Weather Logistics UK predicts close to average winter temperatures, but a stormy polar maritime air flow will add a raw "real-feel" factor

As November draws to a close, the weather pattern is beginning to show signs of a major change. A strong cold temperature anomaly in the mid-Atlantic is building as our polar jet stream is being flung northward along the southwest of Greenland. Intense storms and flooding have affected many parts of northern England and Scotland, with the cool and wet weather expected to continue into December. By Christmas it is likely that our weather will return to more changeable and milder conditions, with wintry showers largely confined to the north. Our last weblog article gives some insight into what is causing this wacky weather, however this article presents our latest long-range outlook of what this winter has in store …

Abstract from our Kindle Weather Forecast for Winter 2011 to 2012  -

Includes 2/9 pages – The full document comprises of weather graphics for several regions of the United Kingdom & Ireland and is available to Purchase.

It will be available from 16:00 BST on 1^st December.

Valid from: 1^st December until 29^th February 2011
Issued: 30th November, 2011 (Final Edition)

Blocking Index: 7.3/10.0 (upper third on Atlantic blocking index)

During the course of November, major blocking patterns have been observed over north-west Europe, located between a two-way diverted jet stream. The increased southerly component of airflow relative to north has seen temperatures climb well above average for many parts of the UK and Ireland. In our previous forecast we anticipated the weather to swing into a more negative NAO phase, with an increased prevalence of cold weather blocking patterns. Although this cold transition has not yet been observed in November, the formation of a strong warm dipole either side of the North Atlantic indicates a change toward intense blocking features over the mid-Atlantic Ocean. The northerly placement of the polar jet stream located southward of Greenland is beginning to spin-up deep and compact low pressure systems that have stormed into Scotland and Northern England. This north-westerly polar airflow is expected to dominate the scene for at least the first half of the winter period, with brief Arctic blasts bringing wintry showers to the north and the exposed coastlines. The long-range forecasts indicate that snowfall is likely in England during the middle of December, with a biting northerly wind-chill.

The current atmospheric signal also indicates that we are in an active period of cyclonic activity, although the tendency toward north-westerlies will bring a distinct chill factor and with temperatures typically around the long-term average. Night time frosts are likely to be infrequent throughout the early part of the winter, due to high winds and increased cloud cover.

General Overview:

During the winter 2011-12 season, temperatures are likely to be close to average for much of the United Kingdom and Republic of Ireland with an anomaly of +0.2 ± 1.5°C (+0.4 ± 2.6°F). Much of this warm anomaly is due to a strong climate signal in the North-west Atlantic Ocean, whilst a cold dipole in the mid-Atlantic Ocean is indicative of a distant and strong blocking pattern with cold north-westerly winds. The energy required for heating a household or work-place is likely to be below average with an anomaly of -2 ± 13%, indicating less heating demand than during the inter-annual winter average spanning the years 1961 – 1990. Since land surface temperatures have slowly been increasing over the British Isles over the past few decades, this represents a cool winter (1 in 3) compared to the base period of 2005 to 2010. A combination of above average storminess and higher temperatures indicate close to average snow day events across much the UK and Ireland, with an anomaly of -1%. These calculations were achieved using the SnowCast predictions at Weather Logistics UK

Near-average precipitation is likely to be experienced this winter across all counties. Precipitation anomalies are likely to be around 104 ± 22% (with precipitation events +2% above average). Although there is a great deal of uncertainty at this stage, the North Atlantic Oscillation (NAO) is generally expected to be negative later in the season, giving rise to some severe cold at times. At present, the predictions are based on a cautious estimate of a slightly positive index of +0.5, based on NOAA data from the last two months and the trend toward Arctic weather by mid-December. Cold and wintry events are more likely to affect the exposed coasts of the UK and Scotland during the early season, as brisk periods of Arctic airflow take the bite. Easterly winds may develop later in the winter, as the NAO tends to become more negative and if the mid-Atlantic cold anomaly in sea temperatures strengthens further.

In summary: Signals are in favour of a typical winter, with temperatures and rainfall close to average for many parts of the UK and the Republic of Ireland. There is a strong possibility of Arctic winds or easterly winds dominating the season throughout the British Isles, particularly over Eastern counties of England and Scotland if the long term weather swings to strongly negative NAO indices during the early to mid-season period.

Updates from last month …

Since our last forecast there has been an incremental rise in the predicted NAO from +0.3 to +0.5 that acts to strengthen the north-south trend of temperature and rainfall. The blocking index, based on the most likely positions and intensities of high pressures is similar, but with strong blocking patterns more likely to appear over the mid-Atlantic Ocean at 25°W. North Atlantic Ocean sea temperature anomalies have undergone a significant transition from last month, again confirming that blocking patterns in the mid-Atlantic are very likely. Due to an intensification of the jet stream at the southerly tip of Greenland a strong polar maritime theme will dominate the weather with frequent and heavy wintry showers in Northern England and Scotland. Stormy conditions are also expected for much of the United Kingdom as the weather systems converge downstream. The British Isles temperature anomaly has fallen by 0.8°C to +0.2°C, aided by the tendency toward north-westerly airflow and falling SST anomalies in line with a well established La Nińa. The weather is likely to remain changeable during the winter period, but with extreme periods of cold and snow likely at times as the winds swings toward the north in several sharp Arctic blasts. This is the final update of the winter and supersedes any previous seasonal forecasts.

A Heating Degree Day (HDD) forecast for winter can also be found on our website. To predict your heating energy demand for gas, oil or electricity please view our Weather Media information pages, where you can also find a download link for our free UK climate calculator.

Further contact details …

e. forecasts@weatherlogistics.com ; m. 07949187732

An array of blocking high and low pressures are teleconnected in the northern hemisphere. These winds are diverting the usual course of the low pressure systems at the surface.

Golden leaves fluttering from our wooded oaks, bramble bushes in full bloom, tomatoes flourishing under cover – yet not the typical weather we might expect in Britain during late November. The outdoors play with our perception of flowing time, our shopping behaviour, the very psychology of Christmas decorations – with the weather almost forgetting to prompt us with those crisp frosts and the odd snow flakes that spark our seasonal imaginations. So as we approach the advent month, and as Weather Logistics UK approaches an end to its operational weather forecasting, we investigate the deeper goings on in the atmosphere that are responsible for these little quirks of nature. After two winters of extreme cold and snow, we address the question of what is causing our unseasonable weather …

Intense blocking patterns located over Poland and the northwest Pacific Ocean have played havoc with our seasonal weather, driving southerly rivers of warm air into Britain and the southeast of the US. This wacky weather phenomenon has seen temperatures in England soar into the high teens during November. In a sense the east coast of the US and the UK are often teleconnected in their seasonal weather, as broadly speaking the planetary waves that are spun-out by our Earth’s rotation operate in their most stable state with a 60 to 70 degree wavelength. So it’s rather unsurprising that our northern hemisphere jet stream has deflected wildly northward at both longitudes, rather like two rather aggressive slinky shaker partners.

Yet as the jet stream flings northward, there have been equal arguments in the northwest America where the vigorous north Pacific upper air flow has plunged far south. This has resulted in unseasoned cold and winter storms that have sent soggy shivers across the northwest, with snow reported in parts of California much earlier than the seasonal norm. Meanwhile, short and sharp Arctic bites are expected across the East Coast of America this season. In part, this is due to the prevalence of relatively cooler surface waters over the tropical Pacific that is associated with a La Niña. A deep cold snap over the East coast of the US points toward an abrupt and intense Arctic-type weather scenario in the UK in 14 to 21 days time. This Arctic deep freeze will see temperatures plummet, with significant widespread snowfall and perhaps blizzard conditions across Britain during mid-December 2011.

The good news, is that our resident high pressure blocking patterns are weakening somewhat and slipping southward. As the meandering wobbles in our upper air flow give way to a more laminar flow, unsettled weather with high winds and heavy rainfall are anticipated to in parts of northern of England and Scotland. Yet again the cause is our jet, situated around 11km above the surface it is fiercely blowing along the western shores of Greenland, introducing a cool Arctic airflow downstream. As this air mass comes into contact with the mild and most south-westerlies in the north-east Atlantic it tends to activate instability and rapid cyclogenesis, the process involved in the development of our frontal weather systems. As a result, we expect a series of deep and compact low pressures to storm toward Scotland that may lead to flash floods and structural wind damage.

The cause of all this wacky and topsy-turvy weather patterns perhaps lies in the strength of the polar jet. As the temperature gradient between the subtropics and poles intensifies in a warming climate, we may expect to see an increase in “blocking patterns”, due to the formation of deep lows that spin-up and help maintain neighbouring high pressures.

Location of the NH jet streams during November. Red colours indicate where the upper level winds at 200hPa (11km) are blowing strongest. Adapted from the University of Wyoming GFS model run.

It is foreseeable that emergent patterns may arise in the behaviour of our jet as our climate warms, with long-term and predictable blocking features dominating the regional weather conditions over parts of the globe. One noticeable trend is that the European jet stream has been taking a two-way path of deflection, leading to interesting northerly bands in both weather patterns and temperature anomalies that deviate substantially from our typical climate. One upper air-stream has dived into North Africa and has traveled through the Mediterranean. Toward the North Pole, another jet stream has been observed crossing Scotland and Scandinavia, ending its extensive fetch in central Siberia. The key players in these “blocking weather patterns” are: emerging sea surface temperature anomalies through weakening of the over-turning meridional circulation(1), downward trends in the northern hemisphere ice coverage(2), expansion and poleward migration of the subtropical anticyclones(3), and rapid increases in the land-surface temperatures in contrast to the oceans(4). Low solar activity also plays a role in the meandering of the polar jet stream, with particular implications for extremes of winter cold over Europe(5).

Our growing concern is that science can be lost in the midst of secrecy, that’s why our energy demand calculator is now publicly available. The calculator provides vital information on the current temperature thresholds for several heating increments and is reported for 19 UK towns and cities. It can easily be adapted to predict your household heating bills. The heating degree day (HDD) data is based on the 1961 – 1990 averages that are reported as daily maximum and minimum temperatures. Furthermore, we have also released our jet stream blocking diversion model, which has been produced in the IDL computer programming language. Please stay in tune for our final winter weather prediction.

Further contact details …

e. forecasts@weatherlogistics.com ; m. 07949187732

References:

(1) Mignot, J., A. Ganopolski, A. Levermann, 2007: Atlantic Subsurface Temperatures: Response to a Shutdown of the Overturning Circulation and Consequences for Its Recovery. J. Climate, 20, 4884–4898

doi: http://dx.doi.org/10.1175/JCLI4280.1

(2) Climate Change 2007: Working Group I: The Physical Science Basis, FAQ 4.1 Is the Amount of Snow and Ice on the Earth Decreasing?

 http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-4-1.html [Accessed November, 2011]

(3) Lu, J., G.A. Vecchi and T. Reichler, Expansion of the Hadley cell under global warming, Geophysical Research Letters , 34(6), L06805, 2007

DOI: 10.1029/2006GL028443

see also: Correction to “Expansion of the Hadley cell under global warming”, Geophysical Research Letters, 34(L14808), 2007, doi:10.1029/2007GL030931

http://www.agu.org/journals/gl/gl0714/2007GL030931/2007GL030931.pdf

(4) Sutton, R. T. Dong, B. Gregory, J. M., Geophysical Research Letters, Land/sea warming ratio in response to climate change: IPCC AR4 model results and comparison with observations, 2007, 34 (2), pages L02701

http://atmosdyn.yonsei.ac.kr/nrl/seminar/Sutton_etal_GRL2007.pdf

(5) Lockwood, M., Environmental Research Letter, 5(2), Are cold winters in Europe associated with low solar activity?, Environmental Research Letters, 2010

http://iopscience.iop.org/1748-9326/5/2/024001/fulltext