Ross Brown (ECCC) and Patricia de Rosnay (ECMWF)
Snow cover onset: Anomalies of August 2018 - January 2019 snow cover duration (SCD) from the Canadian Meteorological Centre (CMC) operational snow depth analysis (from in situ observations) and the NOAA Interactive Multisensor Snow and Ice Mapping System (IMS24) daily snow cover analysis (from mainly satellite data) are in good agreement (Figure 1). The main features are: (1) earlier onset of snow cover over most of eastern North America, central Europe and the Tibetan Plateau, and (2) much later than normal snow cover onset over northern China, and slightly later than normal onset over a large region covering most of eastern Eurasia and western North America. The late start to the snow season was an initial concern for the California snowpack but this has since reached 100% from late January storms. Early winter snow cover was below normal in the European Alps causing a slow start to the ski season and disruption of World Cup ski events.
Winter snow accumulation: Anomalies of January mean snow depth from the CMC operational analysis are more or less consistent with the onset anomalies indicating above-average snow depths over regions with early onset (most of Canada and large areas of Europe) and below-average snow depth over northern China and western North America (Figure 2). Unusually cold and heavy snowfalls in early January across large regions of North America and Europe made headlines with several deaths attributed to heavy snowfall and avalanches in parts of Europe. Heavy snowfalls in 2019 over both continents have contributed to predominantly positive snow depth anomalies as of 19 February (Figure 3) with noticeably deeper than normal snow accumulations over Quebec and Labrador, and over parts of the Tibetan Plateau. Heavy snow and rain in Quebec have contributed to a number of roof collapses.
Hemispheric-averaged anomalies: The GCW snow anomaly tracking products from CMC and GlobSnow (Figures 4 and 5) show that 2018-2019 season snow cover extent has been close to normal but snow water equivalent (SWE) is running substantially above average. This follows the pattern of previous years which saw above-normal SWE followed by rapid spring melt. The higher SWE values obtained by CMC are partly related to masking out of mountain regions in the GlobSnow product.
Spring snow cover duration: Anomalies of January-June snow cover duration (SCD) from the CMC operational snow depth analysis (from in situ observations) and the IMS24 daily snow cover analysis (from mainly satellite data) are in good agreement (Figure 6). The main features are longer than average spring snow cover over central Asia and much of the USA and eastern Canada, and below-average snow cover over Korea and northern China. Most of Europe also had below-average spring snow cover in spite of unseasonably cold spring temperatures. The Arctic had near normal spring snow cover over most of the region with the exception of the Yukon and southeastern Alaska area where spring snow cover was 20-30 days below normal. At the hemispheric scale, June snow cover extent was approximately 1.5 standard deviations below the 1981-2010 reference period in the NOAA climate data record (CDR) product. NH spring (April, May, June) snow cover extent has decreased significantly (0.05 level of significance) over the period 1972-2019 at a rate of 0.865 million km2 per decade, or -5.1% per decade with respect to (as a percentage of) the 1981-2010 reference period average (Figure 7).
March snow accumulation: Anomalies of March mean snow depth (the month of maximum snow accumulation over the NH) from the CMC operational analysis are more or less consistent with the spring SCD anomalies indicating above-average snow depths over central USA and eastern Canada, and below-average snow depths over Europe, China, Korea and Japan (Figure 8). March snowpack average snow water equivalent (SWE) over southern Quebec was 75 mm above normal, the third highest March SWE in the past 57 years of the historical record. The above average spring snowpack combined with heavy spring rainfall contributed to extensive flooding across the Great Plains and in the Ottawa and Montreal regions.
Snow season history: The GCW NH snow anomaly tracking products from CMC and GlobSnow (Figures 9 and 10) show that 2018-2019 snow cover extent was close to normal but the peak SWE was substantially above-average, particularly over North America (see https://ccin.ca/ccw/snow/current for CMC tracker results for North America and Eurasia). North American SWE was also characterized by very rapid melt in March (Figure 11) that contributed to widespread flooding across the U.S. Midwest.
