Broken-S signature this morning?

Hi, all – With this high-shear / low-CAPE event starting to wind down, I wanted to post some screen grabs from earlier this morning, including what looks to be a broken-S signature over our northern CWA. Radar imagery from both KRAX and TRDU (terminal Doppler at RDU) showed weak rotation tightening up a bit over western Person county, along a narrow convective band embedded in the larger pre-frontal rain shield. A portion of this band bowed out concurrent with the very brief strengthening of the velocity couplet. The circulation became broad and diffuse once again on the next scan, although the line itself retained a broken-S signature. We have not yet receive any reports of damage, although this area is pretty rural so any light tree damage might go unnoticed. I did grab some longer loops from both KRAX and TRDU for those interested.

-Gail Hartfield, NWS Raleigh

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4 Responses to Broken-S signature this morning?

  1. Chris Wamsley says:


    Interesting images. Not sure if this is a broken S or just a notch from stronger winds on the northern flank of the line. I have to continue to look at past events…but seems unless there is a high reflectivity on the northern flank of the notch, you usually will not see a tornado develop…or the straight line winds are stronger than the actual tornado to make it hard to find. I have been looking at past event over the TN VLY…including the louisville office past storm events. Seems like last night, there were only a few tornado sightings with plenty of notch signatures that never spawned a tornado (or were too weak to find)…granted surveys today will let us know more.

    I think if we can ask them there theories/discoveries…it may help us on our end in the long run on what to warn on and not (tornado wise).


    • nws-pat moore says:


      Interesting comment about not getting tornadogenesis unless there is high reflectivity on the northern (trailing?) segment of the QLCS break. I don’t think we know for sure by what mechanism these weak short-lived tornadoes develop. A rear inflow jet is most probably involved in breaking the QLCS into segments. There should be cyclonic vorticity generated at low levels on the left flank of the rear inflow jet. Could it be that an updraft associated with the northern flank of the notch is stretching this vorticity vertically and spinning up the tornado in those cases? An updraft would also account for the high reflectivity. The lack of high reflectivity might indicate a lack of an updraft, so the vorticity could not be stretched upward.

      • Pat,
        Your conceptual model as to how these weak tornadoes might spin up certainly sounds reasonable. No, we didn’t warn on it, in part because the rotation was rather broad and weak, despite being gate-to-gate and evident at multiple levels. No damage reports were received. The 1310Z four-panel reflectivity did show a small area of higher reflectivity associated with the circulation at 0.5 deg and 0.9 deg, but not at 1.3 deg, although we’ve certainly seen weak spin-ups with shallow updrafts before, and this did occur near the edge of a weak residual stable pool over the northwest Piedmont, where low level shear may have been enhanced (0-1 km SRH was 500-600 m2/s2 here). The mesoanalyses archive is linked below.


  2. nws-pat moore says:


    Well, it certainly looks like there is a funky break happening in the line, even if it is not exactly “S-shaped”. A reflectivity loop would be more revealing. Did you warn on that reflectivity feature? If so, what type of warning? Let us know if there is any late-breaking damage reports.

    We were intently watching the evolution of the QLCS to our west and hoping that it would run out of gas as it made it into our forecast area. Thankfully, it starved for lack of any bouyancy in the environment over the western Carolinas. That was not the case across the Hunstville and Nashville areas. I saw numerous breaks in the QLCS on the KOHX, KHTX, and KJKL radars. The tornadoes in the Nashville area look interesting.

    Did anything happen in the Huntsville area?

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