chapter 11: winter weather heavy snowfall in st. louis is typically associated with a developing low...

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


What Happened?


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


Impacts

4,000 stranded on highways


System Evolution Charts

Surface Pressure 850 mb Heights and Isotachs

500 mb Heightsand Vorticity

300 mb Heightsand Isotachs


System Evolution 18 UTC January 30


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


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 ....



Surface System and Snowfall 18 UTC Jan 30

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


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)


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 heavy snowfall in st. louis is typically associated with a developing low...

Documents

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