The Pembrokeshire Dangler
Winter weather produces numerous challenges for forecasters, especially because the fine line between rain and snow is crucial in terms of hazards and potential impacts. But smaller mesoscale convective features can also provide a headache:

The beginning of November 2016 saw a rather chilly spell - nothing too unusual, but certainly giving parts of the UK their first real taste of the winter. North to north-easterly winds brought cold air from Scandinavia across the country, with widespread hard frosts, as well as some significant accumulations of snow over northern upland areas. 




























UKMO surface pressure analysis for 0000 UTC on the 6th November 2016. Credit: Met Office. 

The above surface pressure analysis illustrates a brisk NNE flow around the back of a low in the North Sea. These sort of pressure set-ups tend to only bring cold spells that last a few days, as the low will eventually move to the east, and milder air is then likely to move in from the Atlantic. In this case, the chilly air hung around for about 4-5 days, as a weak ridge of high pressure built in behind, allowing the cold air to stagnate over the country for a few more days. Really cold conditions in the winter require a persistent flow from the east or north, bringing very cold arctic or polar continental air masses persistently over the country for a week or more. These set-ups are not common however, and often require a weak or southerly-displaced jet-stream. Sudden stratospheric warming events can sometimes lead to such conditions developing.

Interestingly, cold airflows from the north, such as the one above, can lead to the development of a band of convective showers down the Irish Sea. This is marked on the map above by the convergence line in the Irish Sea and stretching over west Wales and south-west England. This weather phenomenon forms in the same way as lake effect snow in the North American Great Lakes in the late autumn and early winter. Essentially, in a northerly flow, a convergence line develops down the Irish Sea, due to a combination of land-breezes developing off the cold landmasses of Britain and Ireland, as well as frictional effects, which cause more of a NNW wind flow over Ireland. This all results in the wind converging over the sea, and being forced upwards to form large convective clouds. The convection is enhanced by the large temperature difference between the cold northerly air and the relatively warm sea, leading to cumulus congestus or even cumulonimbus clouds to form. These form a band of showers that stretch southwards, often affecting Pembrokeshire and south-west England, depending on the wind direction. 

In the past few days (at the time of writing), a Pembrokeshire Dangler dumped 30mm of rain in Milford Haven in less than 24 hours, while places few miles either side remained mostly dry. It can lead to some extreme local variations in the weather. In very cold conditions, heavy snow can fall, with up to 8 inches falling in Cornwall in the past.
































Radar image for 1130 UTC on 21st November 2015. Credit: weatherweb.net

In the case of the radar image above, the Dangler is positioned a little further east, with the showers over parts of Ceredigion, Carmarthenshire and west Devon. It's an interesting little feature that you should always keep an eye out for when the wind is in the right direction! 







































​​​Visible satellite image of the Pembrokshire Dangler from 6/11/2016. Credit: NERC Satellite Receiving Station, Dundee University, Scotland. Link: http://www.sat.dundee.ac.uk/