How much warning?
It depends on how big the lahar is, where on the mountain it starts, and which valley it takes, so no single number is right. These are the figures the USGS has published, with their sources.
Inside the park
For a lahar to reach residential areas inside Mount Rainier National Park, such as Longmire (USGS).
Once a lahar is detected
The warning the detection system gives most communities to move to safety (USGS, 2020).
A worst case, simulated
In 2022 the USGS published computer simulations (the D-Claw model) of the largest likely landslide-triggered lahars from the weak, altered rock of Rainier's west flank: 260 million cubic metres, about the size of the Electron Mudflow of roughly 500 years ago, behaving as mobile flows. Two starting points gave these arrival times (George, Iverson and Cannon, 2022, USGS Open-File Report 2021-1118):
From the Tahoma Glacier headwall Nisqually
to Ashford, and about 50 minutes to the head of Alder Lake.
From the Sunset Amphitheater Puyallup
to the Orting lowlands, arriving as a flow front about 4 metres deep moving about 4 m/s (9 mph): slow enough to outrun on foot only if you have already started.
Hazard zones: Hoblitt and others, 1998, USGS Open-File Report 98-428. Red: small lahars, more often than every 100 years. Orange: moderate, every 100–500 years. Yellow: large, every 500–1,000 years.
From detection to sirens
A lahar starts
From an eruption, or with no warning at all from a landslide of weakened rock, as the Electron Mudflow did. It can also start from heavy rain or a glacial outburst on a smaller scale.
The valley stations hear it
Seismometers and infrasound sensors along the Puyallup, Carbon, White and Nisqually pick up the flow. Data from each site reach the 24-hour emergency operations centres at Washington Emergency Management and South Sound 911 within 10 seconds (USGS).
The alert goes out
All Hazard Alert Broadcast sirens in the towns from Orting to the Port of Tacoma, the Emergency Alert System, television, radio and NOAA Weather Radio (USGS).
People walk to high ground
Not drive: roads jam. Schools in the Puyallup valley practise it every year in what the USGS calls the world's largest lahar evacuation drill. Know your route: Pierce County.
Lahars Rainier has already sent
At least nine large lahars in the last 5,600 years (USGS). The towns of the Puyallup, White and Carbon valleys are built on their deposits.
| Lahar | When | Size | What started it | Where it went |
|---|---|---|---|---|
| Osceola Mudflow | 5,600 years ago | 2–3 km³ | Avalanches of hydrothermally altered rock from the summit and northeast slope, during eruptions | Down the White River to Puget Sound near Auburn, burying about 550 km² |
| Round Pass Mudflow | 2,500–2,600 years ago | — | Collapse of the Sunset Amphitheater and Tahoma Glacier headwall | Down the Puyallup, and the Nisqually via Tahoma Creek |
| National Lahar | 2,200–2,300 years ago | “Among the largest” | An explosive eruption | Down the Nisqually at least 100 km |
| Electron Mudflow | About 500 years ago | — | A landslide of altered rock, no eruption | Down the Puyallup about 100 km to the Puget Sound lowland; Orting stands on it |
Source: USGS, Significant lahars at Mount Rainier. The USGS gives no volume for the Round Pass and Electron flows on that page.