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Lomond Snowstorm's Winter Forecast 2012/13



Well it’s that time of year again when I, and what seems like half the world, publish their thoughts on the coming winter. Many of us have already had a taste of winter, with fairly significant snow down the east coast, away from the now famous snow shadow here in central eastern Scotland. The northern hemispheric pressure patterns that delivered that snowfall and which have been showing up for some time now consistently on the models has borne a striking resemblance to those of 09/10 and early 10/11, with the PV significantly disrupted and blocking prevalent towards Greenland. This has, justifiably, prompted many to ask if this is a harbinger for the coming winter, and whether this is likely to become the defining theme of the winter of 2012/13. This forecast will look holistically at most of the major known drivers of Northwestern European winters, both longer term global dynamics such as ENSO, the QBO and GLAAM along with more localised leading indicators such as Snow Cover, Stratospheric Temperatures and the current Arctic and North Atlantic Oscillations to give a detailed forecast for the winter ahead.
[b] El Nino Southern Oscillation[/b]

Most people should be relatively familiar with the concept of El Nino/La Nina, the temperature anomaly of an area of ocean in the Southern Pacific that has been found to be a major driver of seasonal and annual climatic patterns around the world. We are currently at the very weakest possible El Nino state, with forecasts indicating that ENSO is likely to become neutral, perhaps more likely positive than negative, but essentially such a weak signal indicates that this will not be a major driver this winter.

[b] Northern Hemisphere Snow cover[/b]

The relationship between the extent of Autumn snow cover across the Northern Hemisphere, in particular Eurasia, and Northern Hemispheric pressure patterns has been well documented.
[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image004.jpg[/img] [i]Correlation between October snow cover and Sea Level Pressure as a spatial anomaly map. The bright oranges over the poles indicate that greater extent of NH snow cover is correlated with higher Sea level pressure over the poles and therefore greater blocking[/i]
[i]Spatial correlation between October NH snow cover and 30mb Stratospheric Temperature (more on stratospheric temperatures later). Again, There is a significant positive correlation for the poles and negative one for southern latitudes, which again shows an increased tendency for upper latitude blocking in winter following Octobers with greater snow cover extent.[/i]

So what does that mean for this winter? Well this October’s snowcover extent has been the 8[sup]th[/sup] snowiest in the Northern Hemisphere in the last 45 years, being above average both across Siberia, Scandinavia and North America, after a September that was 36[sup]th[/sup] out of 44 years. This rapid gain of snowcover is a very strong indication of a continuation of the predominance of high latitude blocking over the last 3 or 4 winters, which of course is a strong indication of a colder than average winter.
[b] Global Angular Momentum[/b]

This is probably the most complex index covered here and one which has only relatively recently been widely used in longer range forecasting (an explanation of GLAAM can be found here [url="http://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281971%29028%3C1329%3AGAMBET%3E2.0.CO%3B2"]Newton, C 1971[/url]). Currently, Global Angular Momentum is in a negative state and, in part due to the weak or slightly negative ENSO state, is expected to remain negative.

[b] QBO[/b]

The Quasi Biennial Oscillation is another key tool in our armoury, as its state fluctuates between easterly and westerly over a roughly 2 year period. We are currently in a negative (easterly) phase, which peaked around August and is starting to decline.

A negative QBO tends to lead to a weakened Jetstream, and is perhaps another indicator of a more blocked pattern this winter.
[b] Current Weather Patterns[/b]

As most of us are probably aware of, this Autumn has featured some particularly potent northerly blocking, with the NAO and AO both being persistently negative throughout October:


In particular, there is a strong correlation between the Autumn Arctic Oscillation and winter pressure patterns, which suggests that the current prevalent blocking over the northern hemisphere is likely to persist into the winter months:

[b] Stratospheric Temperatures:[/b]

Developments in this field have probably contributed the most to improvements in seasonal forecasting over the last few years. Currently, upper latitude stratospheric temperatures are below average and cooling quickly:
Given that generally cooler stratospheric temperatures at upper latitude are associated with a stronger Polar Vortex and therefore a more zonal westerly flow, this does not seem especially encouraging news given the high weight placed on the stratosphere in recent years. However, given the background of an easterly QBO and low Angular Momentum I would expect the stratospheric situation to change as the winter progresses, especially given that highly vaunted Sudden Stratospheric Warmings tend to occur when temperatures have effectively ‘bottomed out’.
[b] Model Forecasts[/b]

There is no strong signal either way in terms of temperature anomalies across northwestern Europe according to the CFS forecasting model.
[img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image018.jpg[/img]More interestingly, and something which I haven’t as yet touched upon, is the rainfall anomalies, as there is a reasonable signal for above average rainfall, which would suggest a more Greenland or mid Atlantic based blocking rather than further east. [img]file:///C:\Users\Calum\AppData\Local\Temp\msohtmlclip1\01\clip_image020.jpg[/img]

[b] Forecast:[/b]

This is based on years which are similar to the consensus view of the key drivers outlined above:

[i]Temperatures are expected to be near or below average for northwestern Europe, though there is a strong signal for a very cold January:[/i]
[attachment=144301:winter 2013 temperature.png]
[attachment=144302:2013 precip.png]

[img]http://www.esrl.noaa.gov/psd/tmp/composites/[/img][b][i]This is suggestive of lower than average precipitation for northern parts and greater than average for the far southwest and into southern Europe, which is consistent with a continuation of a southerly tracking Jetstream.[/i][/b]
[b][i]Pressure patterns[/i][/b]
[b][i]A very strong signal for a negative AO/NAO, with low pressure centred over southwestern Europe and blocking across the arctic.[/i][/b]
[b]Textual Forecast (warning: excessive precision is largely for descriptive purposes and I won’t be held to account for snowfall being a day or so late)[/b]
[b]I’m expecting another early start to the cold, with strong blocking to set up sometime around the 2[sup]nd[/sup] week of December. The first week is likely to be fairly unsettled, with temperatures around or slightly below average, and precipitation largely above average. Initially, blocking is more likely to be centred towards the west, with a fairly potent three day northerly around the 8[sup]th[/sup]. A period of a slacker north-northwesterly flow will follow, bringing fairly cold but mostly dry conditions, before a shortwave drops out of the Arctic. Temperatures in this period will be well below average and there will likely be widespread snowfall across northern and eastern parts of the British Isles and another more transient one further south as the jet tries to undercuts the block. The snow is likely to relent somewhat as the high drifts southwards and a drier, milder spell is possible as the block sits just to our west, with rainfall and some hill snow towards northern Scotland. However, around Christmas look for the blocking to reform, leading to a more sustained period of northeasterlies as blocking establishes properly over Greenland. This should be a fairly snowy period, with at least one major snowfall affecting the bulk of the country.[/b]
[b]Temperature: With the source of cold generally being northerly rather than easterly the cold will be centred further north and east, though it will still be colder than average across England .Scottish Mean: 1.8C CET 3.7C[/b]
[b]Precipitation: Generally average for the north and east, below average for the south and east.[/b]
[attachment=144304:january 2013 pressure.png]
[b]January will begin with a continuation of the cold, with blocking centred to the northwest and the flow generally being north of east, though largely slack for much for the first week. However, this relative calm will give way to the jetstream, bringing above average precipitation but still generally below average temperatures, leading to some major but transient snowfalls. Blocking will be more peripheral, with the low pressure dominant through mid month and with some milder interludes at times, but by the end of the month the block will re-establish and extend across to Scandinavia to bring a potent snowy easterly and the coldest period of the winter will be the final third of January into the start of February. [/b]
[b]Temperature: cold start, cool middle and a very cold end, with the cold generally centred over the spine of the country rather than the north. Scottish Mean: 1.1C CET 1.3C[/b]
[b]Precipitation: Well above average across the country[/b]
[attachment=144305:february 2013 presssure.png]
[b]February will begin with intense cold, with temperatures struggling to get above 0C anywhere with widespread snow cover. It will be mostly dry, with heights again transferring southwards, but this will gradually lead to a thaw and temperatures will start to recover towards average by mid month. With the jet still on a southerly trajectory, though, the block will not sink and some battleground setups with renewed cold uppers from the still frozen continent giving snowfall to eastern areas before giving way to mild southerlies by the end of the month. [/b]
[b]Temperature: very cold start, cool to average middle and much milder towards the very end: Scottish Mean 2.6C CET 3.7C[/b]
[b]Precipitation: Around or slightly above average for the south, Below average further north.[/b]

[b]Note: this forecast was completed in a bit of a hurry so content is subject to slight tweaks over the next week(also if the images don't work I'll get that fixed too), but basically this will be my definitive winter forecast.[/b]


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LS - A most impressive forecast. The content is very pleasing but the compilation is excellent! Everything explained and backed up with evidence. Gold star young man!
I like the idea of the cold hanging off until the middle of December. I have always maintained that this has a better chance of presaging a longer, harder winter - (what do I know - other than a gut feeling and observations over the years!)
I hope it works out for you (and us). Well done!
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Cheers, I know it does look rather optimistic to keep forecasting cold winters but once again there's a lot of evidence pointing in that direction. Always the possibility of another 'close but no cigar' winter too though, it's almost impossible to tell whether or not that's likely at this stage. It looks better than last winter did at this time, the building blocks are already in place whereas last year a lot was against us at the very start.
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Interesting thoughts LS , like the SET and CET figures, cannot see many of the images as of yet.

A cold February certainly is a trend that is worth looking out for , CFS has been loving this idea for some time.
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When I read your forecasts and, indeed, your posts, I'm always amazed at how young you are! An interesting, enticing and informative forecast. Thank you!
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nicely composed LS, I'd be happy enough if your forecast comes to even 50% of all that cold potential. Here we go again eh !
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Recap: details off by quite a bit but the general theme of below average though unsettled first fortnight looks about right, along with probably milder spell around mid month up to just before Christmas. Certainly not a bust yet and the December anomalies might not be too far off if we can get a northerly based cold spell around Christmas.
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Second update: failure of Greenland heights to establish in December is probably the main error here, otherwise it looks at this stage to be pretty close: much colder in Scotland than elsewhere, cold starting early but easing off in mid month and (hopefully) a colder blast to finish the year again.
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