RICHMOND, Va. (WTVR) – Prior to the tornado outbreak in late April 2011, the April 3-4, 1974 “Super Outbreak” was the biggest in modern-day. Now April 25-28, 2011 holds the top spot for the largest tornado outbreak (on record) for a single 24-hour period, and has taken the name “Super Outbreak.” (Click here to learn about historic tornadoes and outbreaks)
But before 2011, the 1974 event was considered by meteorologists to be the “outbreak of the century.” Virginia was among the thirteen states that reported tornadoes, including Alabama, Georgia, Illinois, Indiana, Kentucky, Michigan, Mississippi, North Carolina, Ohio, South Carolina, Tennessee, and West Virginia. One person died as a result of the outbreak on April 4 in Washington County, Virginia. That tornado was rated a F3 (click here for the old Fujita Scale wind ratings) with winds in the range of 158-206 mph. The other two April 4, 1974 Virginia tornadoes were a F1 (est winds 73-112 mph) north of Staunton, Virginia and a F2 (est winds 113-157 mph) mile-wide tornado in the Roanoke area as it entered Salem, Virginia. Click here for a full list of the 1974 Super Outbreak tornadoes.
(Side notes: Virginia’s greatest tornado outbreak in a single day was from Hurricane Ivan’s remnants September 17, 2004 when 40 tornadoes were reported in the Commonwealth. The deadliest Virginia tornado outbreak on record is May 2, 1929 when 22 people were killed.)
There is an extensive case-study published here about the 1974 outbreak that you can read online for free. I’ll pull a few highlights for you from that paper to post here for brevity. The paper is titled, “THE SUPER OUTBREAK: OUTBREAK OF THE CENTURY,” by Stephen F. Corfidi, Jason J. Levit and Steven J. Weiss with the NOAA/NWS/NCEP/Storm Prediction Center, Norman, OK.
“A sampling of statistics only partially conveys its enormity:
- 148 TORNADOES: 95 F2s or stronger, 30 F4s or F5s
- 48 KILLER STORMS: 335 dead, more than 6000 injured
- PATH LENGTHS UP TO 90 miles (145 km)…Total path length > 2500 mi (4000 km)
- F2s OR GREATER PRESENT FOR EACH THREE HOUR PERIOD BETWEEN 12 UTC / 3rd AND 15 UTC / 4th
- AT ONE POINT, 15 TORNADOES IN PROGRESS AT SAME TIME
- TEN STATES DECLARED FEDERAL DISASTER AREAS
Instead of a single “smoking gun,” it appears that several factors which came together more or less by chance were largely responsible for the Super Outbreak. Not to be discounted, the unusually strong upper level jet streak associated with the progressive
Great Basin trough certainly set the stage for a severe weather event by creating large scale conditions favorable for expansive lee cyclogenesis. The jet maximum not only provided the necessary shear for intense, sustained supercells, but also helped focus the mesoscale areas of ascent that assisted in convective initiation.
At the same time, the warm sector during the Super Outbreak was not only very broad but also unusually warm and moist for early April. These characteristics were established, in part, when the trailing frontal system associated with the previous shortwave impulse failed to effect a significant air mass change over the Southeast. This enabled simultaneous occurrence of both dry line storms (“Band Two”) and jet exit region activity (“Band Three”) as the Great Basin disturbance progressed east northeast into the
Plains. Oftentimes, because of the limited aerial extent of surface-based instability, severe weather episodes associated with “ejecting” lee cyclones are limited to dry line/lee trough activity on the initial day of ejection, and jet exit region storms on the succeeding one.
Climatological data suggest another factor that may have contributed to the “quality” of the warm sector boundary layer over the southeastern states on 3 April 1974. Mean constant pressure charts for the previous few weeks (not shown) depict the presence of a persistent upper level ridge over the Caribbean Sea and southern Gulf of Mexico. The ridge had restricted cold frontal penetrations into the region for much of the month of March. This may have allowed a warmer and moister boundary layer to evolve over the Gulf than is usual for the time of year.
Yet another factor which set the Super Outbreak apart was the well-defined gravity wave or bore which provided a “bonus” third source of organized uplift. This feature helped initiate deep convection in areas that may otherwise have remained capped to development. The bore, in conjunction with daytime heating, also helped rejuvenate existing storms in Georgia and the Carolinas.
Returning to the kinematic environment, it is important to note that the fast but low amplitude nature of the upper jet pattern was also influential in the event. This pattern not only (1) allowed for rapid, undiluted advection of the elevated mixed layer plume eastward
from the Plateau into the Mississippi, Ohio and Tennessee Valleys, but also (2) delayed /
discouraged storm evolution toward linear convective modes. By enabling storms to remain discrete for a maximum period of time, the potential for both tornado development and longevity were enhanced.
The low amplitude flow also likely limited the coverage of high level, “warm conveyor belt” clouds which often restrict diabatic heating in more highly amplified regimes. Low amplitude, fast flow patterns have been associated with other significant severe outbreaks in recent years (e.g., Arkansas, 21 January 1999; Tennessee and Ohio Valleys, 10 November 2002).
Finally, it is worth noting that the diurnal cycle was favorably timed with respect to the arrival of rich boundary layer moisture and the intensification of deep shear over the Ohio and Tennessee Valleys during the Super Outbreak.”
Again, those were excerpts from the expert analysis from Stephen F. Corfidi, Jason J. Levit and Steven J. Weiss with the NOAA/NWS/NCEP/Storm Prediction Center, Norman, OK.
There are extensive pictures and videos from the event posted at a website here.
We at the CBS 6 Storm Team encourage you to prepare now for severe weather. Make an emergency kit and plan now. The American Red Cross has excellent tips here.
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