Discussion: The late April 2023 snow cover across the northern Great Plains and Upper Midwest (Fig. 1) is rare but not unprecedented. Since 1998 (when satellite snow cover is easily attainable) late April significant snow cover across the northern Great Plains and Upper Midwest occurred in 2013, 2017, and 2022. The aerial coverage of snow cover in 2013 was farther south than 2023 (Fig. 2). The complete snow cover dissipation date was May 7 (Fig. 3).
In 2017, significant snow cover was observed over the far northern Great Plains and Upper Midwest at a lower aerial coverage than 2023 (Fig. 4). The complete snow cover dissipation date was April 29 (Fig. 5). Finally, last year produced late season snow cover most concentrated on the northern and western Dakotas (Fig. 6). The snow cover dissipation date was May 4 (Fig. 7).
During each analog year, ENSO was in a weak to strong La Nina climate based on multi-variate ENSO index (MEI). The analog years averaged a strong upper trough across central and western Canada characteristic of negative West Pacific oscillation (-WPO). In 2023, the upper trough is biased farther to the west, characteristic of negative Pacific North America (-PNA) index.
Based on the analogs, the average snow cover dissipation date in May 3-4. Current operational forecasts remain chilly through early next week with a risk of some additional snowfall. After early May, melting conditions are generated although complete snow cover dissipation is likely just-after the May 3-4 analog dates around May 5-7 with northeast Minnesota last to clear.
Fig. 1: Current snow cover/snow depth across the U.S.
Fig. 2-3: U.S. snow cover on April 24, 2013 and following northern Plains/Upper Midwest snow cover dissipation date.
Fig. 4-5: U.S. snow cover on April 24, 2017, and following northern Plains/Upper Midwest snow cover dissipation date.
Fig. 6-7: U.S. snow cover on April 24, 2022, and following northern Plains/Upper Midwest snow cover dissipation date.