Analyzing the Roles of Low-Level Forcing and Instability in Significant Severe Weather Outbreaks in the Eastern United States Neil A. Stuart NOAA/NWS Albany NY NROW XVI 9 November

Definitions Significant severe weather outbreak – Large hail ≥ 2 Inches – Winds ≥ 65 MPH – Tornadoes ≥ F2/EF2 Forecast bust – Moderate Risk indicated in SPC severe weather outlook with isolated non significant severe weather or no severe weather – Tornado Watch with no severe weather Outlier event – Significant severe weather in a weak low-level forcing environment – Significant severe weather observed in a weak instability/shear environment Mid-Atlantic U.S. – North Carolina to Maryland and Delaware Northeastern U.S. – Pennsylvania, New Jersey and New York through New England 2

What we will be looking at Character of cold fronts – key to low-level forcing – Low-level wind shift, dew point boundary and leading edge of cold advection often displaced Low level forcing – Need a parameter that encompasses moisture and temperature – Minimize discontinuities from the friction layer 850 hPa ϴ e gradients – accounts for temperature and moisture 850 hPa wind maxima – maximum low-level jet energy 3

What we will be looking at Instability – 4-layer best lifted index Upper dynamics - Eastward progression of 500 hPa vorticity maxima and attendant low level features (such as 850 hPa winds and ϴ e ) Upper jet structure – 250 hPa wind maxima 4

Cases studied 41 significant severe weather events listed in Banacos and Ekster (2010) and Stuart (2012) 9 outlier events between 1973 and forecast busts from 2001 through

Let’s start with 500 hPa heights June 2011 – Springfield, MA EF3 tornado -Progressive eastward moving -Crossed east of Appalachians in 24 hours June 2015 forecast bust -Very little eastward movement -Just about reached the Appalachian Mountains in 24 hours 6

Next – 250 hPa winds June 2011 – Springfield, MA EF3 tornado - Northeastern U.S. in right entrance region of cyclonically-curved relatively weak upper jet June 2015 forecast bust Upper jet a bit too far displaced to the northwest of the Northeastern U.S. 7

Next – 850 hPa parameters – wind cores exit east of the Appalachians June 1953 – Worcester, MA F4 tornado 8 April 2011 – Mid-Atlantic multiple EF2+ tornadoes (1 st of 2 outbreaks in April 2011) Northeastern U.S. Composite of 7 events Mid Atlantic U.S. Composite of 13 events

Next – Instability – values exceeding -2 C during period of maximum instability May 1985 – PA/OH multiple F2+ tornadoes April 2002 – LaPlata, MD F4 tornado 9 Northeastern U.S. Composite of 7 events Mid-Atlantic U.S. Composite of 14 events

Next – 850 hPa parameters - ϴ e March 1984 – Multiple F2+ tornadoes in NC/VA Note ϴ e gradient of ≥ 25K over the mid-Atlantic region associated with a tight ϴ e gradient K  K K  K July 1989 – Multiple F2+ tornadoes NY and New England

Special case of Progressive Derechos – 850 hPa winds - 29 June 2012 – Confluence and initiation in IA/IL/WI - region of tightest 850 hPa ϴ e gradient Extreme instability in region of initiation and downstream through OH valley and mid-Atlantic region 11 Composite of 7 progressive derechos that affected the eastern U.S.

Forecast Busts – 1 st – May 2003 Low-level jet core never crossed the mountains Low-level ϴ e gradient outran the low-level jet core 12

Forecast Busts – NY and New England 8 June 2015 Low-level jet core right around 35 kt threshold Low-level ϴ e gradient well below the ≥ 25K threshold 13 Marginal instability

Outliers – Significant severe weather with weak forcing – NCEP/NCAR October 1979 – Windsor Locks F4 tornado Low-level wind core close to the 35 kt threshold Marginal instability – 4 Layer LI -0.5 C Low-level ϴ e gradient below the 25K threshold 14

Outliers – Significant severe weather with weak forcing – NARR Analyses October 1979 – Windsor Locks F4 tornado Low-level wind core closer to the 35 kt threshold HIRES NARR – 4 Layer LI near -2 C Low-level ϴ e gradient below the 25K threshold 15

Outliers – Significant severe weather with weak forcing – NCEP/NCAR vs. NARR 28 July 2014 – Revere, MA EF2 tornado – NCEP/NCAR Low-level core 14 m/s -1 southeast of Boston Low-level wind core 17m/s -1 (35 kt) threshold southeast of Boston Low-level ϴ e gradient did not meet the 25K threshold in both events (not shown) July 2014 – Revere, MA EF2 tornado - NARR

Outliers – Significant severe weather with weak forcing – NCEP/NCAR vs. NARR 28 July 2014 – Revere, MA EF2 tornado – NCEP/NCAR Low-level ϴ e gradient did not meet the 25K threshold in both events (not shown) July 2014 – Revere, MA EF2 tornado - NARR 4 Layer LI peaks at -3 C 4 Layer LI peaks at -6 C

A first look at Historical Soundings Hysplit parcel trajectories - 1 June Very similar to 8-9 June May – Willard Bay, MI tornado – EF1 1 June - Springfield, MA tornado – EF3 18

A first look at Historical Soundings Hysplit parcel trajectories - 1 June 2011 Springfield, MA tornado - EF3 19

A first look at Historical Soundings 31 May 2011 – Detroit, MI 20

A first look at Historical Soundings 1 June 2011 – Albany, NY 21

In summary It is not often that the low-level forcing, shear and instability exist simultaneously in one region east of the Appalachian Mountains That is why significant severe weather outbreaks are so rare in the mid- Atlantic and northeastern U.S. Significant severe weather outbreaks occur when all these parameters are present – 500 hPa and 850 hPa systems cross east of the Appalachian Mountains within 24 hours – 500 hPa system may be a well-defined impulse tracking around the periphery of a parent upper low – Favorable upper jet structure – Coincident Low-level features and instability Core of ≥ 35 Kt 850 hPa winds passes through the region 850 hPa ϴ e gradient of 25K tracks through the region in 24 hours 4 layer Lifted Index exceeding -2 22

In summary Each severe weather event is unique – – Weaker tornadoes, < 2” hail and wind damage can occur if one threshold for one parameter is met – Important that 850 hPa features cross east of Appalachian Mountains in 24 hours Outlier significant severe weather events can occur under much weaker atmospheric conditions but are extremely rare Analyses of instability, 850 hPa winds and ϴ e varies depending on the resolution of the data set you are using Real-time parcel trajectories at different levels could be important in tracking EMLs and regions of instability 23

Thank you for your time – Questions, comments or suggestions? June 1953 Worcester, MA - Courtesy Worcester Telegram and Gazette May Hermitage, PA Courtesy Harkphoto.com – Hermitage, PA May 1985 June 2011 Springfield, MA - Courtesy Matt Putzel June 2011 Agawam, MA - Courtesy Paulina Dusza September 2001 University of Maryland - Courtesy Ecampus.com April 2002 LaPlata, MD tornado crossing Chesapeake Bay – Courtesy Johns Hopkins Univ. April 2011 Wilson, NC – Courtesy Steven Hoag April 2008 Suffolk, VA - Courtesy Coastal Carolina Weather Examiner July New York City, NY June 2011 Windsor, MA - 4” hail June 2011 Shaftsbury, VT - 3” hail Acknowledgments: Thanks to NCEP NOMADS and ESRL for Global/NARR Reanalysis wind and Lifted Index plots, Storm Prediction Center for Upper Plots and Plymouth State archive for Global Reanalysis and observed ϴ e plots 24