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transcript
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