Archives For beast from the east

SUSPECT #1: HIGH PRESSURE OVER THE NORTH POLE

global atmospheric circulationCold air sinks and this builds HIGH pressure over the Polar regions as part of the “normal” global atmospheric circulation.  Hot air rises over the Equator creating LOW pressure.  Tropical and Polar air flows in a series of complex but fairly predictable patterns in between.  These cells power global weather.  Polar air meets Tropical air in the mid-latitudes at the Polar Front.  Fronts, low pressure systems and jet streams are all a product of this unhappy meeting of two the different air masses.  The jet stream (fast flowing ribbons of westerly winds at altitudes of 15 km) usually acts like a belt and keeps the Polar air inside the high latitudes.  The mid-latitude jet usually sits somewhere near the UK during “normal” winters and brings in relatively mild westerly winds circulating around depressions with rain and wind.  2013 has seen higher than usual pressure over the Polar regions and these have pushed the jet stream and attendant LOW pressure systems further south than usual, somewhere over the Mediterranean which has seen more rain and wind than usual.  Winds blow from HIGH to low pressure and, without the belt-like effect of jetstreams to keep them in, frigid polar air has flooded out across the mid-latitudes.

blocking highs and AO

Anywhere located north of the jetstream has been left exposed to these incursions of the Arctic air mass.  Winds from the Pole tend to blow from the NE or East rather than straight from the North to South because the spin of the Earth deflects them to the right … the coriolis force.  Hence, “the Beast from the East” in the UK.

 

 

 

Arctic Oscillation NAOThe index measuring the balance between HIGH pressure in the north and LOW to the south is called the Arctic Oscillation and this has been at record breaking negative figures this Spring: meaning the pressure over the Pole is unusually high compared to the low pressure over the mid-latitudes.  Blocking HIGH pressure prevented warm air getting to the UK on several occasions this Spring and, with this fresh in our minds, we must consider BLOCKING HIGHS as “caught red handed” in the act of causing our cold spring.  However, it seems that there is always a bigger fish and we must find the master criminal controlling the BLOCKS to get nearer the real killer of Spring2013!  We must find the cause behind these increased heights over the Pole? The answer might be the loss of Arctic sea ice.

Climate Cluedo!


The “Russian ice bear” is escaping again and will arrive in Reigate from Tuesday!  A HIGH pressure developing over Scandinavia and a LOW over Europe will open the cage and let the Russian ice bear gallop across Northern Europe and arrive freezing and growling in the UK from mid-week.  Expect cold easterly winds on the back of the bear.  The frigid continental interior, where the bear lives, has had the whole winter to cool down: the source region is now -10ºC and frequently lower. (View the Urals meteorite shower youtube clips again to get an idea of how cold the source region of this air mass is!) There is the potential for some of the coldest temperatures of this winter yet to be recorded in the SE as the bear bites back.  The week looks mostly dry but snow showers are also possible, especially further east.  The easterly winds warm up on their journey west and also pick up some moisture from the Baltic and comparatively warm North Sea; any warming causes instability (a tendency for air to rise and form cloud and rain/snow) and snow showers are therefore possible on the east coast, some of these could reach further inland on the strong easterly winds forecast.  Whilst the duration of this cold snap is not certain, the end of this sort of episode often sees westerly winds return with substantial snow on the leading edge of fronts.  How and when the bear returns to it’s cage remains uncertain,  but it will be prowling around Reigate from Wednesday on wards this week.

Some models show the cold easterlies continuing through to late December (e.g. CFS graph left) but the GFS sees westerly Atlantic conditions (warmer) breaking through much earlier than that – end of next week infact – and warming things up for a “green” Christmas. As fronts from the Atlantic move in there is a high certainty of considerable snowfalls on the leading edge of any precipitation as the warm Atlantic air meets the frigid cold air sitting over the UK: next Friday 14 Dec could be the first of these snowy Atlantic incursions but that’s a long way ahead to be certain and Reigate may or may not see a big dump of snow as a result.

The Beast from the East – pussy cat or lion? The trend below from the 850hPa temperature forecast for London ECMWF shows a return to “normal” around next weekend (ignore the temperature scale).  It is too early to say which models will turn out accurate but a white Christmas doesn’t look any more likely now just because of this cold snap: there are signs that warmer Atlantic conditions will return before the big day.

london 850hPa for beast from the east

note: the abbreviations are for different computer forecast models commonly used for weather forecasting:

  • ECMWF = The European Centre for Medium-Range Weather Forecasts
  • GFS = Global Forecast System (NOAA)
  • CFS = Climate Forecast System