a bit of confusion in terminology in the text of the yellow warning for heavy rain event over the west of Scotland. In the warning text they use the terms hill and mountain but is there a difference between the two? If there is when does a hill become a mountain? The Scottish Mountaineering Club (SMC) term a Munro as a mountain of 3,000 feet (914.4 m) or higher, and in that regard there are 282 of them across Scotland.
The Met Office are obviously very interested in the weather over the hills and mountains, not just in Scotland but across the whole of the UK. In Scotland they have SIESAWS at the top of Cairngorm and the Cairnwell, as well as close to the top of Aonach Mor in the Nevis range. This being the case why is that they always seem to underestimate the strength of wind across the mountains? For instance in this warning they say that there’s a potential for gusts of 55-65 mph around coasts and hills which is significantly under forecasting the actual speeds today. Here are the winds from Aonach Mor for the last 24 hours when there have been any number of gusts in the range 80 to 85 mph, in fact the actual mean speeds here are closer to the forecast gust values in the warning.
Here’s another example of under forecasting wind speeds in today’s mountain forecast from the Met Office, this time further east in the northern Cairngorms.
At 15 UTC today the mean wind speed was forecast to be 54 knots with gusts to 70 knots at 1300 metres, but on Cairngorm at that time it was meaning 62 knots with gusts to 86 knots. The forecast values look to be around 20% lower than the observed values. These SIESAWS have been around collecting wind data for many years now, and you would have thought by now that the NWP modellers at the Met Office would have fined tuned their higher resolution NWP to cater for winds like these. I suppose you could argue that there’s not much difference between 70 and 87 knots, but I would imagine the accurate forecasting of wind speeds is very important for the many ‘wind farms’ that have now sprung up across the hills right across the UK.