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National Weather Service Weather Forecast Office

Springfield, MO

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	Storm Spotter Online Training NWS Springfield, MO
Squall Lines
Squall Line Basics  |  Shelf Cloud  |  Event Examples

Training Objectives

This training was designed to provide volunteer SKYWARN spotters, dispatchers, emergency management and the public in general with:

• An understanding of squall lines. 

• An understanding of the primary hazards associated with squall lines. 

• An understanding of how identify key features of squall lines.

Understanding Storm Structure

Understanding storm types and structure equips spotters to:

• Take positions in the best viewing areas.

• Spot more safely by knowing where to be and not to be.

• Anticipate the type and magnitude of severe weather hazards.

Knowing storm structure will result in safer and more effective storm spotting.

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Squall Line Basics

Squall lines are the most common type of severe storm that affect the Missouri Ozarks and southeast Kansas.  While not always severe (winds ≥ 58 mph/50 kts), there is a threat of damaging straight line winds and hail.  Tornadoes are rare but not unprecedented along squall lines.

Squall lines are comprised of storms that organize into a linear structure and can be more than 100 miles long.   A squall line is a long lasting multi-cell storm that may be aligned into one or more segmented lines if storms. 

The primary threat is straight line damaging winds. Although squall lines can produce hail and weak tornadoes.

The squall line is often preceded by a shelf cloud.  A shelf cloud is a triangular shaped cloud mass which marks the leading edge or the updraft/downdraft region of a squall line.

Damaging winds may occur as the leading cloud mass or shelf cloud passes, especially if the storms are bowing outward.

Supercell storms can occasionally be embedded or precede a squall line which increases the potential of severe weather.

The worst is first. Strong winds possibly over  60 mph will accompany the passage of the leading of edge (shelf cloud) of the squall line followed by heavy rain.

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Shelf Cloud

The above is a shelf cloud approaching the NWS Springfield office from the northwest.

The shelf cloud marks the leading edge of the cool moist downdraft capable of producing strong to damaging straight line winds.  This feature is also referred to as a "gust front" or "outflow boundary."

The key to squall line identification and spotter positioning is the shelf cloud.

The shelf cloud marks the intersection of the downdraft and updraft portions of the storm, and the leading edge of the cool moist downdraft.

The shelf cloud or gust front will typically mark the onset of the storm and precede the heavy rainfall. 

On some occasions the gust front may surge well ahead of the parent storm.

         

Looking southwest                                                          Looking northwest                           

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July 4th and 5th, 2004 Squall Lines

Squall lines that produce extreme winds and significant damage, and have a long life span of several hours or more are called, "derechoes".   Two of  these events hit portions of the area on July 4th and 5th of 2004.

This squall line produced widespread wind damage on July 4, 2004.  The strongest winds in excess of 70 mph occurred in this portion of the storm.

Area of strongest winds

These storms produced significant wind damage with cost estimates near $750,000.  The extreme winds also resulted in injuries.

These storms produced up to 80 mph winds and significant damage with cost estimates near $700,000.   The extreme winds downed numerous trees at the Truman Lake campground resulting in 48 injuries.   One fatality also resulted as the winds swept off a boat on Truman Lake.

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Squall Line Summary

Squall lines can produce strong to damaging straight line winds, but can also produce hail, weak tornadoes and flash flooding.

Winds speeds can well exceed 60 mph which can produce as much damage as a tornado.

Look for the shelf cloud to identify the approach of a squall line.

For more detailed information about squall lines and other meteorological phenomena go to  JETSTREAM, the National Weather Service online weather school.

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Thank You!

The National Weather Service thanks all of those who volunteer their time and energy in providing crucial storm reports. 

Your reports are critical to providing life saving warnings.