Update: this feature is under discussion on met-forums as to whether it was technically a polar low or not! Some features (e.g. satpic) suggest it was but other features, such as upper temperatures, suggest it may have been a small scale feature with some similar characteristics (referred to below). References below contain more details. This post still valid as it describes an interesting weather feature with several polar low characteristics, albeit jury is out on final definition here!
Polar lows are small but intense maritime meso-scale cyclones, a few hundred km across, that form quickly in cold polar or arctic air advected (moved) over relatively warmer water. They are much smaller than our usual mid-latitude frontal depressions. They usually occur in winter in Arctic or Polar airmasses streaming Equatorwards and they form characteristically beautiful swirls of cumulus cloud and a comma cloud formation visible in satellite pictures (see above VIIRS sat pic 29/01/2015). Sometimes these swirls form an eye reminiscent of tropical cyclones, to which they have surprising things in common. Heaviest snow / rain occurs near the “eye”. This similarity is why they are sometimes called “Arctic Hurricanes”, though they do not always produce hurricane force winds. Despite their scary name they are relatively common over ice-free mid-latitude waters, they can produce rain or snow (due to “warm core” see below) and one was recorded moving down the North Sea in Dec 1995, so they are uncommon but not unheard of around the UK.
Polar lows tend to go through a rapid life cycle of a day or two which previously caught Arctic explorers unawares with hurricane force winds blowing up from nowhere and creating high seas in hostile Arctic waters. Gale force winds wrap tightly into these features. Polar Lows were invisible to meteorologists previous to satellite pictures and only with very recent upgrades in Numerical Weather Prediction models have Polar Lows been “visible” at all on charts. They are still hard to predict and models sometimes struggle to track their intensity and path.
Polar Low circulations do not last long and, like their tropical cyclone cousin, they tend to decay rapidly once they move from warm sea over cooler land, because the convective energy and steep lapse rates driving the system are lost. Cold upper air temperatures and a warm sea assist steep lapse rates that can cause thunderstorms with active convection. Lightning strikes were recorded in this 29/01/2015 low as it came ashore over Northern Ireland, typical of polar lows.
Charts modelling the evolution of this low, albeit rapidly in the last few hours, have predicted tracks moving the system SE across the UK overnight. Areas especially at risk from snow include Northern Ireland, N Wales and Midlands and possibly parts of the Southern England into early Friday. However, the system is likely to fill rapidly overland as the sensible heat flux available for convection is lost over the colder land. Additionally, the land is rougher than the ocean and this increased roughness increases surface convergence (air arriving faster than it lifts) and this causes the central pressure to rise and the system to decay.
Like hurricanes, Polar Lows form over oceans and gain much of their energy from them. Polar Lows usually form in places where there is a rapid change in temperature and/or pressure horizontally, this is known as a baroclinic zone. Edges of ice sheets or simply where warm and cold air meet are prime locations.
Warmer ocean water is another essential trigger for polar lows. The warm surface water provides essential mositure and lift that creates convection, condensation in frigid air and the release of latent heat that develops cumulus and cumulonimbus clouds. The clouds then wrap into a tight circulation around a rapidly deepening low core not dissimilar to a hurricane. Polar lows are much smaller and more transient than a regular mid-latitude depression. Polar lows tend to form on the eastern side of a high pressure ridge and to the east of a decaying occluding mother low. Both these features can be seen on charts.
An important feature of a polar low is the formation a warm core. Charts below show this as milder surface temps at 950hPa and the theta e chart which appears to show warm air too. Internal evolution of this system seem to suggest it has a warm core perhaps comprising Arctic flow chasing down a long-track of milder airmass originally sequestered by the extremely active cold front that swept through this location yesterday secluding a warm pool in the core of the mother low sat over Scandinavia. This warm pool appears to have advected east to meet incoming Arctic air. An area of positive vorticity (spin) contributed to the evolution by adding spin to the air that caused the low to form.
The polar low that developed today NW of Scotland has many of the hallmark characteristics of a Polar Low and seems to be generally accepted as such mainly because of the defining characteristic swirl of cumulus clouds round the low pressure core.
Note: met community not all agreed whether this was a Polar Low or not : has many characteristics but some consider it too warm with rain in some areas rendering it a meso-scale slide low or similar.
some links on Polar Lows