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Analyzing the Roles of Low-Level Forcing and Instability in Significant Severe Weather Outbreaks in the Eastern United States

Neil A. StuartNOAA/NWS Albany NY

NROW XVI9 November 2015

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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

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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

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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 ϴe)

• Upper jet structure – 250 hPa wind maxima

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Cases studied

• 41 significant severe weather events listed in Banacos and Ekster (2010) and Stuart (2012)

• 9 outlier events between 1973 and 2015

• 7 forecast busts from 2001 through 2015

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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

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

June 2015 forecast bustUpper jet a bit too far displaced to the northwest of the Northeastern U.S.

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Next – 850 hPa parameters – wind cores exit east of the Appalachians

June 1953 – Worcester, MA F4 tornado

April 2011 – Mid-Atlantic multiple EF2+ tornadoes (1st of 2 outbreaks in April 2011)

Northeastern U.S. Composite of 7 events

Mid Atlantic U.S. Composite of 13 events

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Next – Instability – values exceeding -2 C during period of maximum instability

May 1985 – PA/OH multiple F2+ tornadoes

April 2002 – LaPlata, MD F4 tornado

Northeastern U.S. Composite of 7 events

Mid-Atlantic U.S. Composite of 14 events

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

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

325-330K300-305K 330-335K305-310K

July 1989 – Multiple F2+ tornadoes NY and New England

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Special case of Progressive Derechos – 850 hPa winds - 29 June 2012 – Confluence and initiation in IA/IL/WI - region of tightest 850 hPa ϴe gradient

Extreme instability in region of initiation and downstream through OH valley and mid-Atlantic region

Composite of 7 progressive derechos that affected the eastern U.S.

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Forecast Busts – 1st – May 2003

Low-level jet core never crossed the mountains

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

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Forecast Busts – NY and New England8 June 2015

Low-level jet core right around 35 kt threshold

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

Marginal instability

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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 ϴe gradient below the 25K threshold

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Outliers – Significant severe weather with weak forcing – NARR AnalysesOctober 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 ϴe gradient below the 25K threshold

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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 ϴe gradient did not meet the 25K threshold in both events (not shown)

28 July 2014 – Revere, MA EF2 tornado - NARR

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Outliers – Significant severe weather with weak forcing – NCEP/NCAR vs. NARR

28 July 2014 – Revere, MA EF2 tornado – NCEP/NCAR

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

28 July 2014 – Revere, MA EF2 tornado - NARR

4 Layer LI peaks at -3 C 4 Layer LI peaks at -6 C

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A first look at Historical SoundingsHysplit parcel trajectories - 1 June 2011 -

Very similar to 8-9 June 1953

31 May – Willard Bay, MI tornado – EF1 1 June - Springfield, MA tornado – EF3

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A first look at Historical SoundingsHysplit parcel trajectories - 1 June 2011

Springfield, MA tornado - EF3

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A first look at Historical Soundings31 May 2011 – Detroit, MI

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A first look at Historical Soundings1 June 2011 – Albany, NY

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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

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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 ϴe 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

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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 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 ϴe

plots