Gavin's weather blog update
Winter 2016/17 With UK Met Office Contingency Planners Guidance
[Updated - 29/11/2016]
The excellent UK Met Office Contingency Planners Guidance was released yesterday. This update covers the three monthly period December to February 2017 - So we cover the full Winter 2016/17 period with this update. You can read the UK Met Office Contingency Planners Guidance for yourself here
For today's blog update we're going to discuss the latest update and see what kind of weather the UK Met Office contingency planners guidance is expecting for Winter 16/17.
The headline from this months update is that the UKMO is expecting a cold start to the Winter followed by moderation through the later part of Winter 2016/17. The unusually extended temperature summary says this:
During December below-average temperatures are more likely than above-average temperatures. The risk of impacts from cold weather is considerably higher than normal.
Predictions for UK-mean temperature for the whole of the winter season (December-January-February) show only a slight shift from the normal range of expected conditions. Nevertheless, this unremarkable outlook conceals a change from an increased risk of a cold start to winter to a greater likelihood of milder conditions later on. These different phases tend to balance the probability of above- and below-average conditions in the overall 3-month average, but the risk of cold weather impacts in the first half of winter is considerably greater than normal.
So quite a clear signal from the Contingency Planners Guidance for early Winter to potentially be significantly colder than average with the chance of "cold weather impacts" being "significantly greater than average" through the early part of the Winter!
The "context" section of the guidance helps us to "drill down" and find out exactly what is going on why Winter 16/17 is likely to have a colder start and a milder end.
Sea surface temperatures in the tropical Pacific Ocean remain slightly cooler than average, close to the threshold for a La Nina event. While temperatures imply a "neutral" state of the El Nino-Southern Oscillation, tropical Pacific rainfall patterns do appear to be similar to those expected during La Nina. Relatively little change is expected in the coming months. Weak La Nina conditions slightly increase the probability of blocking over the North Atlantic and Europe in early winter, but slightly increase the chance of more mobile, westerly weather in late winter. This would imply a shift in likelihood towards colder- and drier-than-average conditions for the UK early in winter and the reverse later on
Arctic sea ice extent is at a record low level for the time of year. The largest sea ice deficits are in the Barents and Kara Seas. Recent research suggests that lack of sea ice especially in the Eurasian sector, may increase the likelihood of blocking patterns and the negative phase of NAO occurring in winter. In the North Atlantic Ocean, sea surface temperatures are well above average in the western Atlantic near Newfoundland. This pattern of sea surface temperatures is thought to moderately increase the probability of above-average pressure in the central North Atlantic, leading to an increased frequency of northerly or northwesterly winds over the UK. At this time of year such a pressure pattern is often associated with below-average temperatures.
And in conclusion the temperature context part of the UKMO contingency planners guidance says this:
During December, the factors described above suggest an increased likelihood of negative NAO and blocking, which usually bring below-average temperatures for the UK. Predictions from the Met Office seasonal prediction system, along with those from other global forecast centres, offer strong and consistent support for this view. The chance of a prolonged spell of cold weather taking hold in December is high compared to normal, although more usual types of winter weather are not ruled out.
Through the early part of the 3-month period, colder-than-average conditions are more likely than milder-than average. Later in the winter, particularly into February, prediction systems signal a shift towards less likelihood of blocked weather patterns, which would imply a reduction in the chances of cold conditions. The effect of these different phases is to bring probabilities for the 3-month UK-average temperature into closer balance.
So we've got a very strong signal for a cold December with a risk of significant weather impacts (snow, frost, ice, fog, etc.) The second half of the Winter is a little more vague and seems to be mostly based on long range forecasting models (such as GloSea5 and EC seasonal) which have been fairly consistent throughout their updates this season in predicting the chance of a milder later Winter period.
So when might the change from cold to milder conditions take place? The guidance doesn't suggest a definitive date but the following section (talking about the QBO and polar vortex) is quite interesting:
The Quasi-Biennial Oscillation (QBO), an oscillation of the equatorial winds in the stratosphere, remains in a westerly phase. The QBO influences winter conditions over Western Europe by modulating the strength of the stratospheric polar vortex (SPV) and thereby the phase of the North Atlantic Oscillation (NAO) at the surface. The westerly phase of the QBO tends to favour a stronger SPV, particularly in early winter, leading to a higher likelihood of a positive phase of the NAO. Despite the westerly QBO, the SPV is actually much weaker than normal this year and is expected to only slowly strengthen during December. A weak SPV tends to influence surface conditions with a delay of a few weeks. This implies an increased likelihood of negative NAO and blocking during December and January, which would increase the probability of cold weather in the UK during this first part of winter.
Although the emphasis with this month contingency planners guidance is very much towards the risk of cold conditions during December, this section implies the risk of cold weather persisting into January as well. So it's possible we could have two appreciably cold months this Winter with only February turning out to be milder - We'll find out more on that next month?
The Contingency Planners Guidance precipitation forecast once again backs up the overall prognosis from the UKMO wioth the headline summary suggesting a drier than average December and potentially quite a dry Winter too.
During December below-average precipitation is more likely than above-average. For December-January-February as a whole there is only a slight shift from the normal range of expected conditions, with below-average precipitation slightly more probable than above-average.
The "context" part of the guidance tells us more:
Factors such as a weak stratospheric polar vortex and tropical Pacific conditions, suggest an increased chance of blocked weather patterns during December. In addition, the Met Office seasonal prediction system and systems from other global forecast centres consistently show an increased chance of the UK experiencing these types of weather patterns. At this time of year, blocking is associated with a decreased frequency of Atlantic depressions crossing the UK and as a result below-average precipitation. Spells of very wet or stormy weather are still possible, but are likely to be less frequent than usual for the time of year.
For the season as a whole (December-January-February), there is a slightly higher chance of below-average precipitation than of above average precipitation. Through the early part of the period, drier than average conditions are more likely than wetter-than-average, given the increased likelihood of blocking. Thereafter, an increasing probability of more westerly types of weather results in more balanced probabilities for the season as a whole.
So a really fascinating update. Obviously the focus in the immediate future will be on the possibility of weather and wintry conditions through December. Later in the Winter we need to see how far into January we can keep the cold conditions going and then we'll see whether February does indeed "flip" to mild.
Next months Contingency Planners update will take us through the late Winter (Jan to Mar) period and should have more detail on the possble change to milder conditions for late Winter.
In the meantime GWV Winter 2016/17 Forecast will be released on Thursday evening. Keep checking back for more.