Shawn M. MilradDepartment of Atmospheric and Oceanic

SciencesMcGill University

Montreal, Quebec, Canada

Eyad H. Atallah and John R. GyakumDepartment of Atmospheric and Oceanic

SciencesMcGill University

Montreal, Quebec, Canada

OutlineMotivation and BackgroundCase comparison: Ottawa, ON (28 January 2010) vs. Calgary, AB (3-4 December 2011)Case overviewsSynoptic-dynamic analysis

Forecasts and warningsFuture work

MotivationThe climatology, dynamics and impacts of lake effect

snow squalls (bursts) throughout the U.S. have been well-documentedGreat 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 burstsThundersnow 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)

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Source: UCAR/COMET

MotivationMotivation/Background Case Overviews Dynamics Forecasts

Future Work

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

MotivationMotivation/Background Case Overviews Dynamics Forecasts

Future Work

Nicosia et al. (2009)Impacts in

Pennsylvania

Previous WorkMotivation/Background Case Overviews Dynamics Forecasts

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Ottawa, Ontario: 28 January 2010

(Photos are courtesy of the Ottawa Citizen)

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Calgary, AB: 3 December 2011

(Photo is courtesy of the Calgary Sun)

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Small snow accumulation: 3.6 cm (1.4 in.)

Near-zero visibility

DynamicsSynoptic-scale forcing for ascent (Q-vector convergence)Mesoscale forcing for ascent: Arctic frontJust enough moisture to create a problem

ThermodynamicsVery steep low-level lapse ratesConvective instability (CI) and Conditional Symmetric

Instability (CSI)Soundings: Deep (300-400 hPa) Moist Absolutely

Unstable Layer (MAUL)

Ottawa: 28 January 2010Motivation/Background Case Overviews Dynamics Forecasts

Future Work

Ottawa: 28 January 2010Motivation/Background Case Overviews Dynamics Forecasts

Future Work

t=0 h (1800 UTC)

Ottawa, ON (CYOW): 28 January 2010

***Ottawa International Airport (CYOW): black star***

Calgary: 3-4 December 2011Motivation/Background Case Overviews Dynamics Forecasts

Future Work

Larger snow accumulation: 8.9 cm (3.5 in.): Below snowfall warning criterion6.1 cm on 3 December2.8 cm on 4 December

Near-zero visibility

Dynamics and Thermodynamics Similarities and differences?

t=-1 h (2310 UTC)

***Calgary International Airport (CYYC): black star***

Calgary: 3-4 December 2011Motivation/Background Case Overviews Dynamics Forecasts

Future Work

t=+1 h (0110 UTC)

Calgary: MeteogramsMotivation/Background Case Overviews Dynamics Forecasts

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Calgary, AB (CYYC): 3 December 2011 Calgary, AB (CYYC): 4 December 2011

Dynamic Analysis: StrategySnow bursts are essentially a form of

wintertime moist convection

Ingredients-based methodology: Moist convectionDoswell et al. (1996); Schultz and Schumacher

(1999); Wetzel and Martin (2001)

Three main ingredientsLift (synoptic-scale and mesoscale)**MoistureInstability**

Convective (CI): (dθe/dz) < 0 Conditional Symmetric (CSI): MPV*

g < 0

Motivation/Background Case Overviews Dynamics Forecasts Future Work

t=0 h (1800 UTC 28 January 2010)

850-500 hPa Q-vector divergence (shaded, cool colors convergent), SLP (hPa, solid), 1000-500 hPa thickness (dam, dashed)

Lift: Synoptic-scaleMotivation/Background Case Overviews Dynamics Forecasts

Future Work

t=0 h (0000 UTC 4 December 2011)

Ottawa 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).

Mesoscale lift: Ottawa vs. CalgaryMotivation/Background Case Overviews Dynamics Forecasts

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t=0 h (1800 UTC 28 January 2010)

Ottawa Calgary

t=0 h (0000 UTC 4 December 2011)

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).

Motivation/Background Case Overviews Dynamics Forecasts Future Work

t=0 h (1800 UTC 28 January 2010)

Saturated equivalent geostrophic potential vorticity (MPV*g, m2 s−1 K kg−1, shaded

for negative values) and θe (K, solid contours).

Instability: CI and CSI (Calgary)

t=0 h (0000 UTC 4 December 2011)

Motivation/Background Case Overviews Dynamics Forecasts Future Work

t=+3 h (0300 UTC 4 December 2011)

Bryan and Fritsch (2000) argued that a sixth static stability state existsMoist absolutely unstable (γs > Γs)

Moist Absolutely Unstable Layers (MAULs)Short-livedRare: 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 convectionIndications of (and caused by) intense mesoscale

vertical motion

Instability: MAULMotivation/Background Case Overviews Dynamics Forecasts

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t=0 h (1800 UTC)

Ottawa (CYOW): 28 January 2010

Instability: MAUL (Ottawa)

Motivation/Background Case Overviews Dynamics Forecasts Future Work

t=0 h (0000 UTC)

Calgary (CYYC): 4 December 2010

Instability: MAUL (Calgary)

t=+6 h (0600 UTC)

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Synoptic-dynamic Conclusions: CalgaryDynamics

Synoptic-scale forcing: *descent* Mesoscale forcing: Arctic front (frontogenesis)Just enough moisture to create a problemUpslope flow?

Thermodynamics: Unstable, no matter how you slice itVery steep low-level lapse ratesConvective instability (CI)Conditional Symmetric Instability (CSI)Soundings: Deep (300-400 hPa) Moist Absolutely

Unstable Layer (MAUL), especially after t=0 h

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Upslope flow?Motivation/Background Case Overviews Dynamics Forecasts

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t=0 h (0000 UTC)

t=+3 h (0300 UTC)

Forecasts and WarningsFrom 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 StatesSnow accumulations too lowWinds not high enough for severe criteriaNot the season for “convection”Not in lake effect regions

In Canada, however….

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Forecasts: Snowsquall WarningWarning

Open-waterLocation Threshold Criteria

National, except extreme Northern Quebec*

When, down wind of large bodies of water, like the Great Lakes,snow squalls are imminent or occurring with one or more of the following conditions being produced:•Localized, intense snowfall producing snowfall amounts of 15 cm or more in 12 hours or less.•Reduced visibility (less than 400 metres) caused by heavy snow with or without blowing snow for 3 hours or more.Note that local snow accumulations may be significant.

FrontalLocation Threshold Criteria

National, except extreme Northern Quebec*

When there is a brief period (less than one hour) of very poor visibility (400 metres or less), caused by heavy snow and blowing snow, and accompanied by strong, gusty winds of 45 km/hr or greater, is expected to occur with the passage of a cold front.

Motivation/Background Case Overviews Dynamics Forecasts Future Work

Forecasts: CalgaryFrom 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

Motivation/Background Case Overviews Dynamics Forecasts 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 CanadaDynamic analysis and synoptic typingIndex and forecast decision tree development

Future WorkMotivation/Background Case Overviews Dynamics Forecasts

Future Work