Halloween Night HSLC Tornadic QLCS – A SHERB Success!

All,

During the late evening of Halloween 2013, the Ohio Valley experienced a fairly widespread and typical HSLC tornadic QLCS between 02Z and 06Z. Forward speed of the convective system was on the order of 65 mph, and the narrow convective line became more organized with time across the state of Ohio. There were four (4) confirmed tornadoes in the WFO ILN CWA, with widespread wind damage. Measured wind gusts were mostly between 50 and 60 mph at ASOS/AWOS sites across the area as the line passed, which caused considerable tree and minor structural damage.

The environment was one characterized by extremely high shear attendant to a 90kt southwesterly low level jet which was centered near 825mb. With 925mb flow around 50kts, this produced low level SRH (0-1km) in excess of 600 m2/s2.  Below are some screen captures of the SPC Mesoanalysis data of the 0-1km SRH, and SBCAPE valid as the line approaching Ohio from eastern Indiana.  Instability was negligible, in the ambient environment ahead of the convective line, but the narrow ribbon of 100-200 J/kg of surface-based buoyancy did develop into Ohio in front of the convective line.

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The 01.00Z KILN sounding, taken just a couple of hours before warnings were issued for eastern Indiana/western Ohio confirmed the very high shear/low instability airmass in place.

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The regional radar mosaic imagery valid at 0336Z is shown below. Note that no lightning strikes were present along the convective line owing to the meager/shallow instability.

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The radar presentation of the convective line was particularly interesting from a FAA (TDWR) perspective. In particular, the Dayton (TDAY) and Columbus (TCMH) radars picked up numerous brief/shallow velocity couplets – most lasting on the order of 3-5 minutes. There were a large number of couplets that did not produce tornadic damage, and review of the TDAY data (image below valid at 0357Z) argues that a radar operator could have easily issued 8-10 simultaneous tornado warnings for these brief/fleeting entities.

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It is important to note that most of these couplets (mostly located on the gust front cusp) were not sampled well by the KILN WSR-88D – instead being shown as marginally enhanced “shear zones” in different locations in some instances from the TDWR along the convective line.  The image below (valid at 0344Z) is a 0.5 SRM image from the KILN WSR-88D.

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While the radar/warning perspective continues to be a significant challenge for operations, the awareness of this event’s potential to produce wind damage and QLCS spinup tornadoes was greatly enhanced by the recently implemented SHERBS3 parameter at WFO ILN.  For 4-5 days prior to this event, the SHERB parameter across a wide platform of NWP solutions (ECMWF, GFS, NAM) indicated a high likelihood of an environment supporting wind damage/spinup tornadoes over the Ohio Valley. The signal was strong and stable particularly among the deterministic ECMWF and GFS models, which both showed SHERB values increasing in coverage and magnitude to values over 1.0 as the night wore on across the Ohio Valley. This is likely tied to the bombing low pressure system over southern Michigan into Ontario, and the lapse rate reduction and low level shear increase in response to that process.

Below is a comparison of NAM (left)/ECMWF (right) SHERB parameter data valid at 01.06Z three days prior to the event (forecast hour 66).  This is similar to data that had been presented in previous days with a SHERB maximum in/near Indiana/Ohio.

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At two days out (below – forecast hour 54)  the signal continues to focus on Indiana/Ohio and increase in agreement/magnitude. (Keep in mind the GFS – not shown – was showing a very similar signal.)

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At day 1 (below – forecast hour 30) the signal repeats, giving confidence to forecasters.

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These clues led to WFO ILN awareness and success in preparing for the event with increased staffing – well above what would normally be in place on a midnight shift given SPC Day 1 Outlooks of 15% Wind/2% Tornado probabilities.  Decision Support Services were ramped up to the State of Ohio and County EMA partners two days in advance on an event that typically would not generate that level of internal WFO response.  When coupled with CIPS Analog data that suggested the potential of an organized/large severe thunderstorm event, the SHERB data was instrumental in raising awareness to the scope/magnitude of the event when traditional severe weather forecasts (SPC Outlooks, CAPE, etc) were not as suggestive of a higher-end event.

Special thanks to the NC State / NWS High Shear Low CAPE CSTAR group that has performed valuable and important research in this area, to WDTB for recent training delivered to the WFOs, to Dan Baumgardt (SOO ARX) for providing code for AWIPS implementation, and to Jonathan Blaes for an invitation to post on this forum. Please send any questions to seth.binau@noaa.gov    — Seth Binau (SOO ILN)

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