Polar Low 'Potential' For Ireland This Weekend

Polar low forms off the coast of northern Norway.  
The following article relating to the potential for a ‘polar low’ to develop and affect Ireland on Saturday is based on current weather model data, which is subject to some change as we move closer to the day in question. Confidence in this polar low developing is around 60% at present. This will be amended to reflect future model output. Analysis by iWeather Online’s synoptics forecaster Fergal Tierney. 

The 12Z GFS is hinting at a possible Polar Low developing to our north late Friday and moving southwards off our west coast during Saturday. A Polar Low (also known as an Arctic Hurricane), is a potent small-scale low pressure system that forms when an Arctic airmass moves out over a warmer sea. It is a common wintertime feature in Arctic regions, and Norwegian sea areas are affected by more than twenty such systems in an average winter, but they are a rare beast in these parts. They can bring strong blizzard conditions and vast snowfall in a short time, but they usually dissipate on reaching land, and the average lifetime of such a system is around 24-36 hours.

These lows are similar to tropical hurricanes in that they are warm-cored, ie. the centre contains warmer air than its surroundings. This is due to latent heat of vaporisation when strong warm moist air from the sea surface rises and condenses, releasing the latent heat. Heavy precipitation results. The isobars become tightly packed, causing the strong winds that form the blizzards. Some polar lows even contain an "eye" at their centre, like a tropical hurricane.

Friday's setup shows all the ingredients present for a polar low. The temperature at the 500hPa level (around 18,000ft) will be below -40°C, There is a strong upper low to the southeast, which will aid development of the surface feature. The sea surface temperatures will be around 10-11°C off our west, sustaining this mass evaporation and resultant precipitation.

The best chart to view it is the 850hPa theta-w (wet bulb potential temperature) overlayed on surface pressure, shown below. We can see that on Saturday afternoon, there is a tightly-packed low to the west, with warmest theta-w values at its centre.
The forecast surface winds are 45 knots in its northwest quadrant, well above the 27 knot threshold required to classify as a polar low.
Polar Low winds
Precipitation wraps around the centre, possibly clipping the western part of the country. With cold air in place, this could fall as snow away from immediate coasts.
Polar Low precipitation
It must be stressed that such details are subject to change so far out, so this future model runs will need to be monitored. The high resolution NAE model, which forecasts out to 48 hours, will be a key supplement when it comes into range on Thursday's runs. There is no guarantee that it will occur at all, but the possibility is there and warrants a mention. We could have the rare chance to witness this beast up close, but just remember to keep your hands out of the cage!!

For more information on Polar Lows, see this very useful guide from the Austrian Met Office.

iWeather Online's Fergal Tierney is an Applied Physics and Chemistry graduate and holds qualifications in Aviation Meteorology.