Identifying key features to predict significant severe weather outbreaks in

the northeastern U.S.

Neil A. StuartNOAA/NWS Albany NY

NROW XIII3 November 2011

Inspiration for this study

• Study of mid-Atlantic significant severe weather outbreaks presented at SLS 2004

• Focus on comparison/contrast of significant severe weather outbreaks and forecast busts

• Recent 2 outbreaks in April in the Carolinas and Virginia and 1 June outbreak in the northeastern U.S.

• Similarities between mid-Atlantic and northeastern U.S. significant severe weather outbreaks and forecast busts

Definitions• Significant severe weather outbreak

– Large hail of ≥ 2 Inches – Winds of ≥ 65 MPH– Tornadoes ≥ F2/EF2

• Forecast bust – Moderate Risk indicated in SPC severe weather outlook– 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

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 ϴe gradients – accounts for temperature and moisture

• 850 hPa wind maxima – maximum low-level jet energy

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

• Upper jet structure – 250 hPa wind maxima

Cases studied

Cases studied

Cases studied

Let’s start with 500 hPa heightsApril 2002 LaPlata, MD F4 tornado- Progressive eastward moving - Crossed well east of Appalachians in 24

hours

May 2003 forecast bust- Very little eastward movement- Just about reached the Appalachian

Mountains in 24 hours

Let’s start with 500 hPa heightsJune 2011 – Springfield, MA EF3 tornado- Progressive eastward moving - Crossed well east of Appalachians in 24

hours

June 2010 forecast bust- Very little eastward movement- Just about reached the Appalachian

Mountains in 24 hours

Next – 250 hPa windsMay 2002 mid-Atlantic widespread winds and large hail- Mid-Atlantic in right entrance region of strong upper jet

May 2003 forecast bustEastern U.S. not in favored region of much weaker upper jet

Next – 250 hPa windsJune 2011 – Springfield, MA EF3 tornado- Northeastern U.S. in right entrance region of cyclonically-curved relatively weak upper jet

June 2010 forecast bustNortheastern U.S. not in favored region of relatively strong upper jet

Next – 850 hPa parameters - winds

April 2002 – LaPlata, MD EF4 tornadoApril 2011 – Mid-Atlantic multiple EF2+ tornadoes (1st of 2 outbreaks in April 2011)

Note wind cores exit east of the Appalachian Mountains

Next – 850 hPa parameters - winds

June 1953 – Worcester, MA F4 tornadoJune 2011 – Springfield, MA EF3 tornado

Note wind cores exit east of the Appalachian Mountains

Next – 850 hPa parameters - ϴe March 1984 – Multiple F2+ tornadoes in NC/VA

April 2011 – Multiple F2+ tornadoes NC/VA (2nd outbreak of 2011)

Note Δϴe of ≥ 25K over the mid-Atlantic region associated with a tight ϴe gradient

Next – 850 hPa parameters - ϴe May 1985 – Multiple F2+ tornadoes in OH/PA

July 1989 – Multiple F2+ tornadoes NY and New England

Note Δϴe of ≥ 25K over the northeastern U.S. associated with a tight ϴe gradient

Next - InstabilityMay 1995 – Great Barrington, MA F4 tornado

May 1998 – Mechanicville, NY F3 tornado

Note Lifted Indices exceeding -2 over the northeastern U.S. during the period of maximum instability

Next - InstabilityMay 1998 – Mechanicville, NY F3 tornado

November 1992 – Multiple F2+ tornadoes NC/VA

Note Lifted Indices exceeding -2 over the northeastern U.S. (left) and mid-Atlantic (right) during the period of maximum instability

Forecast Busts – 1st – May 2003

Low-level jet core never crossed the mountains

Low-level ϴe gradient outran the low-level jet core

Forecast Busts – 2nd – May 2004

Low-level jet core weak and never crossed the mountains

Low-level ϴe gradient weakened and outran the low-level jet core

Forecast Busts – 3rd – June 2010

Low-level jet core right at the 35 kt threshold

Low-level ϴe gradient well below the ≥ 25K threshold

Outliers – Significant severe weather with weak forcing

August 1973 – Stockbridge, MA F4 tornado

Lack of low-level wind core

Lack of low-level ϴe gradient

Impressive instability

Outliers – Significant severe weather with weak forcing

October 1979 – Windsor Locks F4 tornado

Low-level wind core close to the 35 kt threshold

Marginal instability

Low-level ϴe gradient below the 25K threshold

Outliers – Significant severe weather with weak forcing

August 1993 – Petersburg, VA F4 tornado

July 2003 – Multiple F2+ tornadoes in NY/PA

Low-level wind core well south

Low-level wind core met 35 kt threshold

Low-level ϴe gradient did not meet the 25K threshold in both events

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 ϴe gradient of 25K tracks through the region in 24 hours• 4 layer Lifted Index exceeding -2

Caveats

• 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

• Analysis of 850 hPa winds and ϴe can be subjective depending on if it is based on observations or Global Reanalysis sets

Future Work• Resolving the different analyses of low-level features and instability

between observations and Global Reanalysis• Historical RAOB data to analyze CAPE, shear and elevated mixed

layers– Source of Skew-T analyses important as different sources show different

CAPE, shear and midlevel lapse rates• HYSPLIT 24 hour Parcel trajectories - 1500m (~850 hPa), 3000m

(~700 hPa) and 5000m (~500 hPa) to investigate advection of elevated mixed layers, rising and sinking motions

Thank you for your time – Questions, comments or suggestions?

June 1953 Worcester, MA - Courtesy Worcester Telegram and Gazette May 1985 - 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 1976 - New York City, NY

June 2011 Windsor, MA - 4” hail June 2011 Shaftsbury, VT - 3” hail

Acknowledgments:Thanks to NCEP NOMADS for Global Reanalysis wind and Lifted Index plots, Storm Prediction Center for Upper Plots and Plymouth State archive for Global Reanalysis and observed ϴe plots