A strong late September cold front pushed through the Appalachian Mountains during the day, Saturday, Sept 21, with long zone of mainly rain and rain shower just ahead of it from New York to the Gulf Coast. Extensive cloud cover ahead of this limited the instability during the day, however as this zone of lift along the front moved east of the Blue Ridge during the afternoon, very weak instability from limited daytime heating was just enough to help intensify some of the showers. Still, no lightning was detected by networks that the National Weather Service monitors across this region in North Carolina or Virginia, so updrafts were still relatively weak and radar echoes remained quite shallow. However, there was moderate wind shear present in the lower levels of the atmosphere (surface-1km), and several of these intensifying showers did develop weak and fairly broad rotation with them. In one case that we are aware of, in King NC (far southwest Stokes County), this rotation tightened up just briefly enough to spin up a brief EF-1 tornado around 6pm, as determined by a NWS Storm Survey Team. Details of the storm survey are below. Unfortunately, this spin-up took place below the lowest radar beam and possibly between 5-minute radar scans, and so no significant signal indicating an immediate tornado threat was detected with this cell. In fact, other heavier showers in the vicinity actually developed slightly stronger but still very broad and relatively weak rotational signatures, and no tornadic damage was associated with any of these (although there were a handful of reports of isolated trees down across other parts of the Piedmont, and one report of localized damage to some structures, all of which were most likely due to downburst straightline winds.
There are many similarities between this event and a tornadic event that struck this same town in October of 2010. Both occured in high shear but weak instability, and these types of events are more difficult to detect by radar given they are so shallow and develop so quickly, unless they happen to be within about 20 miles of the radar.
Figure 1: Surface Map with Radar Mosaic showing a broad band of rain with a some embedded heavier showers just ahead of a cold front just before the time of the tornado in King, NC.
Below are two maps which show the combination of very marginal instability (Figure 2), yet fairly strong low-level wind shear (Figure 3) which helped the rapid low-level spin-up despite weak upfrafts. These are valid at 5pm, about an hour before the tornado touched down.
Figure 2: Surface Based CAPE (5pm), which is a measure of how unstable the atmospere is for developing updrafts and thunderstorm potential. In this case, it was very weak, and no lightning was detected with storms in this area.
Figure 3: SFC-1km Wind Shear (5pm), indicating potential for any updrafts that do form to develop some rotation. There was an enhanced region of fairly strong shear just ahead of the cold front at this time. Only shallow updrafts with weak and broad rotatin were indicated on radar during the afternoon and evening.
Below is an image and link to an animation of both reflectivity and Doppler velocity information
from the KFCX WSR-88D radar (located in Floyd County, VA), zoomed in on an area of more intense rain
showers over the foothills and Piedmont of NW North Carolina. There is a zone of low-level shear
indicated all along this line (reds and greens fairly close together), and there are some subtle
signatures of couplets of slightly stronger shear or rotation at various locations along this, but the
area that passed over King near the center of these images is no stronger than any of the other ones,
and no other tornadoes were reported anywhere else on this day. These thresholds are well below values
the NWS would typically consider for tornado warnings, even in these strong shear environments, meaning
we either have to issue many tornado warnings in a situation like this, or none at all. Still, we are
working with other offices and NC State University to examine a database of similar kinds of high-shear,
low instability events to learn if there are any more subtle indicators of what may ultimately lead to
tornadoes in these kinds of situations, and what subtle indicators may be used to differentiate between
which shallow cells are more likely to produce a brief tornado.
Figure 4: 2131Z Radar Image from September 21st, 2013 (Loop)
Figure 5: Tornado Damage Path on the west side of King (U.S. 52 is the highway on the right side of the image).
Figure 7: Damage to a cornfield, and beyond, what was left inside a barn, with trees snapped in the distance.
LOCATION...KING IN STOKES COUNTY NORTH CAROLINA DATE...09/21/2013 ESTIMATED TIME...558 PM EDT MAXIMUM EF-SCALE RATING...EF1 ESTIMATED MAXIMUM WIND SPEED...90 MPH MAXIMUM PATH WIDTH...90 YARDS PATH LENGTH...0.61 MILES * FATALITIES...0 * INJURIES...0 * THE INFORMATION IN THIS STATEMENT IS PRELIMINARY AND SUBJECT TO CHANGE PENDING FINAL REVIEW OF THE EVENT(S) AND PUBLICATION IN NWS STORM DATA. ...SUMMARY... THE NATIONAL WEATHER SERVICE IN BLACKSBURG VA HAS CONFIRMED A TORNADO OCCURRED NEAR KING IN STOKES COUNTY NORTH CAROLINA ON SEPTEMBER 21ST 2013. THE TORNADO INITIALLY TOUCHED DOWN NEAR THE STOKES AND FORSYTH COUNTY LINES ALONG NEWSOME ROAD...AND DAMAGED A SUNROOM...KNOCKING DOWN THE EXTERIOR WALLS TO THE NORTHWEST. SEVERAL TREES WERE ALSO SNAPPED OFF IN THIS AREA. FROM THIS POINT...THE TORNADO MOVED TO THE NORTHEAST...COMPLETELY DESTROYING A BARN AND UPROOTING SEVERAL TREES NEAR KATES FARM ROAD AND INDUSTRIAL DRIVE. THE TORNADO CONTINUED TO MOVE IN A NORTHEASTERLY DIRECTION FOR AN ADDITIONAL THIRD OF A MILE...CROSSING HIGHWAY 52...DAMAGING THE GARAGE DOOR OF AN INDUSTRIAL OUTBUILDING AND DAMAGED SEVERAL MORE TREES. THE TORNADO LIFTED NEAR CAMPBELL DRIVE. IN TOTAL THREE STRUCTURES WERE DAMAGED BY THIS TORNADO. THE NATIONAL WEATHER SERVICE WOULD LIKE TO THANK STOKES COUNTY EMERGENCY MANAGEMENT...AND THE CITY OF KING POLICE AND FIRE DEPARTMENTS FOR THEIR ASSISTANCE IN COMPLETING THIS DAMAGE SURVEY. THIS INFORMATION CAN ALSO BE FOUND ON OUR WEBSITE AT WEATHER.GOV/RNK. FOR REFERENCE...THE ENHANCED FUJITA SCALE CLASSIFIES TORNADOES INTO THE FOLLOWING CATEGORIES: EF0...WIND SPEEDS 65 TO 85 MPH. EF1...WIND SPEEDS 86 TO 110 MPH. EF2...WIND SPEEDS 111 TO 135 MPH. EF3...WIND SPEEDS 136 TO 165 MPH. EF4...WIND SPEEDS 166 TO 200 MPH. EF5...WIND SPEEDS GREATER THAN 200 MPH.