An Analysis Of Friday's Weather - 17 Dec 2010

iWeather Online's Fergal Tierney writes: "Let's have a look at the state of play this evening as Ulster suffers its worst snowfall in 25 years, and much of the Republic continues to receive those heavy showers in off the Atlantic."

The satellite picture below shows an area of enhanced cloudiness over Ulster, western Scotland and northern England, with snow and hail showers feeding into the northwest and west of Ireland. The green lines represent the height of the 500hPa pressure level (500hPa geopotenial). Because pressure falls as you climb through the atmosphere, the 500hPa pressure level will be at a certain height. This height is referred to as the 500hPa gepopotential, and the lines join areas of equal geopotential. They are labelled in decameters, dm (1 decameter = 10 metres), so the 524 line joins points where the geopotential is 5,240m (ie. 5.24kms or around 17,000ft). Geopotential charts are an important tool for figuring out what are the driving forces in the upper atmosphere that will affect the weather at the surface. Upper features like kinks in the 500hPa geopotential lines (ie. upper troughs) can cause large areas of air to rise from below, causing clouds and precipitation. This is precisely what caused the widespread snowfall in Ulster today.

The geopotential lines in the picture circulate anticlockwise around an area centred near eastern Scotland. This is called an "upper low" as it is an area where geopotentials are at their lowest. This area contains very cold air, with temperatures at the 500hPa level well below -40°C. Cold air at this level promotes the development of showers and thunderstorms. Looking closely we can see that there's a pronounced curve or kink in the geopotential lines (ie. an upper trough) just west of northern Scotland. This trough caused air to its south to rise, as shown by the light blue area, which is the vertical velocity at the 700hPa level (around 9,000ft). Such areas are a sign that there is significant scope for widespread precipitation, and this turned out to be the case. The blue area started off further west earlier in the day and moved east throughout the afternoon, ending up off Antrim by 1800GMT. By then several inches of snow had been dumped throughout the province, marking the worst (or best, depending on your point of view!) snow event in 25 years.

Another area of upward motion can be seen just west of Kerry, and this corresponds to the developing low pressure system I mentioned last night. As the models are having a hard time figuring out what will happen 24 hours out, this low has actually formed much further south than the NAE had shown this time yesterday. It is currently bringing a band of hail, rain and snow to Co Kerry this evening.

For the rest of us tonight, we can expect a continuation of the showery activity in the western half of the country, with some of these showers making it across to the east. There is a system developing to the south of Iceland that we should keep an eye on as it moves southwards overnight, but this cold spell has turned into a nowcast situation in that the models are chopping and changing their mind on every run. Keep an eye out for Peter's Forecast tomorrow morning, which will have the latest info at hand.