Snow Assessments

Snow Assessment for Winter 2023-2024, Northern Hemisphere and Regional Aspects

07 October 2024

Northern Hemisphere Continental Snow Cover Extent: 2023/2024 Snow Season Update

David A. Robinson & Thomas W. Estilow (Rutgers University, Piscataway, New Jersey, USA)

Snow cover extent (SCE) over Northern Hemisphere (NH) lands for the August 2023–July 2024 period averaged 24.0 million sq. km. This is 0.9 million sq. km. less than the 1991-2020 mean and 1.0 million sq. km below the full period of record mean (Table 1). This ranks this recent snow season as having the 54th most extensive cover in the 57-year period of record. Monthly SCE during the season ranged from 46.9 million sq. km. in January 2024 to 2.6 million sq. km. in August 2023. North America (NA) annual SCE ranked the least extensive in 57 years and Eurasia (EUR) 38th most extensive.

Three figures are presented to depict SCE this past season compared to normal. Figure 1 tracks weekly NH SCE over the course of the past season compared to the period-of-record mean and extremes. August SCE was, as usual, quite minimal over NH lands. The snow season got off to a rather average start in September and October with SCE in the middle tercile. November SCE ranked 18th most extensive in EUR while advancing at an average rate over NA. Above average SCE continued over EUR in December while NA cover fell far behind the average seasonal advance, ranking 55th most extensive, just two places below a record minimum. The script flipped in January with EUR in the lower tercile and NA in the highest. The two continents fell into general agreement for the remainder of the snow season, each maintaining a lower tercile ranking from February through season’s end. For the NH as a whole, the months ranked no more extensive than 44th of 57 years (May) and April had the lowest extent on record. No individual continent ranked more extensive than 38th during the February to June period, that being EUR in February. The most extensive month in NA during this period was March at 44th most extensive with February, April and May from 52nd-55th most extensive.

SStandardized monthly NH SCE anomalies for the 2023/2024 season were generated using Z-scores, with results shown in Figure 2. Results show that the November positive monthly anomaly was the most pronounced of the 2023/2024 season followed on the negative side by April (which also ranked as the least extensive April during the 57-year period of record) and December. In Figure 3, 12-month running means of NH, EUR, and NA SCE anomalies show a pronounced decline in SCE in the mid to late 1980s that has generally remained consistent since that time as declining spring SCE has been balanced by increasing fall and slightly increasing winter SCE.

SCE is calculated at the Rutgers Global Snow Lab (GSL) from daily SCE maps produced by meteorologists at the US National Ice Center, who rely primarily on visible satellite imagery to construct the maps. Maps depicting daily, weekly, and monthly conditions, anomalies, and climatologies may be viewed at the GSL website (https://snowcover.org).

Northern Hemisphere Terrestrial Snow Mass, Winter 2023-2024

Kari Luojus (Finnish Meteorological Institute), Patricia de Rosnay and Kenta Ochi (ECMWF, UK), and Vincent Vionnet (ECCC, Canada)

The WMO GCW Snow Watch expert team has developed several trackers for the cryosphere. Terrestrial snow cover is tracked in regard to snow cover extent and water equivalent of snow cover (SWE) based on satellite data and complemented by model-based information and in-situ snow depth measurements. The trackers provide a quick look at the current state of the cryosphere relative to the mean state of the last 2-3 decades.

The FMI/GCW SWE Tracker is a product of the Finnish Meteorological Institute (FMI), based on GlobSnow SWE. It was developed in collaboration with the GCW Snow Watch expert team. This tracker illustrates the current Northern Hemisphere snow water equivalent relative to the long-term mean and variability. The input data consist mainly of satellite-based passive microwave radiometer data, which is combined with ground-based observations in an assimilation framework (Luojus et al. 2021). The methodology based on passive microwave observations is not able to track snow conditions for the mountains, thus they are omitted from this analysis.

The FMI tracker indicated slightly below average snow mass for the Northern Hemisphere during winter 2023-2024. The below average conditions were driven by slightly negative anomalies over the North American sector. The anomalies for winter 2023-2024 were well within the standard deviation of the 30-year baseline time series for 1982-2012, as seen in Figure 4.

The Environment and Climate Change Canada (ECCC) GCW Snow Water Equivalent Tracker provides an estimate of current Northern Hemisphere SWE relative to the 1998-2011 period. It is based on the Canadian Meteorological Centre operational daily snow depth analysis with SWE estimated using a density look-up table (Brasnett, 1999; Brown and Mote, 2009; Brown et al., 2010). The CMC analysis uses real-time surface snow depth observations and model-derived information and is not a satellite-derived product; it tracks also the snow conditions for the mountains.

The ECCC SWE Tracker, shown in Figure 5, deviates slightly from the FMI/GCW SWE Tracker and indicates slightly above average snow mass for the Northern Hemisphere for the full duration of winter 2023-2024. The average snow mass reached values of approximately one standard deviation over the long-term average. The ECCC tracker includes the snow mass for mountains, while the FMI tracker does not, this explains the difference between the results of the two trackers.

The ECMWF ERA-5 based NH SWE tracker is shown in Figure 6. It relies on a global reanalysis using model and data assimilation (Hersbach et al., 2020). The SWE tracker is computed for the NH, excluding glaciers areas and Greenland icesheet. It indicates slightly below average snow mass for the winter 2023-2024 compared to the reference period 2004-2023.

Figures 1-3 indicate that the NH SCE for winter 2023-2024 was clearly below average. North America (NA) annual SCE ranked the least extensive (lowest snow cover) in 57 year of recorded history with Rutgers Global Snow Lab, while Eurasia showed somewhat below average snow extent. According to FMI and ECMWF analyses shown in Figures 4 and 6, the NH SWE was below its long-term average with a clear negative anomaly seen in NA SWE, while ECCC analysis (Figure 5) indicates slightly above average conditions for winter 2023-2024. The overall synthesis indicates a below average snow conditions over the Northern Hemisphere, with a significant negative anomaly over the North American continent in both SWE and SCE.

The snow season in the Northern Hemisphere mountains, 2023-2024

Simon Gascoin (Centre d'études spatiales de la biosphère (Cesbio), France)

Was 2024 a good or bad snow year in the North Hemisphere mountains? A new interactive atlas shows the snow water equivalent anomalies on April 01 based on ERA5-Land. In the European Alps, there was a widespread positive anomaly but not in the peripheral areas of lower elevation. The anomaly was negative in the Pyrenees and Carpathian Mountains. There was also a marked positive anomaly in the Caucasus mountains. Note the anomaly gradient across the Scandinavian mountains, with a positive anomaly in the south and a negative anomaly in the north-west. In the High Mountain Asia region the data show the snow deficit in the southern arc and the above normal conditions in the north. In the Western North America there was a clear north-south gradient with a lack of SWE in the Cascades and Canadian Rockies, but an excess of SWE in the Sierra Nevada and southern Rockies.

Regional Snow Assessments, Winter 2023-2024

Assessment of the snow-covered area in the central Andes of Chile and Argentina during the first half of the 2024 winter season

Mariano Masiokas, Leandro Cara, Ricardo Villalba (Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Mendoza, Argentina), René Garreaud, Duncan Christie (Center for Climate and Resilience Research, Santiago, Chile)

The snow that accumulates each winter in the Andes between ca. 28° and 37°S (Figure. 8A-B) represents a crucial water resource for most human activities in central Chile and central-western Argentina. This snow regulates the flows of mountain rivers, allows the existence of glaciers, and provides water for recharging the aquifers used in the populated lowland areas on both sides of the Andes. Since 2010 the accumulation of snow in this region has declined substantially, resulting in an extended dry period (locally known as the “Megadrought”) that is unprecedented in the instrumental record.

Daily 2000-2023 MODIS snow-covered area (SCA) data from the study area show that during the winter of 2023 (April-October; Figure 8C), the overall snow extent was markedly negative to the north of ca. 33°S, and less negative further south. Conversely, the first months (April-June) of the current 2024 winter season show a more promising scenario, with above-average SCA anomalies for most of the region (Figure 8D).

Figure 9 below shows daily SCA time series aggregated for the basins on both sides of the continental divide (a limit which coincides here with the international border between Chile and Argentina). Since mid-April and until the end of June 2024, snow conditions have remained above the long term mean and near the 90% percentile on both countries. Monitoring the SCA variations during the second half of the year will remain highly relevant given the region’s strong dependence on snowmelt for most human activities, and the high chances for the occurrence of La Niña conditions projected for late 2024 – early 2025.

Assessment of the snow cover in the Third Pole region

Lijuan Ma (National Climate Center, China Meteorological Administration, Beijing, China)

In winter of 2023/2024 December-February (DJF), the snow cover extent (SCE) in the Third Pole region was 1299.3×103 km², which was 40.9×103 km² (around –3.1%) lower than normal (relative to 2005-2020). Although SCE in last December was close to normal, it was the lowest in January (19.9% below normal) and the 5th highest in February (12.1% above normal) when comparing with that in the past 20 years.

Spatially, the number of snow cover days (NSCD) in the season was below normal except for the northeastern part of the region and the central part of its core area (TPCR, region within black contour denoted in figures). Most of the positive anomalies of NSCD exceed 20 days, while in southern Kazakhstan and in areas south of Tianshan Mountains, along Gangdise Mountains, west of the Pamirs, and eastern TPCR, the negative anomalies exceed –10 days, with part of them lower than –20 days (Figure 10). In December, the most significant negative anomalies occurred in the southern Kazakhstan, while it exhibited positive anomalies in the central part of TPCR. Compared to that in December, NSCD in January became much lower than normal in vast area of the middle of TPCR, especially along Mount Gangdise. While in February, the NSCD anomalies in most areas of south of 45ºN became positive, and the magnitude of negative anomalies along Mount Gangdise was also significantly reduced (Figure 11).

Central Asia Snow Cover Assessment, 2023-2024

Joel Fiddes (Institute for Snow and Avalanche Research, Davos, Switzerland)

The winter 2023-24 saw the first release of the regional snow tracker for Central Asia, called Snowmapper, a collaboration between the Swiss Agency for Development and Cooperation funded projects “CROMO-ADAPT” and “SAPPHIRE”. This near-realtime tracker, shown in Figure 12, is forced by ERA5T that is downscaled to 500m and then forces a physically based snow model. Results are presented against a 24 year climatology (1999-2023) to assess anomalous conditions (see Fiddes et al. 2019 for further details on the model chain).

Syr Darya: Current season 2023-2024 showed largely normal conditions in the Syr Darya basin. After a slow start to the season in autumn and early winter (Sept 2023 - Jan 2024), strong snowfall events in February saw conditions largely approaching norm values throughout most of the basin.

Amu Darya: Current season 20023-2024 in the Amu Darya basin, in contrast was extremely anomalous, and at times below the 5% quantile, throughout the accumulation season and past peak SWE (snow water equivalent), the point in Late March where snow accumulation is maximum before the ablation season commences. It should be noted there were strong precipitation events at the beginning of May 2024 which triggered landslides and mudflows throughout Northeastern Afghanistan and Tajikistan. These are reflected by the spike in SWE shown in the tracker at the beginning of the ablation period.

The snow season in the Swiss Alps, 2023-2024

Christoph Marty (WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland)

Snow depth in the Swiss Alps in the 2023/2024 season was well above average, mainly due to large precipitation amounts, which mostly fell as snow above 1500 m. The figures 15-16 are based on modeled 1 km gridded data from the Swiss Alps (elevation > 500 m) since the hydrological year 1962 (1961/1962), which are provided by MeteoSwiss and SLF.

The Shkhara glacier retreat illustrates well the accelerated melting of the Georgian glaciers over the last half of century. The data from LANDSAT and from field observations of the hydrometeorological department of Georgia show that the retreating speed of glacier Shkhara has increased from approximately 6.5 m/year to approximately 14.7 m/year. (Figure 12). A similar change was observed for other large glaciers of Georgia.

References

Brasnett, B. 1999. A global analysis of snow depth for numerical weather prediction, Journal of Applied Meteorology 38:726-740.

Brown, R., Derksen, C., and Wang, L. (2010), A multi-data set analysis of variability and change in Arctic spring snow cover extent, 1967–2008, J. Geophys. Res., 115, D16111, doi:10.1029/2010JD013975.

Brown, R. D., and P. W. Mote, 2009: The Response of Northern Hemisphere Snow Cover to a Changing Climate. J. Climate, 22, 2124–2145, https://doi.org/10.1175/2008JCLI2665.1.

Estilow, T. W., A.H. Young, and D.A. Robinson, 2015: A long-term Northern Hemisphere snow cover extent data record for climate studies and monitoring. Earth Syst. Sci. Data, 7, 137–142, doi:10.5194/essd-7-137-2015.

Reference: Gascoin, S., Monteiro, D., & Morin, S. (2022). Reanalysis-based contextualization of real-time snow cover monitoring from space. Environmental Research Letters, 17(11), 114044. https://doi.org/10.1088/1748-9326/ac9e6a.

Helfrich, S. R., D. McNamara, B. H. Ramsay, T. Baldwin, and T. Kasheta, 2007: Enhancements to, and forthcoming developments to the Interactive Multisensor Snow and Ice Mapping System (IMS), Hydrological Processes 21: 12, 1576-1586. doi:10.1002/hyp.6720.

Hersbach, H., B. Bell, P. Berrisford, et al.: The ERA5 Global Reanalysis, QJRMS, 146, 1999-2049,2020, https://doi.org/10.1002/qj.3803.

Luojus, K., Pulliainen, J., Takala, M., Lemmetyinen, J., Moisander, M., Mortimer, C., Derksen, C., Hiltunen, M., Smolander, T., Ikonen, J., Cohen, J., Veijola, K., and Venäläinen, P.: "GlobSnow v3.0 Northern Hemisphere snow water equivalent dataset". Scientific Data 8, 163 (2021). https://doi.org/10.1038/s41597-021-00939-2.

Acknowledgements

The section on Northern Hemisphere Mountain SWE is a contribution from the joint body on mountain snow cover of the International Association of Cryospheric Sciences. (https://jb-smsc.github.io/gmba-swe/)