Monday, February 23, 2015

Climatology of 2015 Iditarod Route

Due to poor snow conditions in the southern mainland, the Iditarod race start was moved from Willow to Fairbanks. This is the second time in history (1973-2014) that the race start was moved due to low snow in the southern portion of the route. The other year that the race start was moved to Fairbanks was in 2003. The official restart will occur on Monday, March 9th. Approximately half way through the race, the 2015 route will join with the traditional northern route at the Koyukon village of Ruby. A map of the 2015 route is shown in Figure 1.


Figure 1. Iditarod route in 2015. Populated places along the route are shown as yellow circles.

Analysis Period

What does climatology tell us about the expected conditions during this year's Iditarod? To answer that question, we need to select an analysis period. In the last 20 years, the winning time for the Iditarod is 9 days, 7 hours, and 28 minutes. The top tier of mushers usually finish in about 10 days. Therefore, I chose a time period of 10 days for my analysis. Since the start date this year is on March 9th, the time period for climatological analysis is March 9th to March 18th. This brings up an important point. The analysis in this post is not a climate analysis of past Iditarods. Previous races usually began on a Sunday and the calendar date differs from year to year. This analysis is looking at the same time period over a number of years to assess what the range of temperature and snow conditions have been like along the route used in 2015.

Stations

There are eleven stations along the 2015 route in the Global Historical Climatology Network (GHCN) database. In this analysis I have restricted the date range to the 1973-2014 time period. This corresponds exactly to the years in which the Iditarod race has occurred. That gives us 42 possible years of data. Some stations, like Fairbanks and Nome, have outstanding data for all 42 years. Other stations have incomplete or interrupted data. A map of the climate stations used in this analysis is shown in Figure 2.



Figure 2. Climate stations along the 2015 Iditarod route.

Temperatures

When assessing the temperatures on a yearly basis, I used an average of all stations for all 10 days of the analysis period. This yields a single temperature value for each year. Since the stations are fairly well distributed geographically, arithmetic averaging is sufficient.

As you can see in Figure 3, a wide range of temperatures have been observed since 1973. Keep in mind that these are average daily temperatures (high plus low divided by two). Four years have seen an average temperature of 20°F or warmer and seven years have seen an average temperature below 0°F. Interestingly, the years since 2000 have been, on average, a little cooler than the years in the 1970's and 1980's. Based on climatology, it is reasonable to expect temperatures to average between +10°F and -10°F.

A note of caution, do not over interpret the trend in Figure 3. Remember that this year's Yukon Quest happened to take place during a brutally cold week during one of the warmest winters on record.


Figure 3. Average temperature for the March 9 to March 18 time period for the 11 stations along the 2015 route shown in Figure 2.

Now that we have seen the annual temperature chart, let's look and see how the warmest and coldest years varied on a day-to-day basis during the March 9 to March 18 time period. Figure 4 shows the three warmest and three coldest years.


Figure 3. Average temperature for the March 9 to March 18 time period for the three warmest and the three coldest years along the 2015 route shown in Figure 2.

Even in the very warmest years, only the last day or two was above freezing – even then only by a couple of degrees. The coldest years were quite cold. While not even close to this year's Yukon Quest, temperatures have average as low as -25°F (1995) with average low temperatures near -40°C/F. Keep in mind that temperatures on the river ice can be 10°F colder than at the climate stations.

On a broader scale, we can look at temperatures on a statewide basis using the ESRL Reanalysis tool. I put together a video of temperatures from March 9 to March 18 for each year between 1948 and 2014. Video 1 shows the time lapse of the temperature departures from normal from year to year.



Video 1. Temperature departure from normal from 1948 to 2014 during the March 9 to March 18 time period. The 1981-2010 climate normals are used as the temperature baseline.

Snow

What does a musher really care about? Snow, of course. How has the snow been during the last 42 years along the 2015 route? The short answer is that it has been consistently good. Figure 3 shows the average snow depth and average new snow for all stations along the 2015 route during the 1973-2014 time period.


Figure 4. Average new snow and snow depth for the March 9 to March 18 time period between 1973 and 2014 along the 2015 route shown in Figure 2.

The average snow depth was 20.8" and the lowest was still 9" (1986). Six years have seen over 30" of snow on average with a peak of 42" in 2009. As for new snow, most years have recorded 2"-5" of new snow during the 10-day period – average is 2.4".

Conclusion

While there are no guarantees in life, history tells us that the route chosen for the 2015 Iditarod is consistently cold and consistently snowy. Mush on!

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