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Chapter 11: Winter Weather Heavy snowfall in St. Louis is typically associated with a developing low pressure system To examine snowfall within a developing.

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Presentation on theme: "Chapter 11: Winter Weather Heavy snowfall in St. Louis is typically associated with a developing low pressure system To examine snowfall within a developing."— Presentation transcript:

1 Chapter 11: Winter Weather Heavy snowfall in St. Louis is typically associated with a developing low pressure system To examine snowfall within a developing low pressure system we will consider the penultimate snow storm for St. Louis: 30 January 1982

2 Chapter 11: Winter Weather What Happened?

3 Chapter 11: Winter Weather Impacts 1-in-70 year event, worst event since 20 Feb, 1912 with 15.5” Began Saturday evening and ended Sunday afternoon Contained 5 hours of lightning (thundersnow) Schools canceled for up to 1 week National Guard was brought in to help

4 Chapter 11: Winter Weather Impacts 4,000 stranded on highways

5 Chapter 11: Winter Weather System Evolution Charts Surface Pressure 850 mb Heights and Isotachs 500 mb Heights and Vorticity 300 mb Heights and Isotachs

6 Chapter 11: Winter Weather System Evolution 18 UTC January 30

7 Chapter 11: Winter Weather System Evolution 00 UTC January 31

8 Chapter 11: Winter Weather System Evolution 06 UTC January 31

9 Chapter 11: Winter Weather System Evolution 12 UTC January 31

10 Chapter 11: Winter Weather System Evolution 18 UTC January 31

11 Chapter 11: Winter Weather System Evolution 00 UTC February 1

12 Important Features Deepening surface/850 mb system Westward tilt with height Low-level (850) southerly wind maximum Change in the orientation of upper-level system Chapter 11: Winter Weather

13 What about the snowfall? The track of the surface low was south of St. Louis (as was the track of the 850 mb system) The snow could not be related to the cold front (it extends to the south of the low) Was it the warm front?  Not exactly.... Chapter 11: Winter Weather

14 Surface System and Snowfall 18 UTC Jan 30

15 Chapter 11: Winter Weather Surface System and Snowfall 00 UTC Jan 31

16 Chapter 11: Winter Weather Surface System and Snowfall 06 UTC Jan 31

17 Chapter 11: Winter Weather Surface System and Snowfall 12 UTC Jan 31

18 Snow North and Northwest of the Surface Low Flow from the south tracks northward ahead of the low Once it nears the low, part of it wraps around the low This flow encounters the warm front so it rides up the warm frontal boundary Chapter 11: Winter Weather

19 Snow and the Surface Low Warm moist air flows northward and wraps around low. Once it encounters the warm front is rises over the cold air (over-running) L Warm moist air flows northward and turns to the east after it encounters the warm front. It rises over the cold air (over-running) Chapter 11: Winter Weather

20 Winter Weather Summary Developing low pressure system Moisture is transported ahead of low and then lifts after encountering warm front:  Wrap around moisture leads to snowfall north and northwest of surface low  Eastward shifted moisture leads to warm advection snowfall ahead of the system Snow requires not just moisture and lift, but cold temperatures to support crystal formation. Chapter 11: Winter Weather

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