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  1. Meteorology 1010Supplement to Chapters 9-11This PowerPoint is not a substitute for reading the textbook and taking good notes in class.

  2. This PowerPoint presentation is the first of two that supports Quiz #4, Fall Semester, 2013.Powerpoint presentations should be viewed as a study guide.

  3. Jet StreamsMeridionalvs Zonal FlowWhen the jet stream provokes storms by mixing cool/dry with warm/wet storminess can occur.Mid-latitude storms often begin with mixing of cool/dry air and warm/wet air as the jet stream(s) undulates more north/south, helping to mix differing air masses. High Low

  4. November 18 Salt Lake weather report indicated that westerly flow was going to be more ‘zonal’ rather than ‘meridional’ so there would not be any increasing intensity that otherwise might occur if ‘storm track’ flow dipped north or south to pick up greater moisture and/or greater contrast.Sunday’s tornadoes in Illinois were unusual because of more-than-normal moisture and heat – November usually exhibits snow storms, with some wind.

  5. Notice that eastward-moving storm is typical - counter-clockwise air in the mid-latitudes

  6. Frontal Weather This one is a cold front. A warm front would have similar features, but more gradual.

  7. Notice that the cold-frontside is more vigorous than the warm front side (right side). Warm front chasing cool air Cold front chasing warm air Faster-moving cold front gradually overtakes warm air and causes warm air to lift.

  8. Life of a Midlatitude Cyclone Notice that the cold front side shows more severe radar returns – more vigorous lifting and more severe weather. Relatively Cold Air Relatively Warm Air

  9. 2 3 1 Notice that in a mid-latitude cyclone, cold and warm air don’t mix at first. As Coriolis force helps turn the air, mixing begins as warm, humid air lifts over cooler, drier air that is more heavy. Rising air provokes condensation, precipitation and strong winds. At the end, warm air is temporarily stable above cold air below. Westerly winds 4 5 6

  10. Here we see how the jet stream (with storm track) helps pull low and high pressure cells toward each other. The difference between warm/wet and cool/dry helps produce rising air, high wind, precipitation, hail, lightning. Warm, wet air rises above cool/dry air classic “frontal” storm.

  11. If you click quickly on the next six slides you can see how a mid-latitude cyclone can develop.

  12. Figure 9.18a

  13. Figure 9.18b

  14. Figure 9.18c

  15. Figure 9.18d

  16. Figure 9.18e

  17. Figure 9.18f

  18. Air-Mass Thunderstorms

  19. Air-Mass Thunderstorms Mountainous regions, such as the Rockies and the Appalachians, experience a greater number of air-mass thunderstorms. Because Utah is fairly dry, we have enough heat for dust devils and brief air-mass thunderstorms. For really severe weather, heat and humidity are needed – not just “cash”, but “credit”

  20. Severe Thunderstorms • Severe thunderstorms: • Heavy downpours • Flash flooding • Straight line wind gusts • Hail, lightning • Wind shear • Can overshoot (enter stratosphere) • Downdraft preceding (gust front)

  21. Supercell Thunderstorms

  22. Supercell Thunderstorms • Supercells • These storms can produce extremely dangerous weather. • They consist of a single, powerful cell that can extend to heights of 20 km or more. • The clouds can measure 20–50 km in diameter. • Mesocyclone: • Vertical winds may cause the updraft to rotate, which forms a column of cyclonically rotating air. • Tornadoes often form.

  23. Squall lines – a line of thunderstorms that may hundreds of miles long – cooler/drier air on one side -- warmer/wetter air on the other side. Colder Drier air Warmer, more humid air

  24. Supercell Thunderstorms • Mesoscale convective complexes (MCC): • An MCC consists of many individual thunderstorms. • It is organized into a large oval to circular cluster. • They cover an area of at least 100,000 km2. • It is a slow-moving complex that may last for 12 hours or more. • MCCs tend to form mainly in the Great Plains.

  25. Tornadoes

  26. The Development and Occurrence of Tornadoes • Tornado development

  27. The Development and Occurrence of Tornadoes • Profile of a tornado: • Average diameter 150–600m • Travels ~45 kph – 28mph (Nov 13 storms moved at 60+ mph) • Path about 26 km long • Most travel to the NE • Exist between < 3 min to > 3 hours • Wind speeds between < 150 kph to > 500 kph • November 17, 2013 storm winds estimated at 190 mph

  28. The Development and Occurrence of Tornadoes No. However, a persistent pattern of more severe weather is consistent with other findings about climate change. Based on this chart, can we use single events to support the theory of global warming?

  29. Tornado Destruction

  30. Tornado Destruction • Tornado intensity A November 2013 tornado reached nearly 200 mph EF4 2013 tornado in Oklahoma probably produced 300 mph wind.

  31. Tornado Destruction

  32. Tornado Forecasting • Tornado watches and warnings: • Watches alert the public. • Tornadoes are possible and conditions are favorable. • They usually cover an area of about 26,000 km2. • Watches can last 3 hours or longer. • Warnings are issued when a tornado is actually sighted or conditions are just right. • There is a high probability of imminent danger. • They are usually for a much smaller area. • Warnings are in effect for a much shorter period, usually 30–60 minutes.

  33. News reports stated that:“Predictions are saving lives.”The truth is - - they weren’t genuine predictions, just better quality warnings.Oddly, hurricanes are so big that we can only generalize about where, what and when - - just a forecast. Oddly also, tornadoes are so small and transient that they are very difficult to predict - - we can forecast better, but not really predict.

  34. Tornado Forecasting Doppler effect: • radar reflections arriving more quickly indicate the target is getting closer. • Radar reflections arriving more slowly indicate the target is getting further away.