Differences Between High Shear / Low CAPE Environments in the Northeast US Favoring Straight-Line Damaging Winds vs Tornadoes Michael E. Main, Ross A. Lazear, Lance F. Bosart Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York Northeast Regional Operational Workshop 7 November 2018

Motivation HSLC severe weather is common in the Northeast US High shear / low CAPE (HSLC) severe weather environments are not as obvious as cases with higher CAPE Tornadoes and straight-line damaging winds (SDW) can pose a threat to life and property Little research has been done to distinguish between tornadic vs SDW events in HSLC environments

Background Northeast US HSLC environments are defined as having: Surface based CAPE (SBCAPE) ≤ 500 J kg-1 (Guyer and Dean 2010) Mixed layer CAPE (MLCAPE) ≤ 1000 J kg-1 (Schneider et al. 2006) Most unstable parcel CAPE (MUCAPE) ≤ 1000 J kg-1 (Sherburn and Parker 2014) 0-6 km wind shear magnitude ≥ 18 m s-1 (Schneider et al. 2006) HSLC reports often occur at the beginning or end of an otherwise higher CAPE event (Dean et al. 2008)

Background Sherburn and Parker (2014) also found that HSLC environments typically have: Low instability Low lifted condensation levels (LCLs) High magnitudes of shear in the lower levels of the atmosphere Quasi-linear convective systems (QLCSs) as the predominant storm mode Sherburn and Parker (2014) looked at tornadic vs null events, and found low-level and mid-level lapse rates to be the best at discriminating between the two

Methodology 80 km 40 km 1-2 h 0-2 h Events from 1 June 2007 – 1 June 2017 were examined Storm reports were taken from NCEP’s Storm Event Database Mesoanalysis archive from redteamwx.com used to give a general idea of reports fitting the HSLC criteria Archived mesoanalysis gridded files obtained from SPC and used to determine exact values for each parameter Environments were sampled in accordance with the “Goldilocks Zone” (Potvin et al. 2010)

Methodology Warm season (April – September) and cool season (October –March) were examined separately Four storm modes were identified: Discrete cells Cellular clusters QLCS events QLCSD events (discrete cells embedded in a QLCS) The mean for each parameter for each event was used for the box and whisker plots A two-tailed T-test was used to test for statistically significant results.

Results

Results Number of Events

Results Effective shear definition (SPC): The magnitude of the vector wind difference from the effective inflow base upward to 50% of the equilibrium level height for the most unstable parcel in the lowest 300 mb. 

Warm Season wind vs wind + tornado: p = .033 wind vs tornado only: p =.0039 wind vs all tornado: p = .00063

Warm Season wind vs wind + tornado: p = 5.4x10-7 wind vs tornado only: p = .0045 wind vs all tornado: p = .00017

Cool Season wind vs wind + tornado: p = .016 wind vs all tornado: p = .072

Cool Season wind vs wind + tornado: p = .91 wind vs all tornado: p = .039

Cool Season 23 OCT 20Z PVD 29 OCT 11Z GON 29 OCT 12Z FMH

Most Significant Parameters

Summary Conventional parameters (i.e. 0–1-km shear magnitude) may not be the most useful in discriminating between HSLC environments in the Northeast which favor SDW events vs those that also support tornadoes. LCLs appear to be the best parameter at discriminating between warm season events with and without tornadoes, while effective shear magnitude is the most skillful parameter for cool season events. There are several other parameters which are statistically different for HSLC environments that favor SDW events vs tornadic events Results appear to be supported by recent HSLC tornadic events

Future Research Evaluate results in real time Expand the dataset Analyze HSLC severe reports vs HSLC environments without severe reports

Acknowledgements Ross Lazear and Lance Bosart, thesis advisors Andy Dean, Storm Prediction Center (SPC) Brian Carcione, Huntsville, AL National Weather Service (NWS)

References Dean, A. R., and R. S. Schneider, 2008: Forecast challenges at the NWS Storm Prediction Center relating to the frequency of favorable severe storm environments. Preprints, 24th Conf. on Severe Local Storms, Savannah, GA, Amer. Meteor. Soc., 9A.2. Guyer, J. L., and A. R. Dean, 2010: Tornadoes within weak CAPE environments across the continental United States. Preprints, 25th Conf. on Severe Local Storms, Denver, CO, Amer. Meteor. Soc., 1.5. Potvin, C. K., K. L. Elmore, and S. J. Weiss, 2010: Assessing the impacts of proximity sounding criteria on the climatology of significant tornado environments. Wea. Forecasting, 25, 921– 930, doi:10.1175/2010WAF2222368.1. Sherburn, K. D., and M. D. Parker, 2014: Climatology and ingredients of significant severe convection in high-shear, low-CAPE environments. Wea. Forecasting, 29, 854–877, doi:10.1175/WAF-D-13-00041.1 Schneider, R. S., and A. R. Dean, 2008: A comprehensive 5-year severe storm environment climatology for the continental United States. Preprints, 24th Conf. on Severe Local Storms, Savannah, GA, Amer. Meteor. Soc., 16A.4. Schneider, R. S., A. R. Dean, S. J. Weiss, and P. D. Bothwell, 2006: Analysis of estimated environments for 2004 and 2005 severe convective storm reports. Preprints, 23rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc., 3.5. Vaughan, M. T., Tang, B. H., & Bosart, L. F. (2015). Climatology and Analysis of High-Impact, Low Predictive Skill Severe Weather Events in the Northeast United States. Weather and Forecasting, 32, 1903–1919. doi:10.1175/waf-d-17-0044.1.

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