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Chapter 11: Winter Weather
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
Chapter 11: Winter Weather
What Happened?
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
Chapter 11: Winter Weather
Impacts
4,000 stranded on highways
Chapter 11: Winter Weather
System Evolution Charts
Surface Pressure 850 mb Heights and Isotachs
500 mb Heightsand Vorticity
300 mb Heightsand Isotachs
Chapter 11: Winter Weather
System Evolution 18 UTC January 30
Chapter 11: Winter Weather
System Evolution 00 UTC January 31
Chapter 11: Winter Weather
System Evolution 06 UTC January 31
Chapter 11: Winter Weather
System Evolution 12 UTC January 31
Chapter 11: Winter Weather
System Evolution 18 UTC January 31
Chapter 11: Winter Weather
System Evolution 00 UTC February 1
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
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
Chapter 11: Winter Weather
Surface System and Snowfall 18 UTC Jan 30
Chapter 11: Winter Weather
Surface System and Snowfall 00 UTC Jan 31
Chapter 11: Winter Weather
Surface System and Snowfall 06 UTC Jan 31
Chapter 11: Winter Weather
Surface System and Snowfall 12 UTC Jan 31
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
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)
LWarm 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
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