Transcript ppt
Great Plains Blizzards North Dakota, March 1966 blizzard: a severe weather condition characterized by high winds and reduced visibilities due to falling or blowing snow. The NWS definition specifies winds in excess of 30 kts (35 mph) and visibility of less than a quarter of a mile for at least 3 hours. Typical tracks of cyclones that form east of the Rockies Where are blizzards most common? Where are blizzards most common? Humans and Blizzards hypothermia: decrease in the body’s core temperature to a level at which normal muscular and cerebral functions become impaired Colorado Cyclones and Blizzards Cold, Wind and Snow Extremely cold air is the first component of a blizzard. • • • • Where would this cold air originate? Why would this air be so cold (-30 to -50˚F)? What happens to the sea level pressure of extremely cold airmasses, and what would this mean for blizzard formation? What type of front marks the leading edge of this extremely cold airmass? A cyclone to provide wind and snow is the second component of a blizzard. • • • • What factors contribute to the formation of a cyclone on the east side of the Rocky Mountains? What happens to the extremely cold Canadian air when it comes in contact with the cyclone? Where would strong winds form in this situation? What is the source of moisture for blizzards associated with Colorado cyclones? Figure 7.7 Movement of airmasses and development of fronts as surface low-pressure center forms Trough of warm air aloft (trowal) Snowfall created by the trowal can exceed 10-12 inches in a 300 mile swath. Why would the snow created by the trowal be particularly susceptible to creating whiteouts and huge drifts? Second factor creating snow north of the cyclone center involves topography How do the high plains and Rocky Mountains enhance snowfall in a blizzard? Hint: how would the temperature of an air parcel cool traveling westward from St. Louis to Denver (elevation gain 4,500 feet) change? Alberta Clippers and Blizzards Upper air conditions •Jetstream orientation •Wave in the jetstream Surface conditions Why would Alberta Clipper blizzards tend to produce less snow than Colorado cyclone blizzards? Mature phase of and Alberta Clipper cyclone Ground Blizzards a blizzard that is caused solely by blowing snow (no new snow is falling) • Under what conditions can a ground blizzard occur? • How deep is a ground blizzard? Winter of Blizzards 19961997 Table 14A Summary of surface data at Fargo, North Dakota, during nine blizzards in the 199697 winter Date of blizzard (199697) Lowest Temperature (F) Highest Temperature (F) Greatest Hourly Sustained Wind (knots) Fasted Reported Wind Gusts (knots) Snowfall (inches) 1617 November 9 26 34 42 13.5 1618 December -8 22 33 43 7.9 23 December -14 -8 21 4.9 45 January 0 20 33 40 10.7 911 January -15 15 24 35 4.8 15 January -8 12 38 49 1.0 2223 January -17 24 34 4.9 34 March -5 31 25 15.5 56 April 9 38 24 56 7.0 Figure 14 B Schoolhouse Blizzard Formed: January 12, 1888 Dissipated: January 13, 1888 Maximum amount:* 6 inches (15 cm) Lowest pressure: 930 hPa Damages: UNKNOWN Fatalities: 235 Areas affected: Mid-Western US * Maximum snowfall or ice accretion Figure 14 C Figure 14 D