Lomond Snowstorm's 2011/12 Winter Forecast
Entry posted by LomondSnowstorm
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I perhaps left this forecast a bit late, to the point that it coincides with the Netweather one, but I felt it was worth doing one anyway even if it overlaps with much of what others are saying. I'll have a look at the factors to consider, then summarise each individual month, both in terms of the general pattern and for Scotland in particular.
[b][u][size=4]Key Factors[/size][/u][/b]
[size=4]ENSO - We're most likely looking at a weak-moderate La Nina, perhaps similar in strength to last year, though the forecast is for a significantly weaker La Nina than was thought a month or so ago [url="http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/images2/nino34SSTMon.gif"]http://www.cpc.ncep....ino34SSTMon.gif[/url][/size]
[size=4]Global Angular Momentum - This is currently weakly negative and is likely to stay generally negative or neutral through the winter.[/size]
[size=4]Solar Activity - We are quite a long way from the relatively long sunspot minimum that many believe was largely responsible for the last few winters( [url="http://www.nature.com/ngeo/journal/v4/n11/full/ngeo1282.html"]http://www.nature.co...l/ngeo1282.html[/url] this too links to my next key factor). This increase in sunspot activity could perhaps contribute to lower stratospheric temperatures and hence discourage blocking. [/size]
[size=4]QBO - Generally speaking, a negative or easterly QBO tends to be disruptive to the polar vortex, but moreso in years of low solar activity (see the stratospheric temperature thread for more in depth analysis of these factors). This is currently also weakly negative and will remain negative through the winter, which could aid blocking as the winter progresses.[/size]
[size=4]I decided to have a go at making my own analoguous composite series using years with both a weakly negative La Nina, weak easterly/ neutral QBO and slightly negative/neutral GLAAM to see if it was similar and indeed it is - [attachment=123819:2011-12 composite pressure.png] pressure is anomalously high to the northeast and low to the southeast. [/size]
[size=4]Stratospheric temperatures - currently these are below average, which means high latitude blocking is likely to be hard to come by in the first half of winter, particularly given the lag time for propagation down through the atmosphere. However, the main crumb of comfort for those looking for blocking is that the composite years favour an anomalously warm stratosphere over high latitudes, which hints at a potential SSW (sudden stratospheric warming) and eventual disruption of the polar vortex into January and February. [attachment=123820:2011-12 composite stratospheric temps..png] This is the key to the winter - if the SSW splits the vortex unfavourably or doesn't come off at all then we're unlikely to see a significant cold spell at all. However, if it does, and the analogues give a good guide to where blocking will set up, we could have a classic spell of lamping on our hands yet again.[/size]
[size=4]Autumn Eurasian snow cover - This also affects winter atmospheric circulation through the upward propagation of Rossby Waves, causing a weakening of the polar vortex ([url="http://climexp.knmi.nl/data/g20111126_2140_11308_1.png"]http://climexp.knmi....140_11308_1.png[/url]). The October snowcover for both Eurasia and the NH as a whole was below average ([url="http://climate.rutgers.edu/snowcover/table_rankings.php?ui_set=1#eurasia"]http://climate.rutge...i_set=1#eurasia[/url]) while in both 2009 and 2010 it was significantly above average. However, November currently looks to be running above average for Eurasia [url="http://climate.rutgers.edu/snowcover/chart_daily.php?ui_year=2011&ui_day=329&ui_set=2"]http://climate.rutge...ay=329&ui_set=2[/url] which does add further support to the idea of a winter of two halves.[/size]
NAO - In the past 3 winters this has been negative, weakly so in 08/09 and strongly so in 09/10 and the first half of 10/11. As a pressure index, it is probably best used as an indicator of the synoptic pattern over a season rather than something which is a driver in itself, but it is worth looking at anyway ([url="http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif"]http://www.cpc.ncep....a/nao.sprd2.gif[/url]). The outlook into the first ten days of next month suggests it will remain positive, which is in line with the generally poor outlook for lovers of northerly blocking (as it should be given these are model generated forecasts).
Model Output - The CFS has given us this [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst/images3/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] which runs counter to a lot of what has been said in the forecast. Should we take note of this? Well looking at the last two winters' outputs in November, the CFS has fairly mixed fortunes to say the least ( [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst_history/200911/images/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] and last year [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst_history/201011/images3/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] )
[b][u]December[/u][/b]
The first third of the month looks likely to be very unsettled, with very low heights to the north and Polar Maritime incursions if and when the trough tilts NW to SE (but of course, the usual health warnings about believing fantasy charts apply). Precipitation widely above average, temperatures generally close to average, though much colder in northern areas. Even here, though, any snowfall will be largely transient even on high ground with frosts certainly coming at a premium. Into mid-month, I'd bank on a return to more settled conditions, with high pressure fairly close to the southeast of the UK, and temperatures generally above average and precipitation below average, though perhaps around average in the northwest. Christmas may well coincide with a return to less settled conditions and perhaps a transient cold spell from the north before milder air returns at the end of the month. Overall, then, a milder than average month for most of the UK, though for Scotland it may well end up being close to average. Precipitation somewhat above average, probably moreso in the north than south, but there may be a slight easing of drought conditions in the southeast. There's a very high chance of a NW/SE split in snowfall prospects, with most of central and southern England seeing no lying snow in December and snowfall being above average in Northwest Scotland. East-central Scotland's snowlovers will suffer badly from lack of northerly exposure, though depending on 'snowshield' formation I wouldn't rule out one significant but short lived snowfall accumulation.
CET punt - 5.1C Scotland mean 3.2C
[b][u]January[/u][/b]
A very mild southerly flow will gradually give way to drier conditions as high pressure builds in from the southeast. This quieter spell of weather will usher in the change to colder conditions as the high retrogrades to northern Scandinavia. Snowfall will initially be confined to southeast England with dull, drizzly conditions further up the east coast and sunshine for western areas.However, as the high moves northwards and colder uppers move westwards across the continent showers will become more widespread and intense to end the month. Precipitation will be around average in southern and eastern England but well below, for once, across Ireland and Western Scotland. Snowfall amounts will depend on the timing of the SSW but again I see a NW/SE split, this time with the southeast most likely to see above average snowfall. Eastern Scotland, particularly coastal parts, could well endure a spell of intensely dull weather mid-month with a chilly southeasterly flow uppers not quite low enough for a 'sunshine and showers' regime before the real cold arrives.
CET punt - 3.6C Scotland mean - 2.6C
[b][u]February[/u][/b]
February will begin with a period of intense cold and snowfall with the south bearing the brunt of the snowfall from 'channel lows'. At times, low pressure will become more dominant, temporarily shifting the focus further north and melting accumulations in the south and near the east coast. Showers will again become confined to the southeast as the high builds southwards, bringing the coldest minima of the winter in deep snowcover. This will then be followed by a reload midmonth from the northeast bringing more convective snowfall right down the east coast.. The month ends with a rather messy breakdown taking place and neither airmass winning out until well into March, ensuring a gradual thaw. Precipitation will again be well below average in the northwest with central, southern and eastern parts seeing the most precipitation. For Scotland, the southeast will receive the brunt of the snowfall, with frontal systems providing most of the snowfall away from the east coast.
CET punt - 1.4C Scotland mean - 1.3C
[b][u]Summary[/u][/b]
The degree of certainty in the forecast is of course exaggerated. Confidence up to around the end of December is fairly high but the remainder of the forecast is dependent on how a fairly probable stratospheric warming plays out. The building blocks of a dramatic switcharound in midwinter are most definitely there, but whether it comes off or ends up, like February 2009, as something of a disappointment remains to be seen. Here's the composite for temperature [attachment=123821:Winter Temp Anomaly composite.png] and precipitation [attachment=123822:2011-12 composite precip.png] .
[b][u][size=4]Key Factors[/size][/u][/b]
[size=4]ENSO - We're most likely looking at a weak-moderate La Nina, perhaps similar in strength to last year, though the forecast is for a significantly weaker La Nina than was thought a month or so ago [url="http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/images2/nino34SSTMon.gif"]http://www.cpc.ncep....ino34SSTMon.gif[/url][/size]
[size=4]Global Angular Momentum - This is currently weakly negative and is likely to stay generally negative or neutral through the winter.[/size]
[size=4]Solar Activity - We are quite a long way from the relatively long sunspot minimum that many believe was largely responsible for the last few winters( [url="http://www.nature.com/ngeo/journal/v4/n11/full/ngeo1282.html"]http://www.nature.co...l/ngeo1282.html[/url] this too links to my next key factor). This increase in sunspot activity could perhaps contribute to lower stratospheric temperatures and hence discourage blocking. [/size]
[size=4]QBO - Generally speaking, a negative or easterly QBO tends to be disruptive to the polar vortex, but moreso in years of low solar activity (see the stratospheric temperature thread for more in depth analysis of these factors). This is currently also weakly negative and will remain negative through the winter, which could aid blocking as the winter progresses.[/size]
[size=4]I decided to have a go at making my own analoguous composite series using years with both a weakly negative La Nina, weak easterly/ neutral QBO and slightly negative/neutral GLAAM to see if it was similar and indeed it is - [attachment=123819:2011-12 composite pressure.png] pressure is anomalously high to the northeast and low to the southeast. [/size]
[size=4]Stratospheric temperatures - currently these are below average, which means high latitude blocking is likely to be hard to come by in the first half of winter, particularly given the lag time for propagation down through the atmosphere. However, the main crumb of comfort for those looking for blocking is that the composite years favour an anomalously warm stratosphere over high latitudes, which hints at a potential SSW (sudden stratospheric warming) and eventual disruption of the polar vortex into January and February. [attachment=123820:2011-12 composite stratospheric temps..png] This is the key to the winter - if the SSW splits the vortex unfavourably or doesn't come off at all then we're unlikely to see a significant cold spell at all. However, if it does, and the analogues give a good guide to where blocking will set up, we could have a classic spell of lamping on our hands yet again.[/size]
[size=4]Autumn Eurasian snow cover - This also affects winter atmospheric circulation through the upward propagation of Rossby Waves, causing a weakening of the polar vortex ([url="http://climexp.knmi.nl/data/g20111126_2140_11308_1.png"]http://climexp.knmi....140_11308_1.png[/url]). The October snowcover for both Eurasia and the NH as a whole was below average ([url="http://climate.rutgers.edu/snowcover/table_rankings.php?ui_set=1#eurasia"]http://climate.rutge...i_set=1#eurasia[/url]) while in both 2009 and 2010 it was significantly above average. However, November currently looks to be running above average for Eurasia [url="http://climate.rutgers.edu/snowcover/chart_daily.php?ui_year=2011&ui_day=329&ui_set=2"]http://climate.rutge...ay=329&ui_set=2[/url] which does add further support to the idea of a winter of two halves.[/size]
NAO - In the past 3 winters this has been negative, weakly so in 08/09 and strongly so in 09/10 and the first half of 10/11. As a pressure index, it is probably best used as an indicator of the synoptic pattern over a season rather than something which is a driver in itself, but it is worth looking at anyway ([url="http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.sprd2.gif"]http://www.cpc.ncep....a/nao.sprd2.gif[/url]). The outlook into the first ten days of next month suggests it will remain positive, which is in line with the generally poor outlook for lovers of northerly blocking (as it should be given these are model generated forecasts).
Model Output - The CFS has given us this [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst/images3/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] which runs counter to a lot of what has been said in the forecast. Should we take note of this? Well looking at the last two winters' outputs in November, the CFS has fairly mixed fortunes to say the least ( [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst_history/200911/images/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] and last year [url="http://www.cpc.ncep.noaa.gov/products/people/wwang/cfs_fcst_history/201011/images3/euT2mProbMon.gif"]http://www.cpc.ncep....uT2mProbMon.gif[/url] )
[b][u]December[/u][/b]
The first third of the month looks likely to be very unsettled, with very low heights to the north and Polar Maritime incursions if and when the trough tilts NW to SE (but of course, the usual health warnings about believing fantasy charts apply). Precipitation widely above average, temperatures generally close to average, though much colder in northern areas. Even here, though, any snowfall will be largely transient even on high ground with frosts certainly coming at a premium. Into mid-month, I'd bank on a return to more settled conditions, with high pressure fairly close to the southeast of the UK, and temperatures generally above average and precipitation below average, though perhaps around average in the northwest. Christmas may well coincide with a return to less settled conditions and perhaps a transient cold spell from the north before milder air returns at the end of the month. Overall, then, a milder than average month for most of the UK, though for Scotland it may well end up being close to average. Precipitation somewhat above average, probably moreso in the north than south, but there may be a slight easing of drought conditions in the southeast. There's a very high chance of a NW/SE split in snowfall prospects, with most of central and southern England seeing no lying snow in December and snowfall being above average in Northwest Scotland. East-central Scotland's snowlovers will suffer badly from lack of northerly exposure, though depending on 'snowshield' formation I wouldn't rule out one significant but short lived snowfall accumulation.
CET punt - 5.1C Scotland mean 3.2C
[b][u]January[/u][/b]
A very mild southerly flow will gradually give way to drier conditions as high pressure builds in from the southeast. This quieter spell of weather will usher in the change to colder conditions as the high retrogrades to northern Scandinavia. Snowfall will initially be confined to southeast England with dull, drizzly conditions further up the east coast and sunshine for western areas.However, as the high moves northwards and colder uppers move westwards across the continent showers will become more widespread and intense to end the month. Precipitation will be around average in southern and eastern England but well below, for once, across Ireland and Western Scotland. Snowfall amounts will depend on the timing of the SSW but again I see a NW/SE split, this time with the southeast most likely to see above average snowfall. Eastern Scotland, particularly coastal parts, could well endure a spell of intensely dull weather mid-month with a chilly southeasterly flow uppers not quite low enough for a 'sunshine and showers' regime before the real cold arrives.
CET punt - 3.6C Scotland mean - 2.6C
[b][u]February[/u][/b]
February will begin with a period of intense cold and snowfall with the south bearing the brunt of the snowfall from 'channel lows'. At times, low pressure will become more dominant, temporarily shifting the focus further north and melting accumulations in the south and near the east coast. Showers will again become confined to the southeast as the high builds southwards, bringing the coldest minima of the winter in deep snowcover. This will then be followed by a reload midmonth from the northeast bringing more convective snowfall right down the east coast.. The month ends with a rather messy breakdown taking place and neither airmass winning out until well into March, ensuring a gradual thaw. Precipitation will again be well below average in the northwest with central, southern and eastern parts seeing the most precipitation. For Scotland, the southeast will receive the brunt of the snowfall, with frontal systems providing most of the snowfall away from the east coast.
CET punt - 1.4C Scotland mean - 1.3C
[b][u]Summary[/u][/b]
The degree of certainty in the forecast is of course exaggerated. Confidence up to around the end of December is fairly high but the remainder of the forecast is dependent on how a fairly probable stratospheric warming plays out. The building blocks of a dramatic switcharound in midwinter are most definitely there, but whether it comes off or ends up, like February 2009, as something of a disappointment remains to be seen. Here's the composite for temperature [attachment=123821:Winter Temp Anomaly composite.png] and precipitation [attachment=123822:2011-12 composite precip.png] .
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