WINTER 2019/20 – Mixed start trending drier and colder later

As always, there is huge interest in what the elements will deliver this winter. Unfortunately, despite some ‘forecasts’ you may read, skill in seasonal prediction remains very limited. So our thoughts for December to February give only the broad trends we expect across the UK, based on all available information. We hope you find it helpful.

THE KEY FACTORS

  • Numerical long-range forecasts from most major centres indicate an enhanced signal for strong Atlantic winds, delivering milder and wetter-than-normal weather for much of the winter. In isolation, this signal would be compelling. However it is worth noting that these models have performed poorly in recent months – with a notably mild bias in November.
  • Sea temperature patterns in the North Pacific and Indian Ocean are thought theoretically to increase the likelihood of a strong westerly jet stream. However, current sea temperature patterns between Canada and Greenland may promote ‘blocking’ of the jet stream across northern latitudes.
  • The jet stream wave pattern has shown greater amplitude than models have indicated in recent weeks, and it may be that these large amplitude waves have helped to impinge on the stratospheric winds at the top of the atmosphere.
  • After a very strong period, stratospheric winds above the arctic have weakened significantly, and models indicate that they will remain relatively weak through December. This increases the chances that the core of the Polar Vortex may be displaced somewhat from the North Pole (as is happening now) or even split. If the latter occurs, a corresponding ‘sudden warming’ of the stratosphere can lead to a reversal of winds lower down the atmosphere. Such an event can increase the chances of severe cold outbreaks across the mid-latitudes later in winter. However this is far from always the case and there are no immediate indications of this happening.
  • Stratospheric winds above the tropics are now slowly transitioning into an easterly phase of the ‘Quasi-Biennial Oscillation’. This phase has also been shown to promote a weakening in the strength of the jet stream lower down the atmosphere in wintertime.
  • We are reaching a ‘minimum’ in the Solar Cycle. These low sun-spot periods have been shown to have some impact on the strength of stratospheric winds, causing a corresponding weakening of the jet stream. Such weakening can promote ‘blocking’ events, and an increased chance of colder periods across the UK.

Based on all the latest available observations, historical analogues and computer forecasts, we suggest the following for December-February:

Further flooding events are possible particularly during any stormier spells

OUR FORECAST

  • During the early part of winter, bursts of strong jet stream energy will alternate with notable periods when the jet stream is slowed or deflected. As such a number of wet and windy spells are likely, interspersed with drier and colder interludes.
  • Further flooding events are possible particularly during any stormier spells. However some snowfall events are also likely during incursions of arctic air.
  • Temperatures are likely to fluctuate greatly in tandem with the ‘stop-start’ strength and direction of the jet stream. Over December as a whole, values may be close to average but this may disguise quite notably mild and cold periods. However prolonged severe cold weather looks unlikely.
  • Later in the winter, there is likely to be a trend towards more sustained ‘blocking’ of the jet stream. Although the position of the block will be crucial, this will likely promote longer drier spells and lower overall temperatures developing through January and February.
  • Should a marked disruption of the Polar Vortex lead to a Sudden Stratospheric Warming event later in December or January, this could increase the chances of a severely cold spell in late winter. However there are no clear indications of this yet.

UPDATES

So – an interesting and possibly impactful winter ahead, we think! We will update this forecast later in the winter, as required, and John Hammond’s Month Ahead will, of course, continue to be updated every week.

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