Tioga Pass & Glacier Point Roads Opening Dates

Post date: Apr 19, 2012 3:0:32 AM

Since the plowing on Tioga Pass and Glacier Point Roads was scheduled to begin this week, I thought I'd post a blog about expected opening dates. There are many sources of information online to use to make an educated guess on the date that the Tioga Pass Road and Glacier Point Road will open in Yosemite National Park. Probably the most important information is the dates that these roads opened in the past. There are two sites that I've found with this information-the National Park Service page and the Mono Basin Research page. The National Park Service page shows the Tioga Pass opening and closing dates since 1980 and the Glacier Point opening and closing dates since 1995. The Mono Basin Research page shows the Tioga Pass opening and closing dates since 1933. Now, what information is out there that would allow someone to predict the opening date for a particular road in a particular year? The most important factor is amount or depth of snow followed by temperature. The two sites that I use that report snow amounts/depths are the Calfornia Data Exchange Center (CDEC) and the Western Regional Climate Center (WRCC). For the purpose of predicting the opening date for the two Yosemite roads, I will only use the CDEC website here because the closest WRCC site is along the Virginia Lakes Ridge south of Bridgeport, CA. The two CDEC data sites that I will look at are Tenaya Lake (TNY) and Dana Meadows (DAN) because these report real-time snow water equivalence (SWE) and they are located very near the Tioga Pass Road. Unfortunately, there are no real-time snow water equivalence data available near Glacier Point. To get to either of these Tioga Pass Road sites (other than the links provided), click on the CDEC link provided above, click on "Station Locator" on the left-side bar, click near the "25" (north of Fresno) on the "Northern California" map (left one), click on the Tioga Pass Road in the upper-center portion of the map (within the green shaded area), and click on the station button (red). If you follow these instructions, TNY should be on the far right side of the map. To get to the DAN button, click "EAST" and it will be on the right side of the map. On the station page will be a table describing the location of the station and below this a table describing the data available. For both of these sites I used the "SNOW, WATER CONTENT(REVISED)" data.

I downloaded the data for both of the sites (back to 1998 at TNY and 1980 at DAN) and the opening dates for the two roads and put them into an Excel Spreadsheet. I looked at the relationships of Peak SWE, April 1 SWE, and April 15 SWE with the opening dates. April 1 corresponds to the last snow survey that California performs during the water year (October 1 to September 30). I included April 15 because in some years the snow has not yet started to melt by April 1 (peak SWE can occur after April 1). After comparing the sites, the TNY data showed better correlation to the Tioga Pass opening dates than the DAN data. The correlations grew steadily stronger from using Peak SWE to the April 1 SWE to the April 15 SWE. Incredibly, the linear R^2 value (metric of how well the data fits a curve, 1 is perfect, 0 is no correlation) for April 15 SWE vs opening date was 0.81! Many scientific papers get published reporting data with R^2's less than 0.5. Therefore, the April 15 SWE at the TNY station is a very good indicator of when the Tioga Pass Road will open. The increase in correlation is most likely due to the fact that you move forward in time from Peak SWE to April 15 (Peak SWE is usually before April 1). Hence, at April 15th the opening date is closer than it is at the date of Peak SWE (in most cases). As the opening date approaches, less factors affect the prediction. Unfortunately, the correlations to the Glacier Point opening dates were not as good. However, based on past observations of the period between the opening of Glacier Point and Tioga Pass for years such as this one (small snowpack), Glacier Point usually opens about a week before Tioga Pass (usually opens more than a week before Tioga Pass in a larger snow year).