Debris Signature with Isolated Tornadic Supercell adjacent to KRAX on March 29, 2014

All,

The severe weather event that unfolded in central NC during the evening hours of March 29, 2014 was notable for several reasons.  Although diurnal destabilization in central NC was severely limited by widespread cloud cover, intermittent showers, and weak mid-level lapse rates during peak heating, strong synoptic ascent /layer-lifting/ in the presence of a convectively unstable airmass (associated with a drier mid-level airmass advecting atop an unseasonably moist boundary layer) resulted in an atypical thermodynamic setup in which marginal to moderate destabilization (500-1000 J/kg mlcape) occurred after peak heating during the early evening hours. This in of itself is worth another post, as I think it highlights some of the inherent limitations in convective parameters and the necessity of performing a thorough environmental analysis using data from a variety of remote sensing platforms, observations, and short-term model guidance.

The animated GIF below shows a 4-panel loop of REF, SRM, VEL and CC from 0.5 to 2.4 degrees of the 0111Z volume scan from KRAX.  This is an excellent example of a subtle tornadic debris signature associated with a supercell observed within several miles of an 88D.  Note that the ‘hole’ in CC (values 0.60 to 0.90) is co-located precisely with both the velocity couplet and the reflectivity ‘ball’ within the hook.  Vertical continuity (including a down-shear tilt) of this feature from 0.5-0.9-1.3-2.4 degrees and the lack of such a feature prior to when the supercell arrived or after it departed provide compelling evidence for a tornadic debris signature.  A weak (EF-0) tornado was indeed confirmed at this precise location/time in southeastern Wake county (see damage survey PNS below).

0.5,0.9,1.3,2.4 degree 4-Panel Loop of REF,SRM,VEL,CC at 0.5,0.9,1.3,1.8 degrees on the 0111Z March 30, 2014 Volume Scan at KRAX

0.5,0.9,1.3,2.4 degree 4-Panel Loop of REF,SRM,VEL,CC at 0.5,0.9,1.3,1.8 degrees on the 0111Z March 30, 2014 Volume Scan at KRAX

In case the aforementioned radar loop did not attach properly, you can also find it at:
http://www.redteamwx.com/2014_03_30_0111_to_0113Z_0.5_to_2.4.gif

March 30, 2014 0111Z 1.3 deg 4-Panel from KRAX with subtle tornadic debris signature

March 30, 2014 0111Z 1.3 deg 4-Panel from KRAX with subtle tornadic debris signature

...TORNADO CONFIRMED NEAR GARNER IN WAKE COUNTY NORTH CAROLINA...

LOCATION...GARNER IN WAKE COUNTY NORTH CAROLINA
DATE...03/29/2014
ESTIMATED TIME...0908 PM EDT
MAXIMUM EF-SCALE RATING...EF0
ESTIMATED MAXIMUM WIND SPEED...80 MPH
MAXIMUM PATH WIDTH...75 YDS
PATH LENGTH...1/8TH OF A MILE
* 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 RALEIGH NC HAS CONFIRMED A
TORNADO NEAR GARNER IN WAKE COUNTY NORTH CAROLINA ON 03/29/2014.

PER EYEWITNESS AND A DAMAGE SURVEY...A WEAK EF0 TORNADO OCCURRED
SHORTLY AFTER 9 PM AT THE INTERSECTION OF TREBOR DRIVE AND CROSS
POINTE LANE. A RESIDENCE SUSTAINED PARTIAL ROOF DAMAGE WITH A
COUPLE OF WINDOWS BROKEN AND A COUPLE OF TREES DOWN.

 

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About bvincentnws

Meteorologist National Weather Service 2003 - Current
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5 Responses to Debris Signature with Isolated Tornadic Supercell adjacent to KRAX on March 29, 2014

  1. I am commenting mostly so that I receive word of any follow ups, but also curious. Your post briefly refers to downshear tilt of the vortex; given such rapid forward motion, is this partly a product of the time delay between scans? I am very interested in the question of the structure of vertical vorticity in these high shear environments.

    • bvincentnws says:

      Matt,

      Excellent question. The radar loop consisted of 4 elevation angles (0.5, 0.9, 1.3, 1.8, 2.4) from the same volume scan, spanning a duration of 1 minute and 57 seconds with the first frame (0.5 deg) beginning at 09:11:30 EDT and the last frame (2.4 deg) ending at 09:13:27. I considered the possibility that any apparent updraft tilt was an artifact of storm motion, however, given the short duration of the loop (1 minute and 57 seconds) and that the best storm motion I could come up with was from 225 degrees at 23 knots (26 mph), I think I dismissed the notion that forward motion significantly contributed to the apparent updraft tilt.

      Fortunately, we can quantitatively assess the contribution of forward motion to apparent updraft tilt and compare that to the actual updraft displacement.

      Duration of Loop:
      1min 57s, or 117 seconds, or 0.0325 hours

      Storm Motion:
      From 225 degrees (SW) at 23 knots (26 mph)

      Apparent Tilt Expected Due to Storm Motion:
      26 mph x 0.0325 hours = 0.84 miles NE

      Updraft Tilt from 0.5 to 2.4 deg (350-1500 ft AGL) as measured via GR2A:
      0.75-0.80 nautical miles or 0.86-0.92 miles

      Assuming my math/logic are correct (please double check me), the above calculations suggest I made a pretty bad assumption 🙂 The apparent tilt due to storm motion (0.84 miles NE) compared to the observed tilt (0.86-0.92 miles) indicates that the observed tilt was almost (if not entirely) an artifact of storm motion.

      Is this consistent with what you’ve calculated or observed, i.e. little if any updraft tilt is present in the lowest 1500 ft AGL? All else being equal, is the updraft tilt in supercells most prominent in the mid-levels?

      -Brandon V.

      • Matt Parker says:

        Thanks, Brandon! You’ve put an impressive amount of work into both the original post and this reply. I’m not sure if Jason Davis still reads this blog, but I think he was finding a similar amount of displacement with height that was attributable to the movement of HSLC vortices. I’m not sure what the “right” amount of tilt is for these vortices, but I think it makes sense for them to be rather upright in the lowest levels (and quite serendipitously, due to proximity, this case provided an unusually good view of the lowest levels).

  2. skeighton says:

    Great case to share Brandon…thank you! Fairly subtle signature, but compelling nonetheless. Given the small amount of damage, I wonder at what range from the radar the low CC signature would have no longer shown up? This was only a few nm from the radar? With the KFCX radar sitting as high as it is, I wonder if we’d EVER see something like this for a brief EF0 unless it is just a few miles from the radar in Floyd County (very rare climatologically). Anything east of the Blue Ridge would have to be a pretty strong tornado to kick up debris high enough to get a signature like this for us. Kind of hope we never have an example, but certainly curious to see what it would take to get one. Again, great post!

    • bvincentnws says:

      Appreciate the feedback, Steve! Given the weak/brief (EF-0) nature of the tornado and the relatively small amount of debris lofted to an altitude of no more than 1500 ft agl (as best as I can tell), I doubt this feature would have been observable at a distance greater than ~20 nm from the radar, the minimum distance from the RDA where the 0.5 deg elevation angle would overshoot 1500 ft agl. Increasing beam width may also have masked the TDS at a further distance from the RDA due to insufficient horizontal resolution to resolve debris.

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