Another Landslide

The news of a major landslide yesterday in Ketchikan elicited groans of "not again" from many people around Alaska, I'm sure.  Landslides have caused loss of life and major damage on several occasions in recent years in Southeast Alaska, including last year's terrible event in Wrangell, a Juneau slide in 2022, and Haines in 2020.

https://alaskapublic.org/2024/08/26/1-person-killed-in-ketchikan-landslide-multiple-homes-destroyed/

It's clear that the latest event was tied to Sunday's heavy rain, although the rain amounts don't seem to have been at all unusual for rain-soaked Ketchikan.  The airport reported a little over 2.5 inches of rain, and this pales in comparison to many much greater rainstorms of the past, including more than 8" in two days in August 2017, and nearly 11" in two days in October 2015.  A more substantial 5" was measured near downtown yesterday, but again it doesn't seem particularly unusual.

What is unusual is that this was the first major rain after a very dry summer.  Prior to this storm, only about 6.5" of rain had fallen since early June, which is far below the normal of over 20" for that period.  The plot below shows that precipitation deficits were developing in late winter and spring as well (note that the latest day's rainfall is not included on the plot):

While concern has been expressed that ocean and atmosphere warming trends are driving an increase in excessively heavy rain events, it seems unlikely that this latest event can be explained so simply, and I haven't seen any quantification of how much the changing climate is contributing to increased risk.  It does seem possible that the last decade's outbreak of slides is just random chance in what has always been a landslide-prone region; but of course the more events we see in a short period (relative to normal), the less plausible "coincidence" becomes as an explanation.

Wild Weather in the South

Weather is never very far from the news in a place as big as Alaska, and the more extreme events aren't usually good news.  Yesterday a major Gulf of Alaska storm barreled into a cold air mass and high pressure over southern Alaska, and the results were dramatic: damaging winds in the Matanuska Valley, and a major landslide down south in Wrangell.  Sadly the landslide brought loss of life, and perhaps worse than the 2020 Haines landslide.

ADN news links:

https://www.adn.com/alaska-news/mat-su/2023/11/21/most-power-restored-and-roads-cleared-following-mat-su-windstorm/

https://www.adn.com/alaska-news/2023/11/21/large-landslide-south-of-wrangell-closes-highway-draws-search-and-rescue-team/

The rain in the Southeast doesn't appear to have been particularly unusual for the region; the map below shows 72-hour totals (click to enlarge).  But it was pretty wet earlier in the autumn, so there may have been a compounding risk.  Sitka is in the top 10 for rainfall since September 1.

As for the winds around Palmer and Wasilla, the map below shows how localized the wind storm was in the greater Anchorage area (showing yesterday's peak gusts in mph).

This looks like a great example of channeled downsloping flow from the east, driven by a strong pressure gradient.  The map below shows the sea-level pressure analysis at 3pm yesterday, courtesy of Environment Canada.  Check out that pressure gradient over the southeastern coast of Alaska; high winds were seen throughout the region.