Record snow blankets the United Kingdom 5-6 January 2010

1. INTRODUCTION Record snow blankets the United Kingdom 5-6 January 2010 By Richard H. Grumm National Weather Service Office State College, PA 16803 A rare winter storm brought heavy snow the United Kingdom during the first week of 2010. Southern England experienced a bout of heavy snow on 5-6 January 2010. As far south as Hazlemere there were reports of over 10 inches of snowfall. An impressive snow fall and well anticipated snow amount for southern England. The United Kingdom Meteorological Office (UKMO) predicted heavy snow in southern England (Fig. 1). This relatively rare event was apparently well predicted by numerical weather prediction models. Snowfall data was difficult to come by. Reports suggest Reading had about 9 inches of snowfall, possibly the largest single snowfall in over 40 years. This was the second significant snowfall of the winter of 2009-2010. The UKMO completed a review of the snowfall event of 17-23 December 2009. This multi-day event produced 16, 13, 17, 20, and 17 cm respectively from 0900 local time on 18-22 December 2009. According to the report, this event too was well predicted. This event and the December 2009 event occurred during an episode of high latitude blocking. The blocking began in December and persisted into January 2010. It likely was related to the unusual winter weather over the Northern Hemisphere. Some of the unusual weather included: One of the longest cold episodes in over 25 years in the eastern United States Sustained cold weather and snow in Western Europe to include England and France. Cold and record snows in eastern Asia to include China and Korea. Figure 1. Snow forecast from the UK Met office for the southern England snowfall. In addition to the blocking the Arctic Oscillation (AO: Wallace and Gutzler 1981; Hurrell 1995) and the North Atlantic Oscillation (NAO: Wallace and Gutzler 1981) went extremely negative during this period of unusual weather (Table 1). The average value of the AO for December 2009 was -2.43, the lowest value since records began surpassing the previous monthly value of -2.354 set in 2000 (Table 2). The running mean AO value for the data in Table 1 was -4.189. The lowest daily AO value was -5.67 set on the 21 December 2009 (Table 1). Table 2 lists the bottom ten lowest AO December values by year. The data in Table 2 suggest that the

Figure 2. Climate Prediction Center (CPC) image showing the height anomalies in the northern hemisphere from 65 to 90 North latitude and a bar graph of the daily value of the Arctic Oscillation. Real time link. NAO followed a similar evolution as the AO. The value of these indices in understanding the potential meteorological impacts and the relationship of the AO to high latitude of blocking are of interest here. Figure 2 shows the relationship of the AO to high latitude blocking. The upper panel shows the height anomalies from 65 to 90N and the lower panels shows the AO. A persistent episode of high latitude blocking is clearly evident in Figure 1 through the troposphere through most of the month of December. In November, positive anomalies were observed in the stratosphere. The negative and strongly negative AO values coincide well with large positive height anomalies in the troposphere. These data show strong positive height anomalies in the higher latitudes from about 12 December through 2 January 2010. It should be noted that the cold weather and snowier than normal conditions in Britain began shortly after the turn toward negative AO values and high latitude blocking in the lower troposphere. High latitude blocking (Rex 1950a, b; Glickman 2000) can impact the value of the AO and NOA. The Rex block (Rex 1950a) is often characterized by a cut-off low beneath high latitude anticyclone. The omega block, so named due to its similarity to the Greek letter Ω is characterized by an anticyclone anchored by to cut-off cyclones on either side. Episodes of blocking can persist for periods of days to weeks.

Figure 3. NCEP GFS 00 hour forecasts of 500 hpa heights (m) and 500 hpa height anomalies (standard deviations) for the 9 day period from 1200 UTC 29 December 2009 through 6 January 2010. Return to text. This paper will show the conditions associated with the strong high latitude blocking episode of December 2009 and January 2009. The focus will be on the pattern and the imp acts on the pattern and weather over the Western Europe and Great Britain. It should be noted during this episode of blocking, unseasonably cool conditions affected Asia and the eastern United States. 2. METHODS and Data All NAO, AO, and Pacific North American index values were retrieved from the Climate Prediction (CPC) teleconnections website. The monthly mean values and the daily values were sorted using Microsoft excel. Plots of blocking and AO values were also obtained from the CPC. No attempt to replicate these images was accomplished. The 500 hpa heights, 850 hpa temperatures and other standard level fields were derived from the NCEP GFS, GEFS, and the NCEP/NCAR (Kalnay et al. 1996) reanalysis data. The means and standard deviations used to compute the standardized anomalies were

the NCEP/NCAR data as described by Hart and deviations from normal, as standardized anomalies. All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies computed as: SD = (F M)/σ ( ) Where F is the value from the reanalysis data at each grid point, M is the mean for the specified date and time at each grid point and σ is the value of 1 standard deviation at each grid point. Model and ensemble data shown here were primarily limited to the GFS and GEFS due to the global nature of the event and critical fields to examine. Displays will focus on the observed pattern and some forecast issues associated with the pattern. Comparison cases were retrieved using the Figure 4. As in Figure 3 except showing mean sea level pressure (hpa) and pressure anomalies. Return to text.

CPC AO and NAO data. Previous strongly negative NAO months are displayed using the NCEP/NCAR reanalysis data. For brevity, times will be displayed in day and hour format such at 29/0000 UTC signifies 29 December 2009 at 0000 UTC. Due to the transition from 2009 to 2010, times with such as 01/1200 and 03/1200 UTC refer to 1200 UTC 01 and 03 January 2010. Figure 5 shows the PW and PW anomalies during the same period. Note how the high PW AR is shunted to the south. 3. Overview i. Large scale pattern Figure 2 shows the 500hPa pattern over Atlantic basin in 12-hour increments from 0000 UTC 29 December to 0000 UTC 06 January 2009. These data show the evolution of the 500 hpa anticyclone over Greenland with attained a 5700 m close contour on 2 January 2009 with height anomalies on the order of 4-5SDs above normal. Anchoring mid-tropospheric cyclones with negative height anomalies were present on either side of the blocking ridge. Several waves from North America crossed the Atlantic, beneath the block to form the semi-persistent trough over Western Europe. The 5880m contour and anomalous ridge over Africa early in the period was another interesting feature. Other features of note over the region at this time included large surface anticyclone (Fig. 4 e-h). Several cyclones and implied easterly flow can be seen over the central Atlantic as storms moved west to east across the basin. In response (Figure 5) surges of high precipitable water air were moved poleward (Fig. 5c) and across the Atlantic (Fig. 5g-i). The implied broad easterly flow in the gradient east of Britain was critical in bringing cold air from Eastern Europe and Russia into extreme Western Europe. The 850 hpa temperatures for this time are shown in Figure 6. These data reveal warm air beneath the ridge and cold air over northern and Western Europe. The abnormally cold air as indicated by the -1SD anomalies settled in around 2 January 2010. Note the surge of anomalously warm air Africa and southeastern Europe from 29 December through 01 January. Figure 7 shows the 850 hpa winds and u-wind anomalies. These data show the easterly flow north of the persistent cyclone track over the central Atlantic and easterly flow out of Russia into Western Europe. A cyclone develops in the trough over the English Channel and closes off over southern England on 6 January. ii. Regional views The evolution of the features of interest over Britain is shown in Figures 8-11. The sea-level pressure field (Fig. 8) shows the anticyclone to the northeast and low pressure to the east. The 850 hpa temperatures (Fig. 9) are quite telling with this event as temperatures were abnormally cold, averaging 1-2SDs below normal and were in the -6 to -8C range potentially cold enough to support snow. The 850 hpa winds and u-wind anomalies (Fig. 10) show the strong northeasterly flow over Britain. Initially, the stronger winds and anomalies are over northern Scotland (Fig. 10e-f). But new wind maximum develops over southern England; Figure 10h-i north of the 850 hpa low which is moving east to west over the Channel. iii. Forecasts Figures 11-13 show NCEP GEFS forecasts of several fields shown in previous section and the quantitative precipitation forecast (QPF: Figure 11). The QPF shows the highest threat for precipitation and snow was over southwestern England. These successive forecasts improved upon the location.

Figure 5. As in Figure 3 except precipitable water (mm) and precipitable water anomalies. Return to text. In addition to the QPF, the 850 hpa winds and u-wind anomalies and mean sea-level pressure forecasts are shown in Figures 12 & 13 respectively. These data imply that in the ensemble mean, a deep cyclone and strong u- winds were correctly predicted over southern England. Shared data (not shown) suggest local WRF runs and UKMO models predicted these features and the QPF and snow potential quite well. 4. Summary and conclusions During a period of high latitude blocking a winter storm and unseasonably cold air impacted Western Europe. A polar low developed over southern England on 5-6 January 2010 and produced record snowfall over a large swath of southern England. Snowfall amounts in excess of 10 inches were both forecast and observed. This unusual snow and cold event occurred in a pattern with an unusually low AO and during a period of strong high latitude blocking. This may be an avenue of further research correlating cold and snow in Western Europe with low AO values and high latitude blocking. The 5700 m anticyclone over

Greenland during this event is quite anomalous. The pattern over England during the snow was similar to those found over other parts of the northern hemisphere. The cold air at 850 hpa, the 850 hpa u-wind anomalies, and the location of the snowfall relative to the 850 hpa cyclone are features commonly associated with heavy snowfall in the eastern United States. Atypical in this case being the 850 hpa cyclone was moving to the west-southwest in this event. There appear to be many common characteristics to snow events through out the northern hemisphere. Clearly, numerical guidance provided useful clues as to the potential for an unusual event. The UKMO forecast (Fig. 1) suggests high situational awareness and good input to users of these forecasts. Knowledge of the pattern may also contribute to the success in forecasting this record event. Figure 14 (courtesy Medium Range forecasting (ECMWF)) shows the snow depth over the British Isles. These data likely reveal good approximate total snow fall in southern England where 1 to 26 cm of snowfall is quite evident. The data over central England and Scotland likely represent accumulations over the entire cold episode and the snows of December 2009. Impressive features on this map include snow over Britain from Channel to northern Scotland and snow in northeastern Ireland. During this event, the heaviest snow was focused over southern England and the Reading area, in good agreement with the UKMO forecasts (Fig. 1). In addition to the snow, extremely cold temperatures impacted the region. This was a high impact winter event for much of Western Europe and the British Isle. The satellite image in Figure 15 shows the impact of a prolonged cold and snow period. As viewed from space, one can clearly see that nearly the whole of Britain was snow covered on 7 January 2010. 5. Acknowledgements Discussions on the block were part of the SUNY-Albany map thread. Dave Schultz provided snowfall information and radar images from the event. Tim Hewson provided information on the event and WRF QPFs before the event. Declan Cannon provided some editorial assistance. 6. References Ambaum, M.H.P., B.J. Hoskins, and D.B. Stephenson, 2001: Arctic Oscillation or North Atlantic Oscillation? J. Climate, 14, 3495 3507. Doty, B. E., and J. L. Kinter III, 1995: Geophysical data and visualization using GrADS. Visualization Techniques Space and Atmospheric Sciences, E. P. Szuszczewicz and Bredekamp, Eds., NASA, 209 219. Colucci, S.J. 1985: Explosive Cyclogenesis and Large-Scale Circulation Changes: Implications for Atmospheric Blocking Stephen.Journal of the Atmospheric Sciences, 42, 2701 2717. Konrad, C.E, and S.J. Colucci, 1989: An Examination of Extreme Cold Air Outbreaks over Eastern North America. Mon. Wea. Rev.117, 2687 2700. Glickman, T. 2000: Glossary of Meteorology, Second Edition; American Meteorological Society, 2000; ISBN 1-878220- 34-9. Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events:

Preliminary Findings. Wea. and Fore., 16,736 754. Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, 2426 2442. Higgins, R. W., A. Leetmaa, and V. E. Kousky, 2002: Relationships between climate variability and winter temperature extremes in the United States. J. Climate, 15, 1555-1572. Hirsh, M.E, A.T. DeGaetano, S.J. Colucci,2000: An East Coast Winter Storm Climatology.J.Climate,14,882-899. Hurrell, J. W., 1995: Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269, 676 679., and H. van Loon, 1997: Decadal variations in climate associated with the North Atlantic Oscillation. Climatic Change, 36, 301 326. Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp. 445 453 Neiman, P.J., F.M. Ralph, G.A. Wick, J. D. Lundquist, and M. D. Dettinger, 2008: Meteorological characteristics and overland precipitation impacts of atmospheric rivers affecting the west coast of North America based on eight years of SSMI/satellite observations. J. Hydrometeor., 9, 22-47. Pelly,J.L, and B.J. Hoskins, 2003: A new perspective on blocking. JAS,60,743-755. Ralph, F. M., P. J. Neiman, and G. A. Wick, 2004: Satellite and CALJET aircraft observations of atmospheric rivers over the eastern north Pacific Ocean during the winter of 1997/98. Mon. Wea. Rev., 132, 1721-1745. Rex, D. F., 1950a: Blocking action in the middle troposphere and its effect upon regional climate. I. An aerological st udy of blocking action. Tellus, 2, 196 211., 1950b: Blocking action in the middle troposphere and its effect upon regional climate. II. The climatology of blocking action. Tellus, 2, 275 301. Stuart,N.A and R.H. Grumm 2007: Using Wind Anomalies to Forecast East Coast Winter Storms.Wea. and Forecasting, 21,952-968. Thompson, D. W. J., and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime

geopotential height and temperature fields. Geophys. Res. Lett., 25, 1297 1300. Wallace, J. M., 2000: North Atlantic oscillation/annular mode: Two paradigms one phenomenon. Quart. J. Roy. Meteor. Soc., 126,791 805. (patterns for AO look like NAO temps) Wallace, J.M. and D.S. Gutzler, 1981: Teleconncections and the geopotential height during Northern Hemisphere Winter. Mon. Wea. Rev.,109,784-812. Wettstein, J.J., and L.O. Mearns, 2002: The Influence of the North Atlantic Arctic Oscillation on Mean, Variance, and Extremes of Temperature in the Northeastern United States and Canada. J. Climate, 15, 3586 3600. Zhou, S., A. J. Miller, J. Wang, and J. K. Angell, 2001: Trends of NAO and AO and their associations with stratospheric processes. Geophys. Res. Lett., 28, 4107-4110.

Monthly Mean AO values 1948-2009 December January February 2009-2.43 1977-3.767 1969-3.114 2000-2.354 1963-3.311 1978-3.014 1995-2.127 1966-3.232 1986-2.904 2005-2.104 1969-2.967 1958-2.228 1976-2.074 1985-2.806 1960-2.212 1985-1.948 1960-2.484 1968-2.154 1950-1.928 1970-2.412 1965-2.084 1969-1.856 1979-2.233 1956-2.029 1952-1.827 1998-2.081 1977-2.01 1996-1.721 1980-2.066 1983-1.806 1958-1.687 1959-2.013 1952-1.747 1961-1.668 2004-1.686 1963-1.721 TABLE 2. Most negative monthly mean AO values sorted for the months of December, January, and February. For each month, the year and the AO value are provided. Data courtesy of the CPC. Return to text.

2009 12 14 2.111 3.821 2009 12 15 1.957 3.89 2009 12 16 1.696 3.61 2009 12 17 1.617 3.744 2009 12 18 1.725 4.238 2009 12 19 1.833 4.651 2009 12 20 1.964 5.341 2009 12 21 1.584 5.821 2009 12 22 0.976 5.503 2009 12 23 0.876 5.577 2009 12 24 0.935 5.256 2009 12 25 1.017 5.052 2009 12 26 0.886 4.406 2009 12 27 0.636 3.724 2009 12 28 0.434 3.696 2009 12 29 0.517 3.836 2009 12 30 0.838 3.969 2009 12 31 1.196 4.1 2010 1 1 1.548 5.546 2010 1 2 2.091 5.0326 2010 1 3 2.387 5.486 2010 1 4 2.04 5.546 2010 1 5 1.479 5.239 2010 1 6 1.32 5.099

Figure 6. As in Figure 3 except showing 850 hpa temperatures. Return to text.

Figure 7. As in Figure 2 except showing 850 hpa winds and u wind anomalies. Negative u winds anomalies are cold colors implying easterly flow. Return to text.

Figure 8. GFS 00 hour forecasts of mean sea level pressure and pressure anomalies in 6 hour intervals from 1200 UTC 4 January to 1200 UTC 6 January 2010. Isobars are every 4hPa. Return to text.

Figure 9. As in Figure 8 except 850 hpa temperatures (C ) and temperature anomalies. Return to text.

Figure 10. As in Figure 8 except 850 hpa winds and u wind anomalies. Return to text.

Figure 11. NCEP GEFS ensemble mean QPFs of accumulated precipitation (mm) from 1200 UTC 5 January through 0000 UTC 7 January 2010. Data in millimeters as indicated by the color bar. Forecasts are initialized at a) 1200 UTC 01 January, b) 0000 UTC 2 January, c) 0600 UTC 3 January, d) 1800 UTC 3 January, e) 06Z04 January, f) 12Z04January, g) 1800 UTC 4 January, h) 0000 UTC 05 January and i) 0600 UTC 5 January 2010. Return to text.

Figure 12. GEFS forecasts of 850 hpa winds valid at 0000 UTC 6 January 2010 and u wind anomalies from forecasts initialized at a) 0600 UTC 3 January, b) 1800 UTC 3 January, c) 0600 UTC 04 January, d) 1200 UTC 04January, e) 1800 UTC 4 January, f) 0000 UTC 05 January g) 0600 UTC 5 January, h) 0600 UTC 5 January and i) 1200 UTC 05 January 2010. Return to text.

Figure 13. As in Figure 12 except showing mean sea level pressure. Return to text.

Figure 14. Snowfall (cm) across the British Isles. Map produce by the European Center for Medium Range forecasting (ECMWF) and provide here by Tim Hewson (ECMWF). Return to text.

Figure 15. High resolution visual image of the Britain on 7 January 2010. The snow cover over the whole of Britain is clearly visible. Images provided by Kerry Emanuel and Richard James. Return to text.

Figure 14. GFS forecasts of QPF. Not used.