1. Appalachian Lee Troughs and their Association with Severe Thunderstorms Daniel B. Thompson, Lance F. Bosart and Daniel Keyser Department of Atmospheric and Environmental Sciences University at Albany/SUNY, Albany, NY 12222 Thomas A. Wasula NOAA/NWS, Albany, NY Matthew Kramar NOAA/NWS, Sterling, VA 37 th Northeastern Storm Conference, Rutland, VT 3 Mar 2012 NOAA/CSTAR Award # NA01NWS4680002

2. Motivation Region of study: Mid-Atlantic Accurately forecasting location, mode and severity of thunderstorms is important, due to proximity of Eastern Seaboard Region is often characterized by weak forcing and ample instability → Mesoscale boundaries important Sea breeze boundary Outflow boundaries Lee trough

3. Analyze the structure of Appalachian Lee Troughs (ALTs) Construct a climatology of warm-season ALTs Analyze the distribution of severe convection in the Mid-Atlantic –Spatial distribution –Temporal distribution –Characteristic CAPE/shear Objectives

4. Data and Methodology 1.Analyzed 13 cases of ALT events associated with warm-season severe convection ─Sterling, VA (LWX) CWA ─0.5° CFSR (Climate Forecast System Reanalysis) 2.Identified common features and used them as criteria to construct a climatology –May–September, 2000–2009

5. PV = −g(∂θ/∂p)(ζ θ + f) (Static stability)(Absolute vorticity) d(PV)/dt = 0 for adiabatic flow Flow across mountain barrier will subside on lee side –Advects higher θ downward → warming –−g(∂θ/∂p) decreases → ζ θ must increase → low level circulation Adapted from Martin (2006) Appalachians Lee Trough Formation: PV Perspective

6. ALTs – Common Low-Level Features MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10 −5 s −1 ), 10-m winds (barbs, kt) 1800 UTC Composite (n=13)

7. ALTs – Common Low-Level Features MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10 −5 s −1 ), 10-m winds (barbs, kt) 1800 UTC Composite (n=13) Winds orthogonal to mountains

8. ALTs – Common Low-Level Features MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10 −5 s −1 ), 10-m winds (barbs, kt) 1800 UTC Composite (n=13) Winds orthogonal to mountains Thermal ridge

9. ALTs – Common Low-Level Features MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10 −5 s −1 ), 10-m winds (barbs, kt) 1800 UTC Composite (n=13) Winds orthogonal to mountains Thermal ridge Negative thermal vorticity

10. ALTs – Common Low-Level Features 0000 UTC Composite (n=13) MSLP (black, hPa), 1000–850-hPa thickness (fills, dam), thermal vorticity < 0 (white, 10 −5 s −1 ), 10-m winds (barbs, kt) Negative thermal vorticity Winds orthogonal to mountains Thermal ridge

11. Domain for Climatology DOMAIN WIND ZONE ALT ZONE

12. Climatology was based on the following 3 criteria: 1)925-hPa Wind Direction –Checked for wind component directions orthogonal to and downslope of Appalachians –Appalachians in the Mid-Atlantic are oriented ~ 43° right of true north →Satisfactory meteorological wind directions exist between 223° and 43° DOMAIN WIND ZONE ALT ZONE  Criterion: wind direction computed from zonal average of wind components along each 0.5° of latitude within Wind Zone must be between 223° and 43° Methodology for Climatology

13. Climatology was based on the following 3 criteria: 2)MSLP Anomaly –Averaged MSLP along each 0.5° of latitude within domain –Checked for minimum MSLP along each 0.5° of latitude within ALT Zone DOMAIN WIND ZONE ALT ZONE Methodology for Climatology  Criterion: difference of minimum and zonal average MSLP must be less than a threshold value

14. Climatology was based on the following 3 criteria: 3)1000–850-hPa layer-mean temperature anomaly –Averaged 1000–850-hPa layer-mean temperature along each 0.5° of latitude within domain –Checked for maximum 1000–850-hPa layer-mean temperature along each 0.5° of latitude within ALT Zone Methodology for Climatology  Criterion: difference of maximum and zonal average 1000–850-hPa layer-mean temperature must be greater than a threshold value DOMAIN WIND ZONE ALT ZONE

15. The three criteria must be met for six consecutive 0.5° latitudes An algorithm incorporating the three criteria was run for the length of the climatology at 6-h intervals (0000, 0600, 1200 and 1800 UTC) ALTs identified by this algorithm were manually checked for false alarms (e.g. frontal troughs, cyclones, large zonal pressure gradients) Methodology for Climatology

16. Each bubble denotes the percentage of time an ALT is recorded under a particular set of MSLP/temperature anomaly constraints Boxes indicate the criteria adopted as the ALT definition ← Stricter Climatology – Results

17. MSLP anomaly 1°C Climatology – Results Over 75% of ALTs occur in June, July and August

18. MSLP anomaly 1°C Climatology – Results Over 75% of ALTs occur in June, July and August Nearly 66% of ALTs occur at 1800 or 0000 UTC –The seasonal and diurnal heating cycles likely play a role in ALT formation

19. Severe local storm reports were obtained from the NCDC Storm Data publication Included all tornado, severe thunderstorm wind and severe hail (>1”) for May– September, 2000–2009 Storm Reports in the ALT Zone – Data and Methodology ALT ZONE

20. 754 unique days with at least one storm report 199 days with > 20 storm reports Most active day: 13 May 2002 (207) Day = 0400 to 0400 UTC Storm Reports – Daily Distribution

21. Controlling for Dataset Inconsistencies “Clustering” – attempt to control for population bias –Overlay a 0.5° by 0.5° grid box over the domain –If a storm report occurs within a certain grid box on a certain day, that grid box is considered “active” for the day Any subsequent storm reports occurring within the active box are discarded for the day The number of active grid boxes for each day are tallied to measure how widespread the severe weather was on that day

22. Storm Reports – Spatial Distribution CFSR composite of top 10% of severe ALT days. MUCAPE (fills, J/kg) and surface to 500-hPa shear (black, kt) n=706 n=48 Percentage of ALT days with >0 active grid boxes (smoothed)

23. Storm Reports – Spatial Distribution n=706 n=48 Storm report max near D.C. coincides with CAPE/shear maxima NC local max more difficult to explain Percentage of ALT days with >0 active grid boxes (smoothed) CFSR composite of top 10% of severe ALT days. MUCAPE (fills, J/kg) and surface to 500-hPa shear (black, kt)

24. CAPE/Shear at First Daily Storm Report To quantify severe thunderstorm parameters characteristic of ALT Zone, CAPE/shear was calculated at location of first daily storm report Dataset: 32 km NARR (8 analysis times daily) Procedure: –Find location and time of first severe report on a certain day (0400–0359 UTC) –Calculate MUCAPE and Sfc–500-hPa shear at location of storm report using nearest analysis time at least 30 min prior to storm report

25. CAPE/Shear at First Daily Storm Report Only included days in which first storm report occurred between 1530 and 0029 UTC Time of 1 st Daily Storm Report (UTC) Corresponding NARR analysis time (UTC) 1530–18291500 1830–21291800 2130–00292100

26. CAPE/Shear at First Daily Storm Report ALT Zone was divided into sectors to minimize the likelihood of the first daily storm report not being representative of the environment CENTER NORTH SOUTH

27. CAPE/Shear at First Daily Storm Report South sector peaks earlier (1800 UTC) than north sector (2000 UTC) Center sector has flat peak between 1800–2100 UTC NORTH CENTER SOUTH

28. CAPE/Shear at First Daily Storm Report Higher median CAPE (shear) for first daily storm report in south (north) sector Higher shear in north sector is likely because it is nearer to the mean warm- season upper jet Whiskers: 10 th and 90 th percentiles // Box edges: 25 th and 75 th percentiles // Line: median NORTH CENTER SOUTH

29. CAPE/Shear at First Daily Storm Report First daily storm report does not concentrate well in CAPE/shear phase-space

30. CAPE/Shear at First Daily Storm Report No storm reports occurred in this phase-space First daily storm report does not concentrate well in CAPE/shear phase-space

31. CAPE/Shear at First Daily Storm Report CAPE (shear) at first daily storm report maximized in June, July and August (May and September) Whiskers: 10 th and 90 th percentiles // Box edges: 25 th and 75 th percentiles // Line: median

32. ALTs form preferentially during diurnal and seasonal heating maxima Summary – Key Points

33. Distribution of storm reports in ALT Zone varies by latitude –First daily storm report occurs 2 h earlier in south sector compared to north sector Summary – Key Points

34. CAPE and shear at first daily storm report vary by latitude and month –Greater median CAPE (shear) occurs in June, July and August (May and September) South (north) sector GREATER SHEAR GREATER CAPE Jun, Jul, Aug May, Sep