Incident Report 3.10.25 P - Part 2

Local Wind/Gust Plots

Unfortunately, there is no publicly available recorded wind plot. The nearest accessible plot is located in a valley just south of the resort. In back-testing with weather models there is a correlation the resort and this lower station. Interpolation of speeds would be higher at the resort. Two days before the incident, gust speed and timing run somewhat parallel to resort forecasted wind. At the time of the incident (at this station) notice, gusts no longer parallel steady-state wind and extreme direction changes at the incident time.

-Yellow bar is incident time-

Forecast Model Available

Using the HRRR - High Resolution Rapid Refresh forecast model covering 2.56 x 2.56 km gridded land size. 

Wind is referenced as: Gusts @ 33’, Wind Speed @ 33’ AGL, 260’ AGL, 9900’el. The resort covers 4 grids. 

In 18 hours the resort could utilize 1,848 HRRR wind forecasts.

Local Wind/Gust Plots   

Due to digital mapping limitations in mountainous terrain, the speed must be interpolated. For that reason, the simple HRRR based National Weather Service three times daily forecast must be adjusted – it will not be accurate unless this is done.

HRRR Grid with Ropeway Location

Chart shows: grids, elevations, wind forecast levels, resort elevations (green line) and Triple chairlift elevations. An interpolation shows gust speed two grids away could be more representative in this terrain. This interpolation is very common in mountainous terrain that shoots above the model’s average elevation. Notice the ropeway covers 2 grids, also these are well above grid forecast elevation.

Local Gust Effects

In gust evaluation three atmospheric levels should be considered. At higher speeds, lower level terrain influence becomes less significant. 

HRRR Gust Forecasts Surrounding Incident Time

The image below shows the HRRR Grid “blocks” with gusts colors and speeds without interpolation. The forecast was produced around midnight. The forecast is for 8AM. Resort is circled.

HRRR Gust Forecast 12 Noon 

This forecast was produced at 11 AM for 12 Noon. A faint topo map is underlaid, as seen elevation interpolation/correction would be needed – see above. The top of the ropeway is circled. Speed forecasted without interpolation is around 55 mph.  

Ensemble Gust Forecasts 

With any forecast it’s important to examine if this was a “quick ramp-up” event, impossible to forecast or if there is model error. The general rule for accuracy is: same grid same speed in 3 consecutive hours equals high accuracy. The forecast chart below is an “ensemble” of 10 slightly different models called HREF High Resolution Ensemble Forecast. This model has been available since 2017. The forecast references every hour, 48 hours before the incident. HREF also must be interpolated. The gust chart was produced 60 hours before the incident. Speeds are +70 mph.

HREF 56 Hours Before Incident

Below is the HRF 56 hours before the morning of the incident, same “ramp-up” of wind with a little larger portion above 70 mph.

HREF - 39

Moving into a window of better accuracy, this chart was produced 39 hours before the 9AM opening showing speeds at 75 mph. 

HREF -27 

27 hours before the 9AM opening, speeds shown at 60 mph.

HREF -3   

3 hours before the 9AM opening, speeds at 60 with some at 70 mph.

 Chart Option - Simple Line Graph

Those with challenges of color differentiation, the chart below covers the same HRRR model and produces a line graph of forecasts. Within that chart is the: model, elevation, time and 6 different wind forecasts. The sample chart below shows gusts in circles and is produced by SPOTWX.com. 

SPOTWX - Grid Locator 

Grid locator is over the resort, different models with grid sizes are shown. 

Wind Force vs Wind Speed

Wind Force is a much better example than speed for reactions with ropeways. Strictly speaking; gusts produce the wind force that can cause a derailment. Wind force and speed do not parallel each other. Looking at speed gauge can give you a false sense of security. 70mph=13lb/sqft     35mph=3lb/sqft 

Weather Summary 

48 hours before the incident: the gust arrival, duration and speed were characteristic of the forecast and did not significantly deviate in those 48 hours. 

The Structure of the Incident 

• Based on tower adjustability, sheave stem clearance and the ropeway being load tested numerous times it is very likely there is nothing inherently wrong with the ropeway. 
• The ropeway operated in a forecasted wind speed exceeding its design. 
• It should be noted that the derailment was not the root of the incident it was one of many results.

 Potential Factors to the Incident 

• The higher risk was not realized of operating a ropeway in a significant forecasted wind event.
• An assumption that the NWS forecast applied to the resort is accurate.
• A grid terrain and elevation correction to the weather model may not have been applied.
• In 2011 this resort had a very similar incident. Insufficient corrective action normalized that event within the resort and with the industry. The persons involved in the incident described the wind coming in strong bursts…
• Ropeway partners have not given enough attention to causal factors surrounding wind incidents. That lack of attention has been normalized.
• Ropeway partners have not addressed forecasting skills in any reference handbook, standards, codes, or formal training. This has been normalized.
• Ropeway partners have been lulled into a false sense of security with rope catcher performance. That high confidence has been normalized.
• Misapplication of weather apps found on phones.
• Misapplication of using wind speed gauges to forecast gusts.
• Application of the wrong weather model.
• The resort’s experienced technician may not have been available.
• The resort may have been lulled into a false sense of security with updated rope catchers.
• The resort’s personal may have formal risk training but it was not applied.
• The resort’s personal may have formal forecast training but it was not applied.
• Assumption that a “clear sky” indicates lower wind speed and less gusts.
• Assumption that a ropeway (from stop switch activation to full stop) stop time will be quicker than the reactions from a gust.
• Assumption that wind speed mirrors wind pressure.
• Riding a ropeway in high wind can be very disconcerting. Consideration must also be given to rider comfort. 

  Wind Operation 

When operating in wind, the supervisor must take into consideration types of wind and its forces. There is intrenched old-timey thinking that you can reliably forecast gusts by steady-state wind speed. That thinking is not true. This is just one of the few reasons forecasters issue separate gust forecasts from steady-state wind. Gusts do not always mirror steady-state wind. Since predicting gusts from steady-state wind cannot-be-done, a forecast must be used. Past incidents have also shown gusts can force a rope from a sheave assembly while not running and gusts have forced riders from carriers.