The Distinguishing Characteristics of Storms that Produce Rain-on-Snow Events in Alaska

A growing body of research has shown that rain-on-snow (ROS) events can have adverse impacts on high-latitude ungulate populations. When rain freezes in the snowpack, it forms ice that blocks access to winter forage and, in extreme cases, contributes to mass die-offs. Research into the atmospheric conditions that cause ROS have shown a relation to anomalously southerly winds and warm, moist air associated with the passage of extratropical cyclones. However, these conditions are common for many winter precipitation events, not just ROS. This study uses the MERRA2 atmospheric reanalysis to differentiate ROS events in Alaska from precipitation events in which only snow falls on a preexisting snowpack (snow-on-snow, or SOS). Although ROS-producing storms are no stronger than SOS-producing storms, for much of Alaska (the interior, southwest, and south central), the tracks of ROS-producing storms tend to be farther north and west than SOS-producing storms. Sea-level pressure in the eastern Gulf of Alaska is significantly higher during ROS than SOS, as well as 72 hours prior to storm events. This suggests that blocking highs in the Gulf of Alaska may help encourage rainfall (and therefore rain-on-snow events) by: 1) diverting storms to the northwest, which places more of Alaska under the influence of southerly flow and 2) strengthening that southerly flow by increasing the pressure gradients on the southeast side of storms. Both mechanisms might contribute to the significantly higher 2-m air temperature and total column water vapor observed for ROS versus SOS. The role of blocking highs may also provide a source of predictability for ROS events in parts of Alaska.