Whilst extreme weather scare stories in the press can be ignored as hot air, there is real confidence growing for a quick icy plunge of Arctic air visiting UK and reaching SE and Reigate next week. This is the first Autumnal cold snap of upcoming winter 2013 so heads-up to dig out the woolly hats and thermals, at least for a good part of next week until it probably warms up to autumnal average again over next weekend. It is a well within the “normal” for late Autumn cold conditions so nothing to get too excited about; in fact, it resembles a similar Omega Block from exactly this time last year (see below). Nevertheless, it will bring the usual frisson of excitement as some substantial snow is due for the hills of N England and Scotland and possibly flurries of wintry weather reaching the Midlands and even over the Downs to as far south as Reigate and SE (though here sleet is more likely but cool certainly). A regressing HIGH in the Atlantic moving to NW of UK will drag in this chilly NW Arctic air courtesy of a northerly blast from the jetstream: this arrives in SE from Monday on ward and reaching Reigate properly by Tuesday, lasting through much of next week. The upper air temps will plunge well below -30c at 5000m above the UK next week – and the “thickness” of the air will fall to below 528dam (decametres) which is a critical threshold for snow and wintry weather. (Warm air masses are “thicker” than cold; warm air takes up more depth in the atmosphere and, rather like a warm fluffy thick duvet, keeps the surface warmer, and usually builds a higher pressure. Imagine thinner cold air-masses as a thin duvet lacking the insulating capacity of warmer thicker air. This is a simple interpretation of “thickness”: warm and thick or thin and cold)
Daytime temps will be above freezing, but not much… perhaps 4-5c and a significant windchill on occasions will make it feel distinctly colder than it has been. It will feel chilly at night, down to -2 or -3c. Any precipitation during this time will be wintry.. i.e. snow or sleet. Not much ppt is forecast for us in the SE during this time except possibly for places near the east coast but check back for updates on this.
Meanwhile, it is usually safe to ignore alarmist headlines… check below and confirm that we are still here, despite the dire warnings from the past!
The initial Arctic blast does not last that long: a toppling HIGH moves over from Iceland and across the N of Scotland later next week to effectively shut the door on the direct Polar air. This pattern of weather can persist and is called an “OMEGA BLOCK” after the similarity in pressure pattern and upper wind flow to that of the Greek letter omega. Unfortunately, this pattern is likely to leave a large cut-off COOL POOL of frigid air over the UK and the near continent which can bring cool easterlies to the east coast which may penetrate inland across the SE giving Reigate continued cool weather through to next weekend 23-24 Nov . COOL POOLS usually warm up slowly but this can spark showers and later lead to slack gloomy conditions weather for the East coast.
On the other hand, the chart below shows a really strongly negative Arctic Oscillation building up and this would usually mean a strong blocking pattern where Arctic air is allowed to leak periodically from the Poles because of a weaker pressure gradient in the Atlantic and a correspondingly weak jetstream. The jetstream acts like a belt holding cold air into the Polar regions, if it slackens off it can allow the cold air to escape more easily. Cold end of November persisting is a possibility looking at the chart below.