National Weather Service:
- Cumulative Precipitation Threshold
- Rainfall Intensity/Duration Threshold
- Antecedent Wetness Index (AWI)
- Forecast Relative to Threshold
Data is updated every 60 minutes. Updates may be interrupted occasionally by instrument, computer, or network malfunctions.
Monitoring at this site is for researching rainfall thresholds for forecasting landslide potential. Shallow landslides are common on coastal bluffs overlooking Puget Sound. Hundreds of landslides occurred on the bluffs in late December 1996 and early January 1997 as heavy rainfall accompanied by abrupt warming caused rapid melting of recent heavy snow. Smaller numbers of landslides occur every few years during extended rainy periods and intense storms.
Cumulative Precipitation Threshold
The cumulative precipitation threshold for the occurrence of landslides in the Seattle area is based on an analysis of historical landslide and precipitation data from the period 1933 - 1997. The threshold was visually identified after plotting antecedent 3-day and prior 15-day cumulative precipitation amounts associated with landslides that were part of events with 3 or more landslides in a 3-day period. Between 3.5 and 5.2 inches total precipitation during any 18-day period (last 3 days plus the previous 15 days) are required to exceed the cumulative threshold. It is interpreted as a lower-bound threshold below which the specified level of landslide activity does not occur or occurs only rarely. Recent analysis has shown that the probability of landslides occurring on a day when the cumulative threshold has been exceeded is roughly 10%. See USGS Open-File Report 00-469, USGS Open-File Report 03-463, and USGS Open-File Report 06-1064 for details concerning the identification of the threshold and its performance.
Rainfall Intensity-Duration Threshold
The rainfall intensity-duration threshold for the occurrence of landslides in the Seattle area is also based on an analysis of historical landslide and precipitation data from the period 1933 - 1997. This threshold was developed to identify conditions when abundant landslides are likely. Recent analysis has shown that the probability of landslides occurring on a day when the intensity-duration threshold has been exceeded is between 30% and 70%, depending on antecedent soil wetness and size of the area affected by threshold exceeding rainfall. See USGS Open-File Report 06-1064 for details concerning the identification of the threshold and its performance. USGS Fact Sheet 2007-3005 also has more information about the rainfall thresholds and their application to forecasting landslides.