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Abstract Extreme floods and landslides in high‐latitude watersheds have been associated with rain‐on‐snow (ROS) events. Yet, the risks of changing precipitation phases on a declining snowpack under a warming climate remain unclear. Normalizing the total annual duration of ROS with that of the seasonal snowpack, the ERA5 data (1941–2023) show that the frequency of high‐runoff ROS events is a characteristic feature of high‐latitude coastal zones, particularly over the coasts of south‐central Alaska and southern Newfoundland. Total rainfall accumulation per seasonal snowpack duration has increased across western mountain ranges, with the Olympic Mountains experiencing more than 40 mm of additional rainfall over the snowpack in the past eight decades, followed by the Sierra Nevada. These trends could drive an 8% increase in rainfall extremes (e.g., more than 10 mm for 6 hr storm with a 15‐year return period), highlighting the need for resilient flood control systems in high‐latitude coastal cities.
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Synoptic Climatology of Rain-on-Snow Events in Alaska
Rain-on-snow (ROS) events can have adverse impacts on high-latitude ungulate populations when rain freezes in the snowpack, forming ice layers that block access to winter forage. In extreme cases, ROS events have led to mass die-offs. ROS events are linked to advection of warm and moist air, associated with extratropical cyclones. However, these conditions are common to many winter precipitation events, challenging our understanding of the particular conditions under which ROS events occur. This study uses the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) to differentiate ROS events in Alaska from precipitation events in which only snow falls on a preexisting snowpack [snow-on-snow (SOS)]. Over the North Slope and Kotzebue Sound, no clear difference exists between the tracks of ROS-producing and SOS-producing storms. However, in the interior, southwest, and Anchorage, tracks of ROS-producing storms tend to be farther north and west than for SOS-producing storms. The northwest shift of ROS-producing storms is linked to the position of upper-tropospheric anticyclones in the eastern Gulf of Alaska during ROS events. ROS-producing storms are no more intense than SOS-producing storms, but their association with atmospheric blocking leads to stronger pressure gradients on the east side of storms and thereby stronger advection of positive anomalies in temperature and precipitable water. For several sites, sea level pressure in the eastern Gulf of Alaska is also significantly higher a few days prior to ROS events than prior to SOS events, further implicating atmospheric blocking as a facilitator and potential predictor of ROS events.
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- Award ID(s):
- 1928230
- PAR ID:
- 10137677
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Monthly Weather Review
- Volume:
- 148
- Issue:
- 3
- ISSN:
- 0027-0644
- Page Range / eLocation ID:
- p. 1275-1295
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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