As you requested, we have investigated the weather conditions at 1521 Stevens Road, Pierre, South Dakota on and prior to October 25, 2010, with particular attention to the depth and weight of accumulated snow and ice. The results of our investigation are presented in the following paragraphs.
Our research shows that in Pierre, daily high temperatures ranged from 63 to 72 degrees on October 21, 22 and 23, 2010. The ground was free of any snow and ice on the 23rd and the temperature of the soil was well above the freezing point (32 degrees Fahrenheit). A couple of rain showers moved through the area during the evening of the 23rd.
The weather in Pierre turned much colder on October 24, 2010 as an early season Arctic air mass arrived from the north. At the same time, a slow-moving storm was intensifying over the southern Rockies.
During the predawn hours of the 24th, the sky over Pierre was cloudy and the temperature ranged from 35 to 39 degrees. The weather in Pierre remained cloudy at the start of the daylight hours and steady snow began to fall between 9:00 AM and 10:00 AM. At first, the snow melted as it fell. However, as the morning progressed, the intensity of snowfall increased and the temperature fell to just below 32 degrees. The snow began to whiten the ground between 10:00 AM and 11:00 AM. Between sunrise and 12:00 Noon, the wind blew from the north at an average speed of 10 to 16 MPH.
During the afternoon of October 24, 2010, as the intensity of the snowfall continued to increase and the temperature fell from 31 to the middle 20s, the depth of snow on the ground increased steadily. At 6:00 PM on the 24th, undisturbed ground surfaces were covered with an average of 3 to 4 inches of snow weighing an average of 1.6 to 2.1 pounds per square foot. There was a steady strengthening of the wind, blowing from the north, as the afternoon progressed. The average wind speed increased from 15 to 20 MPH at the start of the afternoon to 20 to 25 MPH at the end of the afternoon. Highest gusts reached approximately 35 MPH by late afternoon. As the temperature fell, the snow became less wet, and the wind speed increased, the accumulated snow began to blow and drift late in the afternoon.
The blizzard grew to its full fury during the evening of October 24, 2010. Snow fell continuously and often heavily and the temperature fell from the middle 20s to about 16 degrees. The wind reached maximum strength during this time, blowing from the north at an average speed of 22 to 32 MPH with highest gusts to between 40 and 45 MPH. Severe blowing and drifting of the accumulated snow resulted. The combination of heavy snowfall and blowing snow due to the high winds caused near whiteout conditions at times. The depth of snow on the ground increased dramatically. At 12:00 Midnight, undisturbed ground surfaces were covered by an average of 12 to 15 inches of snow with a liquid equivalent of 1.10 to 1.40 inches and an average weight of 5.7 to 7.3 pounds per square foot. There were significant variations in the actual depth and weight of the snow cover from place to place due to the severe drifting.
The blizzard continued to rage in Pierre throughout the morning hours of October 25, 2010. Snow fell continuously and often heavily and the temperature ranged from 16 to 18 degrees. The wind blew from the north, averaging 22 to 32 MPH with gusts to between 32 and 42 MPH. Severe blowing and drifting of the accumulated snow continued. Drifts grew to several feet in depth. At 12:00 Noon on the 25th, undisturbed ground surfaces were covered by an average of 24 to 28 inches of snow. The liquid equivalent of the snowpack was 2.35 to 2.75 inches and its average weight was 12.2 to 14.3 pounds per square foot.
The storm eased across central South Dakota during the afternoon of October 25, 2010. The intensity of the falling snow lessened and the wind was not quite as strong. Between 12:00 Noon and 3:00 PM, the wind blew from the north and north-northwest at an average speed of 20 to 25 MPH with gusts to approximately 35 MPH. Between 3:00 PM and 6:00 PM, the wind blew from the north and the average speed diminished to 10 to 15 MPH. The blowing and drifting of the snowpack gradually came to an end. The afternoon temperature ranged from 18 to 22 degrees. The depth of the snow on undisturbed ground surfaces reached a peak during the late afternoon of the 25th, totaling 27 to 31 inches. The liquid equivalent reached 2.80 to 3.25 inches and the average weight of the snow cover peaked at 14.6 to 16.9 pounds per square foot. There were large variations in the depth and weight of the snowpack from place to place due to the drifting during the blizzard.
In summary, at 1521 Stevens Road, Pierre, South Dakota, the ground was free of snow and ice from October 21, 2010 through October 23, 2010 and high temperatures during these three days ranged from 63 to 72 degrees. The temperature of the soil was well above freezing at the end of the 23rd.
A severe early-season blizzard struck Pierre on October 24 and 25, 2010. Snow fell continuously from the early daylight hours of the 24th through the early evening of the 25th. The air temperature was just above 32 degrees when the snowfall commenced and the snow melted on the ground at first. Then, as the snowfall intensified, the temperature fell through the 20's and then into the teens, changing the characteristic of the falling snow from wet to drier and powdery. The total average amount of snow that fell at the site in question during the storm was between 27 and 31 inches. The total average liquid equivalent of the snowpack at the end of the storm was 2.80 to 3.25 inches and the average weight of the snowpack as the storm ended was between 14.6 and 16.9 pounds per square foot.
The severity of the storm was exacerbated by very strong winds from the north that gusted to as high as 40 to 45 MPH at the height of the blizzard. These high winds caused severe blowing and drifting of the powdery snow on the ground, resulting in massive drifts to as deep as 10 feet. The severe drifting caused substantial variations in the depth and weight of the ground snow cover from place to place. After the snowfall ended between 7:00 PM and 8:00 PM on October 25, 2010, there was no further precipitation in Pierre during the rest of the 25th and there was insufficient wind to cause any noteworthy drifting.
To this point in the report, factors specific to the roof in question have not been taken into account. All snow depths and weights given have been average values on undisturbed ground surfaces. The primary factor that affected the amount and weight of the snow on the roof in question was drifting.
According to the roof plan of the Golden Leaf Furniture Warehouse provided to us, there is a ridgeline that runs approximately east to west separating the "showroom" side of the building to the north from the "warehouse" side to the south. There is a ridge cap along the ridgeline that extends 4 to 6 inches above the roof. The slope of the roof toward the north and south is _ in 12. The distance from the ridgeline to each eave is 125 feet. Thus, there is a drop of about 3 feet in height from the peak of the ridge cap to the gutters at the eaves. When the wind blew across the ridge cap, a wind shadow effect was created to the lee (downwind) of the cap and a snowdrift was formed in the wind shadow region.
During the many hours that drifting occurred with wind blowing from a generally northerly direction, drifts formed on the "warehouse" side of the ridgeline. By October 25, 2010, drifts in this area may have reached 3 feet in depth. A drift of this depth would have weighed approximately 44 to 46 pounds per square foot.
The information in this report has been determined from the best sources of weather information available to us at this time and is the result of interpretation by our staff of professional meteorologists and represents our opinions to a reasonable degree of scientific certainty.
We trust that this information is useful to you. If you should have any additional questions or need additional information, please do not hesitate to contact us.
1 This and all other time references in this report are expressed in Central Daylight Time (CDT). Daylight Savings Time ended at 2:00 AM on October 26, 2010.Note: Dates, times, names, and locations may have been changed to protect client confidentiality.
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