**UPDATE** Possible Explanation of the Disassociation of a TDS from a Rotational Signature

***Just a few notes before we continue. First, This post is purely based off radar data only. At the time this was written, there were no confirmed sightings of a tornado. Second, I decided to write this because someone asked a question and I was curious to see what the answer was, or most likely was.***

****UPDATE 5-16-2014: Well NWS Raleigh did their damage survey of this storm today. An EF-1 tornado was confirmed, but not in the area I thought it would be. Unlike what I said below, the TDS was associated with the tornado as it was happening and not a delayed reaction like I thought. Either way i hope those of you who did read this enjoyed it. I have attached the PUBLIC INFORMATION STATEMENT below.****

The origin of this post begins on facebook. Someone reported seeing a possible TDS (Tornadic Debris Signature) on the KRAX radar. The time stamp of the image was 2214z. Someone made note of the face that the TDS was not directly associated with the rotational couplet on SRV (Storm Relative Velocity).

Below is my attempt at finding out why the TDS was not associated with the velocity couplet seen on the original image post. The 4 panel images below show Reflectivity (Z), SRV, KDP, and CC.

2149z

At 2149z the rotation that caused the TDS can bee seen east of Fearrington. In CC low values can bee seen that range from .99-.88. This is most likely due to heavy rain, which is supported when looking at KDP, which had values from 2.95*-1.40*/km.

At 2158, the rotation appears to reach its strongest point as it crosses I-40. CC and KDP appear to remain at the same levels as they did back at 2149.

2209z

At 2209 the rotation appears to weaken and spread out. It also appears at this time in SVR that a new rotation is trying to form. In CC we see out first signs of a TDS with a local minimum of 0.75. In KDP the heavy rain had shifted slightly NW of the rotation.

2214z

Between 2209-2218 the rotation associated with the TDS weakens dramatically and disappears. A new rotation forms just to the NW. In CC the TDS is now clearly visible at 2214, with values as low as 0.61 at 2218. KDP drops below 0.95 near the TDS and is labeled at ND (No Data).

 

2223z

At 2223, the TDS weakens as values rise to 0.84. It also appears to diverge slightly from the original track, appearing to move NW as the new rotation moves NE. By 2228 the TDS is gone.






Using the RHI tool I made a cut across the TDS at 2214 in an attempt to see just how high the debris was lofted. It appears that whatever the debris was, was lofted somewhere between 7000 and 8000 KFT.

2214z

NOTE- The radar beam ranged from 1900-2400 KFT above the ground.

In my opinion, the tornado touchdown was around 2154 and possibly lifted around 2209? This would explain why the TDS was detached from the circulation we see in the above screen caps. The tornado was weak, which is why it took a little while for the debris to be lofted enough to be seen by radar, and also why the debris did not go above 10 KFT.

Again, just my non expert opinion. Hope you all enjoyed....LOL

 ***Again, This post is purely based off radar data only. At the time this was written, there were no confirmed sightings of a tornado. Second, I decided to write this because someone asked a question and I was curious to see what the answer was, or most likely was.***


Some helpful links to learn more about Dual Polarization Radar:

WDTB Dual-Pol Training

Dual-Pol Training Aid 

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE RALEIGH NC
447 PM EDT FRI MAY 16 2014

...TORNADO CONFIRMED APPROXIMATELY 3 MILES SOUTHWEST OF DURHAM IN
DURHAM COUNTY NORTH CAROLINA...

LOCATION...3 SW DURHAM IN DURHAM COUNTY NORTH CAROLINA
DATE...MAY 15 2014
ESTIMATED TIME...610 PM TO 613 PM EDT
MAXIMUM EF- SCALE RATING...EF1
ESTIMATED MAXIMUM WIND SPEED...90 MPH
MAXIMUM PATH WIDTH...150 YARDS
PATH LENGTH...0.8 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 RALEIGH NC HAS CONFIRMED A
TORNADO 3 SW DURHAM IN DURHAM COUNTY NORTH CAROLINA ON MAY 15
2014.

DAMAGE CONSISTED OF DOZENS OF SNAPPED AND UPROOTED TREES...AND
APPROXIMATELY 40 HOMES THAT EXPERIENCED ROOF OR OTHER STRUCTURAL
DAMAGE. MOST OF THE DAMAGE TO THE HOMES WAS CAUSED BY FALLING
TREES AND OTHER DEBRIS IMPACTING THE STRUCTURES. HOWEVER...THERE
WERE AT LEAST A HALF DOZEN HOMES THAT EXPERIENCED MINOR ROOF
DAMAGE CAUSED SOLELY BY THE WIND.

IN ONE CASE...A LARGE OAK TREE WAS UPROOTED...AND IN THE PROCESS
FELL ONTO A HOME AND SUBSEQUENTLY SLICED THROUGH THE ROOF AND AN
EXTERIOR WALL OF THE HOME.

THE EXTENT OF THE HARDWOOD TREE DAMAGE SUGGESTED MAXIMUM SPEEDS OF
90 MPH (EF1) BASED ON THE ENHANCED FUJITA SCALE.

THIS INFORMATION CAN ALSO BE FOUND ON OUR WEBSITE AT
WEATHER.GOV/RAH.