Heavy Rain, Flooding, and Debris Flows at Mount Rainier Park

On Monday, I was contacted by Scott Beason, Park Geologist at Mount Rainier, who was looking for precipitation data.

Why?  

Because of a serious debris flow down "Dry Creek" on the southwest side of Rainier, that closed the Westside  Road on Saturday evening.  Look at the pictures of the aftermath (below).

Just to orient you,  in the image below, the area of interest is shown by the red circle and is just east of Mount Wow.

The debris flow, driven by very heavy rain, was even evident on local seismic data (see below).   Note that the slide action started around 0515 UTC Sept 7 (10:15 PM PDT Saturday, Sept 6).  

It turns out this event was caused by VERY heavy rain on Saturday evening associated with potent thunderstorms.

The regional radars showed substantial thunderstorm/heavy rain activity moving across the Rainier area (an image around 8 PM is shown below)

Looking closer at the weather radar at  9:15 and 10:30 PM below, you can see some heavy rain over the "dry creek" area.  Remember, weather radar can provide a measure of precipitation rate.

 

What were the rainfall totals in the debris flow areas?  There were no nearby observation stations.

We can add up the radar precipitation rates over time to secure the answer (see below).  By 10:28 PM, some locations in the area received as much as 2 inches, at least according to the radar.

So we know what produced the heavy rain that led to the debris flow:  potent thunderstorms.   

Did the forecast models predict them?  

To help answer this question, below is the rainfall total covering the period from the high-resolution UW forecast model for the 24-hour period ending 5 AM on Sunday.  The circle shows the area in question.

Moderate rain was predicted, but clearly the precipitation was underpredicted.  

Getting thunderstorm precipitation right is very, very hard, and accurate local forecasts of such localized heavy rain may never be as skillful as we would like.