Natural Hazards of Spring Opening of Going to the Sun Road


Background | Comparisons | Predicted Opening | Influences on Opening | Resources 
Plowing the Big Drift, GTTS RoadBackground
The Going-to-the-Sun (GTTS) Road in Glacier National Park, Montana is one of the park's premier attractions and the most heavily used facility.  The road traverses Glacier, from the west entrance in West Glacier to the east entrance in St. Mary.  The narrow, 52-mile road crosses the Continental Divide at Logan Pass, at an elevation of 6880 feet.  The Logan Pass section of the GTTS Road is closed every fall due to inclement weather and heavy snowfall.  The scenic, steep, glacially carved terrain the GTTS Road crosses also exposes considerable avalanche risk.  Spring opening of this and other roads throughout the park begin in earnest in April, and are widely anticipated by park employees and the public.

Every spring, park staff work safely to open the Logan Pass section of the GTTS Road in timely manner.  Few consistent records have been kept on snow conditions prior to, and during, snow removal operations with the exception of the "Big Drift" that has been measured as being over 98 feet high.  The most complete snow data set is from Flattop Mountain SNOTEL station, approximately 15 km from Logan Pass. SWE standard deviation and snowpack accumulation-melt cycleAs seen in the figure at left, the Logan Pass section typically opens as Snow Water Equivalent (SWE) is decreasing - while Flattop Mountain and the GTTS Road are melting out.  Also seen in this graph is that variability of SWE from one year to the next (standard deviation between years) reaches its peak shortly after the typical GTTS Road opening - meaning that Logan Pass typically opens just when conditions from year to year are most variable, making planning difficult.  A GTTS Road opening date earlier or later than average results in opening efforts occurring at a different location in the snowpack accumulation and melt cycle, bringing with it a unique suite of safety concerns.  Safety concerns focus on avalanche danger, snow instability, and inclement weather along with rock slides, slope failures (landslides), and surface ice formation - all are integrally connected with the snowpack accumulation and melt cycle.  Melt/freeze cycles for example - snow or ice melting, refreezing, and expanding - occur almost daily from the time of peak SWE (typically mid-April) until the snowpack is melted (typically mid-July) and influence rock fall along the GTTS Road.

Opening Comparisons
Opening day SWE vs Opening Day for the GTTS RoadCorrelation between Flattop Mountain SWE on Logan Pass opening day and the timing of opening day (seen in the graph at right) indicates that detailed investigation of the relationship between snowpack characteristics and road opening may assist Glacier National Park managers in identifying influential and hazardous conditions.  Late Logan Pass-GTTS Road openings often coincide with deeper snowpacks at the Flattop Mountain SNOTEL station.  This relationship is only weakly correlated however, due to the obvious tendency of the snowpack to melt more with later openings.  Also, snowpack at Flattop Mountain may not accurately represent snowpack conditions at Logan Pass.

A statistical correlation also exists between Flattop Mountain SWE on the 1st Monday in April (approximately the first day of plowing) and the Logan Pass opening of the GTTS Road.  In spring 2002, Tuesday April 2 was the 1st day of plowing on the GTTS Road.  During the years of 1970-1999, Flattop Mountain SWE was associated (correlated) with 34% of the variation in Logan Pass opening dates.  Each 1" increase in SWE above 30" was associated with the road opening an average of 0.57 days after the 13th of May.  For example, if SWE on the 1st Monday in April was 49", the typical opening date was 9 June (julian day 160.8).  Whereas if SWE was 50" on the 1st Monday in April, the typical opening date was 10 June (julian day 161.4).  Statistical Details:  opening date = 132.9 + 0.570* SWE" on 1st Monday in April, p-value=0.01, F1,28-test.  Date expressed as julian day (day of year), 95% confidence interval for the SWE multiplier is 0.265 to 0.875.  These relationships are valid only for 1st Monday in April SWE values between 30" & 70".

Predicted Opening
By extrapolating from the relationship established above, the typical Logan Pass opening date for the current year (2002) can be estimated from Flattop Mountain SNOTEL SWE on the 1st Monday in April (1 April 2002). Spring 2001 GTTS Road predicted opening date using Flattop Mountain SNOTEL data The winter of 2001-2002 has been relatively wet and cold, with the Flattop Mountain SNOTEL station recording 51.7 inches of SWE (114% of average) and a snow depth of 153 inches on 1 April.  Temperatures were unseasonably cold - the station recorded a low of -12 F (-23 C) - as they had been through most of March.

The typical opening date can be identified using the following formula, where the date is expressed as julian day (numbered day of year): Opening date = 132.9 + 0.57* 51.7" SWE.  Just as important as this single typical opening date of 11 June (julian day 162) is the range of variability in this calculated opening date - computed in the table at left as approximately ±?? days using a 95% confidence interval.  The graph at left can also be used to identify the typical road opening date:  with SWE for the 1st Monday in April along the vertical axis (51.7" on 1 April 2002) and the intersection with the three red lines indicating the lower confidence interval, typical opening date, and upper confidence interval along the horizontal axis.  Yet with estimates of $1.1 million added to the regional economy when the entire GTTS Road is open, such a wide range in the estimated opening date is of limited use.  For comparison, the earliest opening of the GTTS Road is 16 May and the latest opening is 23 June, with an average opening date of 7 June (for the 30-year period 1970-1999).  Actual and calculated opening dates using this technique are shown for 1998, 1999, 2000 and 2001 at left.  Our estimates for typical Logan Pass opening dates are calibrated using previous years SNOTEL data, which ranged from 30"to 70" SWE.

Influences on Opening
Plowing crews are limited in their ability to work when fog, heavy rain, or snow obscure visibility and prevent avalanche spotters from having a clear view into avalanche start zones.  A much more accurate estimate of the association between Flattop Mountain snowpack, weather conditions, and Logan Pass-GTTS Road opening incorporates SWE on the 1st Monday in April as well as the frequency of days with precipitation during plowing (a proxy variable for visibility and avalanche danger).  These two variables in a linear regression model were associated with 69% of the variation in opening dates between 1979-1999.  Using this model, each 1" increase in SWE on the 1st Monday in April (above 30") was associated with the Logan Pass-GTTS Road opening an average of 0.8 days later and a 10% increase in the frequency of precipitation was associated with the Logan Pass-GTTS Road opening an average of 6.1 days later (from 2 April).  This statistical correlation is interpreted as demonstrating the significance of spring weather on the opening date of the Logan Pass-GTTS Road: while snowpack is influential, years with frequent moist spring weather are typically those when the Logan Pass-GTTS Road opens latest.  The most recent forecast from the NOAA Climate Prediction Center is for average temperatures and slightly above average precipitation for the second half of April 2002.  Statistical Details:  opening date = 92.46 + 6.10*frequency of precipitation during plowing + 0.80*SWE" on 1st Monday in April, p-value<0.001, F2,18-test. Date expressed as julian day (day of year), 95% confidence interval for SWE multiplier is 0.48 to 1.12, & 95% confidence interval for frequency of precipitation multiplier is 2.40 to 9.79.

Every year presents unique challenges in opening the GTTS Road that are impossible to predict.  Snowpack, weather, park management concerns, maintenance needs, and road crew safety are a few of the numerous issues that influence this spring opening.  The snow conditions presented here are from Flattop Mountain, along the Continental Divide; they are quite different from conditions in nearby warmer, drier towns such as Kalispell or Browning.  Please see http://www.nrmsc.usgs.gov/research/ftm_snow.htm for updates on snowpack and weather conditions at the Flattop Mountain SNOTEL Station.

Additional data analysis is providing researchers with a more detailed picture of snow density conditions along the GTTS Road corridor.  Also correlated with road crew plowing progress are changes in snow density, as winter progresses to spring and rain and snow events alter snow density.  This study of snow density has been greatly facilitated by the addition of a snow depth sensor at Flattop Mountain.  With both field observation and spatial modeling efforts ongoing, we intend to further refine our understanding of local snowpack dynamics to provide assistance for the safe opening of the Logan Pass section of the Going-to-the-Sun Road.

Resources
For updates on the status of the Going-to-the-Sun Road, visit Glacier National Park's Website Visitor Center.

For more information on avalanches along Going-to-the-Sun Road, go to the CCME web page: Going-to-the-Sun Road Avalanche Forcasting Program.

For more information on Flattop Mountain SNOTEL data, go to Glacier Field Station web page:  Flattop Mountain SNOTEL Snowpack.

For current and historic weather/snow data from Flattop Mountain SNOTEL.

Fagre, D.B. and Klasner, F.K.  2001.  Application of snow models to snow removal operations on the Going-to-the-Sun Road, Glacier National Park.  In:  Proceedings of International Snow Science Workshop 2000.  pp. 266-272.