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Shawn M. Milrad Department of Atmospheric and Oceanic Sciences McGill University

Return of the snowburst : A diagnostic comparison of two recent wintertime convection events in Canada. Shawn M. Milrad Department of Atmospheric and Oceanic Sciences McGill University Montreal, Quebec, Canada Eyad H. Atallah and John R. Gyakum Department of Atmospheric and Oceanic Sciences

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Shawn M. Milrad Department of Atmospheric and Oceanic Sciences McGill University

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  1. Return of the snowburst: A diagnostic comparison of two recent wintertime convection events in Canada Shawn M. Milrad Department of Atmospheric and Oceanic Sciences McGill University Montreal, Quebec, Canada Eyad H. Atallah and John R. Gyakum Department of Atmospheric and Oceanic Sciences McGill University Montreal, Quebec, Canada

  2. Outline • Motivation and Background • Case comparison: Ottawa, ON (28 January 2010) vs. Calgary, AB (3-4 December 2011) • Case overviews • Synoptic-dynamic analysis • Forecasts and warnings • Future work

  3. Motivation/Background Case Overviews Dynamics Forecasts Future Work Motivation • The climatology, dynamics and impacts of lake effect snow squalls (bursts) throughout the U.S. have been well-documented • Great Lakes • Wiggins (1950); Niziol (1987); Niziol et al. (1995) • Great Salt Lake • Steenburgh et al. (2000); Steenburgh and Onton (2001) • Lake Champlain • Payer et al. (2007); Laird et al. .(2009) • Other studies have examined events dynamically related to snow bursts • Thundersnow events (synoptic-scale cyclones) • Market et al. (2002); Market et al. (2006); Crowe et al. (2006) • C0ld-season severe weather • Holle and Watson (1996); Schultz (1999); Hunter et al. (2001); Trapp et al. (2001); van den Broeke et al. (2005); Corfidi et al. (2006)

  4. Motivation/Background Case Overviews Dynamics Forecasts Future Work Motivation Source: UCAR/COMET

  5. Motivation/Background Case Overviews Dynamics Forecasts Future Work Motivation • However, few studies have focused on snow bursts that occur outside lake effect regions • Snow bursts outside of lake effect regions: • Often not associated with a synoptic-scale cyclone • “Cold-season convection” • Can produce rapid-onset whiteout conditions  hazardous to motorists and aviation • Often occur without warning (DeVoir 2004) • Not large enough snow accumulations to meet NWS warning or advisory criteria

  6. Motivation/Background Case Overviews Dynamics Forecasts Future Work Previous Work • Nicosia et al. (2009) • Impacts in Pennsylvania

  7. Motivation/Background Case Overviews Dynamics Forecasts Future Work Ottawa, Ontario: 28 January 2010 (Photos are courtesy of the Ottawa Citizen)

  8. Motivation/Background Case Overviews Dynamics Forecasts Future Work Calgary, AB: 3 December 2011 (Photo is courtesy of the Calgary Sun)

  9. Motivation/Background Case Overviews Dynamics Forecasts Future Work Ottawa: 28 January 2010 • Small snow accumulation: 3.6 cm (1.4 in.) • Near-zero visibility • Dynamics • Synoptic-scale forcing for ascent (Q-vector convergence) • Mesoscale forcing for ascent: Arctic front • Just enough moisture to create a problem • Thermodynamics • Very steep low-level lapse rates • Convective instability (CI) and Conditional Symmetric Instability (CSI) • Soundings: Deep (300-400 hPa) Moist Absolutely Unstable Layer (MAUL)

  10. Motivation/Background Case Overviews Dynamics Forecasts Future Work Ottawa: 28 January 2010 Ottawa, ON (CYOW): 28 January 2010 t=0 h (1800 UTC) ***Ottawa International Airport (CYOW): black star***

  11. Motivation/Background Case Overviews Dynamics Forecasts Future Work Calgary: 3-4 December 2011 • Larger snow accumulation: 8.9 cm (3.5 in.): Below snowfall warning criterion • 6.1 cm on 3 December • 2.8 cm on 4 December • Near-zero visibility • Dynamics and Thermodynamics • Similarities and differences?

  12. Motivation/Background Case Overviews Dynamics Forecasts Future Work Calgary: 3-4 December 2011 t=+1 h (0110 UTC) t=-1 h (2310 UTC) ***Calgary International Airport (CYYC): black star***

  13. Motivation/Background Case Overviews Dynamics Forecasts Future Work Calgary: Meteograms Calgary, AB (CYYC): 4 December 2011 Calgary, AB (CYYC): 3 December 2011

  14. Motivation/Background Case Overviews Dynamics Forecasts Future Work Dynamic Analysis: Strategy • Snow bursts are essentially a form of wintertime moist convection • Ingredients-based methodology: Moist convection • Doswell et al. (1996); Schultz and Schumacher (1999); Wetzel and Martin (2001) • Three main ingredients • Lift (synoptic-scale and mesoscale)** • Moisture • Instability** • Convective (CI): (dθe/dz) < 0 • Conditional Symmetric (CSI): MPV*g < 0

  15. Motivation/Background Case Overviews Dynamics Forecasts Future Work Lift: Synoptic-scale 850-500 hPa Q-vector divergence (shaded, cool colors convergent), SLP (hPa, solid), 1000-500 hPa thickness (dam, dashed) Ottawa Calgary t=0 h (1800 UTC 28 January 2010) t=0 h (0000 UTC 4 December 2011)

  16. Motivation/Background Case Overviews Dynamics Forecasts Future Work Mesoscale lift: Ottawa vs. Calgary 925 (850-700) hPa frontogenesis (K (100 km)-1 (3 h)-1), shaded), 925-700 (850-600) hPa lapse rate (K km-1, blue solid contours starting at -8 with an interval of .5), 1000-500 hPa thickness (dam, dashed), and 10 m wind (knots, barbs). Ottawa Calgary t=0 h (0000 UTC 4 December 2011) t=0 h (1800 UTC 28 January 2010)

  17. Motivation/Background Case Overviews Dynamics Forecasts Future Work Instability: CI and CSI (Ottawa) Saturated equivalent geostrophic potential vorticity (MPV*g, m2 s−1 K kg−1, shaded for negative values) and θe (K, solid contours). t=0 h (1800 UTC 28 January 2010)

  18. Motivation/Background Case Overviews Dynamics Forecasts Future Work Instability: CI and CSI (Calgary) Saturated equivalent geostrophic potential vorticity (MPV*g, m2 s−1 K kg−1,shaded for negative values) and θe (K, solid contours). t=0 h (0000 UTC 4 December 2011) t=+3 h (0300 UTC 4 December 2011)

  19. Motivation/Background Case Overviews Dynamics Forecasts Future Work Instability: MAUL • Bryan and Fritsch (2000) argued that a sixth static stability state exists • Moist absolutely unstable (γs > Γs) • Moist Absolutely Unstable Layers (MAULs) • Short-lived • Rare: 1.1% of 100,000 soundings in Bryan and Fritsch (2000) • Often shallow; deep MAULs are defined as at least 100 mb in depth with a dewpoint depression of <= 1˚C throughout • Occur in close proximity to moist convection • Indications of (and caused by) intense mesoscale vertical motion

  20. Motivation/Background Case Overviews Dynamics Forecasts Future Work Instability: MAUL (Ottawa) Ottawa (CYOW): 28 January 2010 t=0 h (1800 UTC)

  21. Motivation/Background Case Overviews Dynamics Forecasts Future Work Instability: MAUL (Calgary) Calgary (CYYC): 4 December 2010 t=0 h (0000 UTC) t=+6 h (0600 UTC)

  22. Motivation/Background Case Overviews Dynamics Forecasts Future Work Synoptic-dynamic Conclusions: Calgary • Dynamics • Synoptic-scale forcing: *descent* • Mesoscale forcing: Arctic front (frontogenesis) • Just enough moisture to create a problem • Upslope flow? • Thermodynamics: Unstable, no matter how you slice it • Very steep low-level lapse rates • Convective instability (CI) • Conditional Symmetric Instability (CSI) • Soundings: Deep (300-400 hPa) Moist Absolutely Unstable Layer (MAUL), especially after t=0 h

  23. Motivation/Background Case Overviews Dynamics Forecasts Future Work Upslope flow? t=0 h (0000 UTC) t=+3 h (0300 UTC)

  24. Motivation/Background Case Overviews Dynamics Forecasts Future Work Forecasts and Warnings • From limited research (a few case studies), the models appear to do a decent job in predicting these events (not shown) • The real problem is that despite the high impact of snow burst events, they often do NOT meet warning or advisory criteria in the United States • Snow accumulations too low • Winds not high enough for severe criteria • Not the season for “convection” • Not in lake effect regions • In Canada, however….

  25. Motivation/Background Case Overviews Dynamics Forecasts Future Work Forecasts: Snowsquall Warning

  26. Motivation/Background Case Overviews Dynamics Forecasts Future Work Forecasts: Calgary • From the Prairie and Arctic Storm Prediction Center: FORECASTS FOR SOUTHERN ALBERTA AND THE MOUNTAIN PARKS ISSUED BY ENVIRONMENT CANADA AT 11.00 AM MST SATURDAY 3 DECEMBER 2011 FOR TODAY AND SUNDAY. CITY OF CALGARY.TODAY..SNOW. AMOUNT 5 CM. WIND NORTHWEST 40 KM/H GUSTING TO 70. HIGH PLUS 2.TONIGHT..SNOW. AMOUNT 5 CM. BLOWING SNOW WITH VISIBILITY LESS THAN 1 KILOMETRE THIS EVENING. WIND NORTH 50 KM/H GUSTING TO 70 DIMINISHINGTO 30 GUSTING TO 50 THIS EVENING. LOW MINUS 9. • But, no snowsquall warning

  27. Motivation/Background Case Overviews Dynamics Forecasts Future Work Future Work • Important questions: • What is the frequency of occurrence of snow burst events? • Are they more prevalent in certain regions? • What are the favorable large-scale meteorological conditions? • What are the favored instability regimes? • Predictability: Model forecasts • Objectives: • Assemble an event climatology at representative stations in the northern U.S. and southern Canada • Dynamic analysis and synoptic typing • Index and forecast decision tree development

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