Mountain Snowstorms

• Mountain snow is the primary source of agricultural and urban water supplies in the western U.S.; mountain snow "fuels" the tourist industry - ski & scenery

• Over one-third of the 200 billion kilowatt hours of power generated in the western U.S. Is hydroelectric (50% of the power generated in the Pacific coast states and 80% of all power generated in OR, WA & ID); Figure 15.1

• Interstate 80 through Donner Pass in the Sierra Nevada can be closed for a week or more due to heavy snowfall, stranding trucks and automobile traffic on both sides of the mountain range

• Interstate 70 heading west out of Denver crosses the Continental Divide (11,260 feet MSL) can be closed due to snow, cutting off Denver from the resorts on the west side of the range

• Little cottonwood Canyon connects Salt Lake City with the Alta and Snowbird ski resorts in the Wasatch Mountains; the road is 9 miles long, but traverses no less than 42 avalanche paths, with half the road in avalanche path runout zones

• East of the Rockies: seasonal snowfall determined by latitude & temperature

• From the Rockies to the Pacific coast: seasonal snowfall determined by elevation & orography; Figure 15.2

• Orographic lifting: mountains force air to rise sharply along the windward slopes where a majority of the precip occurs, creating a rain shadow on the lee side; Figures 15.3-6

• Mountain barriers disrupt storm circulations, modify frontal structures and weaken low-level features; upper-level forcing (troughs & jetstreams) continues to trigger cloud formation

• Pacific Coastal Range: first mountain range encountered by air off Pacific Ocean; heaviest precip in contiguous U.S. on coast north of SFO; 180" annually on WA's Olympic Peninsula; coastal peaks usually below the melting level (2000-8000 feet), leading to mostly rain

• Sierra Nevada of CA & Cascades of OR & WA: a high and nearly unbroken mountain barrier, that merge with the Coast Range in B.C. of Canada; air is forced to rise nearly 2 miles, thereby condensing out most of the moisture during ascent; greatest seasonal snowfall in contiguous U.S., with annual totals from 140 to 700" (over 58 feet), but in stormy winters can range from 235 to over 1100" (over 91 feet!); Figure 15.7; in parts of western Canada and SE-Alaska, so much snow falls that it does not all melt in Summer, leading to glaciers

Mountain Snow (continued)

• Wasatch Range, Bitteroot Mountains and the Rockies (Front, Wind River & Teton Ranges): these major interior ranges receive less precip, since most of the precip has been precipitated out over the upwind ranges; temps are colder along these ranges, creating a lower water equivalent in the snow (11:1 to 14:1 ratio, but can be as low as 25:1 to 30:1 ratio); powder snow great for skiing; Utah ski resorts advertise their "Champagne Power" as "The Greatest Snow on Earth"

• East Slopes of the Rockies: "upslope storms"; easterly wind flow forces air to rise nearly 5,000 feet from Mississippi River basin (Pressure: ~1000 mb) to the foothills of the Rockies (Pressure: ~850 mb), where the air is forced up another 2,000 to 7,000 feet (Pressure: ~700 mb); when air is laden with moisture from the Gulf of Mexico, very heavy snowfall can occur (2 to 3 feet, or more)

• Pattern #1: Arctic high pressure to the north or northeast of CO, dropping out of central Canada toward the north-central U.S. Sets up an easterly, upslope flow into the Rockies; typically produce light snowfall (4" or less) and cloud tops are low; Figure 15.9

• Pattern #2: low pressure ("Four Corners Low") to the south of CO, creates an easterly wind flow, that sends moist air from the Gulf of Mexico into the mountains; snowfall is heavier than Pattern #1 (foot or more); Figure 15.10

• Combo of #1 & #2 can lead to exceptional blizzards, especially if the high to the north is feeding in very cold air and the storm to the south is strong; resulting tight pressure gradient sets up the necessary winds for a "blizzard"

• Snowfall distribution in the Denver-Fort Collins urban corridor changes, depending on the prevailing wind direction; NE surface wind flow favors Denver and SE surface wind flow favors Fort Collins; Figure 15.11