Event Summary
     National Weather Service,
     Raleigh NC


March 20, 1998 Severe Weather and Tornado Outbreak





Introduction

The severe weather outbreak that occurred across North Carolina on March 20, 1998 followed a week of active weather. The region had already received heavy rainfall (4.40 inches in the previous 3 days ending at 700 AM, on March 20th, at RDU). The active weather was produced by a split upper flow pattern that transported several weather disturbances across the state that week.

On the day of the severe weather outbreak, a cold, stable air mass was in place across much of interior North Carolina. This cold, stable air mass, wedged into place from the Piedmont area west to the eastern slopes of the mountains, is known as cold air damming. Across the damming region, skies were generally cloudy with temperatures in the 40s and 50s. To the south and east of the damming region, a warmer, more unstable airmass was in place with temperatures in the 60s and 70s. A boundary was in place between the two differing air masses, this boundary, known as a Thermal Moisture Boundary or TMB would play a critical role in the development of severe weather later in the day.

The severe thunderstorms that developed on March 20, 1998 affected several states across the Mid Atlantic region. North Carolina was hard hit with 10 tornadoes, 2 fatalities, and 27 injuries. There were also 46 reports of damaging hail and 9 reports of wind damage. The two graphics below provide additional details on the coverage and extent of severe weather that day.


Figure 1 - Severe weather reports from March 20, 1998.



Figure 2 - Tornado tracks and fatalities from March 20, 1998.


Details on the Severe Weather Outbreak

Several factors lead to the development of severe thunderstorms on March 20, 1998. A cold, stable air mass was in place across much of interior North Carolina with a contrasting warmer and more unstable airmass moving into southern and eastern part of the state ( see Figure 3 ). The Thermal Moisture Boundary or TMB shifted north during the afternoon as the warm, moist and unstable airmass moved into the Foothills and Piedmont. This boundary provided a focus for convection to develop and it created strong low level wind shear supporting super cell thunderstorm formation. A strong upper level low pressure system was moving slowly east across the Tennessee Valley. The upper level low allowed the mid and upper levels of the atmosphere to become increasingly unstable and while increasing the amount of mid and upper level wind shear. ( see Figure 4 ).



Figure 3 - Surface chart from 18Z March 20, 1998 showing a cool stable airmass
over northeastern NC and eastern VA with a northeast wind and an increasingly
unstable airmass over SC and western NC with a west to south wind.




Figure 4 - 500 MB chart from 12Z March 20, 1998 showing the strong upper level
“cut off” low pressure system over the Tennessee Valley.




A line of showers and thunderstorms developed around midday over the western Piedmont and moved quickly northeast across much of the Piedmont and into the Coastal Plain. ( visible satellite loop from 1625Z on March 20, 1998 through 2300Z ). The convection intensified and the structure changed from a line of thunderstorm cells to several supercells as it moved into a more unstable airmass with a tremendous amount of wind shear ( ETA 6 hour forecast of CAPE, 0-6 km mean wind, and helicity valid at 18Z March 20, 1998 and ETA 12 hour forecast of CAPE, 0-6 km mean wind, and helicity valid at 18Z March 20, 1998 ). The severe weather continued into the evening hours with several thunderstorms becoming strongly rotating supercells with tornadoes.


Figure 5 - Visible satellite image from 2220Z March 20, 1998.
Loop of visible satellite imagery Visible from 1625Z on March 20, 1998 through 2300Z


The strongest tornado developed over SE Stokes county shortly before 300 PM local time. Although the touchdown was brief, the storm continued to strengthen as it exited Stokes county. The storm moved rapidly into western Rockingham county and produced another tornado between 313 PM and 325 PM (radar loop from the KFCX WSR-88D between 1837Z through 2032Z March 20, 1998 ) . This tornado would become the strongest tornado ever to affect Rockingham county as it produced F3 damage in the town of Stoneville, killing 2 people ( see Stoneville, NC damage photographs ). The tornado continued on its northeast track at nearly 50 mph as it passed into Henry and Pittsylvania counties in VA. This would be the strongest and deadliest tornado of the day.


Base reflectivity image from the KRNK WSR-88D at 2027Z March 20, 1998





Photograph of the Stoneville Tornado as it crossed US Route 220 just
southwest of Stoneville, NC on March 20, 1998 - photo courtesy of NWS
Blacksburg, VA



Additional thunderstorms continued to develop over the central and eastern Piedmont, the Sandhills and Coastal Plain of North Carolina through the afternoon and evening hours ( visible satellite loop from 1625Z on March 20, 1998 through 2300Z ).

Another supercell thunderstorm developed over Holly Springs in southwestern Wake county around 715 pm. This tornado briefly touched down in a subdivision and produced some damage. The same supercell thunderstorm moved northeast toward Raleigh and produced additional tornadoes in Garner and east Raleigh. The Garner tornado produced (F2) damage at approximately 740 PM (see Garner, NC damage photographs - photo 1, photo 2, photo 3, and photo 4 ). The east Raleigh tornado touched down briefly near Wake Medical Center at around 730 PM, producing (F0) damage.

At about the same time as the tornadoes were touching down in Wake county, another supercell thunderstorm was producing tornadoes over northeastern Durham and western Granville counties. After touching down in far northeastern Durham county an (F2) tornado touched down again near Berea along Highway 158 in Granville county at approximately 730 PM.




References

U.S. Significant Tornadoes, Thomas Grazulus, 1860-1991.

Severe Plot, Historical Severe Weather Report Database, John Hart and Paul Janish, Storm Prediction Center, NOAA.

State of NC, Dept of Crime Control and Public Safety



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