Updated Radar-Based Techniques for Tornado Warning Guidance in the Northeastern United States Brian J. Frugis & Thomas A. Wasula NOAA/NWS Albany, New York NROW XVI – 4-5 November 2015

Motivation CSTAR study has been completed to update the V-R Shear Technique for tornado warning guidance using 8 bit, high resolution radar data – This was originally done for the area around the Albany CWA in , but has been expanded this year for tornadoes over the entire Northeastern US – Will the updated version of this study help detect tornadoes that were previously difficult to predict?

Motivation (continued) Impact-based warnings require knowledge regarding the estimated strength of the tornado – Ongoing studies have been looking at estimating tornadic strength from the maximum height of tornadic debris from events from the Southeast & Plains, but Northeastern United States tornados have not been examined in this matter. – Although limited due to the small amount of tornadic events, are the results for these Northeastern US Events similar to the findings for other parts of the US?

Original V-R Shear Study (LaPenta et al. 2000) Maximum observed gate-to-gate shear below 3 km was found to be useful in identifying tornadic storms (LaPenta et al. 2000) S=V r /(D*1800) Shear (S) is measured in units of s -1, rotational velocity (V r ) in knots and D is the diameter of which S is calculated in n mi. V m is the maximum strength of the mesocyclone, which can be found using a vertical cross-section over a 3.5 n mi width Radar data was 4 bit and had to be range normalized due to radar resolution and beam spreading

Expanded V-R Shear Study 8-bit radar data from 76 tornadoes from were examined using Gibson Ridge Software’s GR2Analyst. This expanded the dataset that was previously just 41 tornadic events near the ALY CWA that was originally completed in null cases (storms with strong rotation that prompted tornado warnings but didn’t produce a tornado) were also looked at. Shear was calculated for each tornado using a diameter of 0.5 n mi for all tornadoes from within 60 n mi of radar (since super-res data was available). The old methodology for determining D was used for tornadoes from The maximum velocity differential of the mesocyclone (V m ) was calculated for each storm Results of S and V m were compared to the nomogram graphic created by the LaPenta study

Using the V-R Shear Method Rotational Velocity of tornadic couplet from 0.5° SRM measured over 0.5 nm width (about 10 nm from KENX radar located just south- southeast of tornadic couplet) V r =42.5 kts, S= s -1 Cross-Section of SRM over 3.5 nm width of Mesocyclone. Max Strength: 89 kts (V m =Max inbound + Max Outbound of Meso)

Updated V-R Shear Study Nomogram for Entire Northeastern United States (n=76) Group I: No Clear Signal Between Tornadic & Non-Tornadic Mesocyclones; no F2/EF2 or stronger Group II: 71% Tornadic & Mainly Weak Tornadoes Group III: 100% tornadic; mainly F1+/EF1+ Group I: No Clear Signal Between Tornadic & Non-Tornadic Mesocyclones; no F2/EF2 or stronger Group II: 71% Tornadic & Mainly Weak Tornadoes Group III: 100% tornadic; mainly F1+/EF1+

Tornadic Debris Signature Detection (TDS) Methodology (Adapted from Entremont and Lamb 2015) 1)Find a tornadic couplet in SRM with gate-to-gate rotation. 2)Have Correlation Coefficient (CC) less than 0.90 co-located with the couplet. 3)Have Reflectivity (Z) values 35 dBZ or greater co-located with the couplet. 4)Have Differential Reflectivity (ZDR) values around zero co-located with the couplet. 5)If all these criteria are met, check next radar elevation slice above and continue the process until they are not met anymore. The top slice where all criteria is met is the Max TDS Height UTC 22 May 2014 Duanesburg-Delanson, NY EF3 KENX Radar 0.5° Elevation Angle Top-left: SRM (kts), Top-right: CC Bottom-left: Z (dBZ), Bottom-right: ZDR (dB)

Tornadic Debris Signature Statistics Northeastern US Events tornadic events from from the Northeastern United States were examined for the presence of a Tornadic Debris Signature (TDS). Out of those 69 tornadoes, a TDS was present in 24 of them (35%). Of these 24, 12 tornadic events occurred within supercell thunderstorms and 12 were within Quasi-Linear Convective System (QLCS) thunderstorms. All EF2+ strength tornadoes occurring in the Northeastern US between displayed a TDS. EF-0EF-1EF-2EF-3EF-4EF

Maximum Height of TDS Average & Median Values For the 24 events examined between , the average & median values increase with increasing tornadic strength, especially once reaching the EF2 level. This is similar to results found by Entremont and Lamb (2015). More storms will be examined in future years as the dataset expands to see if these trends continue.

Maximum Height of TDS vs. EF Scale Strength vs. Range Weak tornadic events (EF0) were generally only see close to the radar, as debris generally didn’t get very high and beam height rising over distance would prevent these further away storms from being seen. Hard to seen much difference between EF0/EF1 storms with this plot.

Maximum Height of TDS Northeastern US Tornados Box and Whisker Plot EF0/EF1 are similar, but jump up seen with EF2+ events. Entremont and Lamb (2015) also saw a similar signal in their data as well (which contained 181 events).

Normalized Rotation (NROT) NROT is a derived product found in GR2Analyst that attempts to assess rotation for each range bin, regardless of distance from radar Values range from -5 to 5 and are unitless – Software documentation notes that over 1.00 is significant and over 2.50 is extreme NROT display from GR2Analyst from 2247Z 29 May NROT values reached a maximum of just prior to tornado touchdown.

Normalized Rotation (NROT) Data & Analysis 137 tornadic events from from the Northeastern United States were examined using Gibson Ridge’s software GR2Analyst. The value of Normalized Rotation (NROT) was recorded at the onset for each tornadic event. In addition, 42 null events were examined as well. Null events showed enough rotation to warrant a tornado warning, but a tornado was not reported.

Normalized Rotation (NROT) As Compared to Storm Strength

Normalized Rotation (NROT) Null vs. Tornadic Events

Conclusions: Expanded V-R Shear Study The updated V-R Shear technique shows similar results as the previous study done by LaPenta et al. in 2000, but now accounts for the improvements in radar resolution. – Expanded dataset over entire Northeastern US helps increase confidence in study results Weak Tornadoes (EF0/EF1) continue to be difficult to detect via radar – Expanded V-R Shear Study still shows little skill differentiating between null mesocyclones & weak tornadoes Strong tornadoes for the Northeast (EF2+) show a strong correlation between increasing amounts of low-level shear and the increased strength within the mid-level mesocyclone

Conclusions: Northeastern US TDS Study As also shown by Entremont and Lamb 2015, the higher Max TDS Heights are correlated with an increase in tornado strength. Most pronounced jump in Max TDS height occurs with transition between weak (EF0/EF1) to strong tornados (EF2+) Small amounts of tornados since launch of Dual- Polarization are a limiting factor, but results are similar to larger study by Entremont and Lamb

Conclusions: Normalized Rotation (NROT) NROT is not always a clear discriminator on its own for tornadic vs. non-tornadic storms. There also isn’t a strong signal seen in increasing tornadic strength with increasing NROT values either. NROT could be useful if combined with other mesoscale information – WFO BGM has done work regarding this & has shown this at previous NROW meetings

Questions?