lake-effect storms - judith currycurry.eas.gatech.edu/.../lake_effect_storms.pdf · shore-parallel...

rearrangement perturbations, which is not possible ifEulerian perturbations are used.

See also

Dynamic Meteorology: Balanced Flows; Potential Vorti-city. Hamiltonian Dynamics. Kinematics. NumericalModels: Methods. Tracers. Wave Mean-Flow Inter-action.

Further Reading

Hoskins BJ (1982) The mathematical theory of frontogen-esis. Annual Review of Fluid Mechanics 14: 131–151.

Hoskins BJ, McIntyre ME, and Robertson AW (1985) Onthe use and significance of isentropic potential vorticitymaps. Quarterly Journal of the Royal MeteorologicalSociety 111: 877–946.

Lamb H (1932) Hydrodynamics, 6th edn. Cambridge:Cambridge University Press.

Marsden JE and Ratiu T (1994) Introduction to Mechanicsand Symmetry. Texts in Applied Mathematics, vol. 17.Berlin: Springer-Verlag.

Norbury J and Roulstone I (eds.) (2002) Large-ScaleAtmosphere–Ocean Dynamics: vol. 1 Analytical Meth-ods and Numerical Models; vol. 2 Geometric Methodsand Models. Cambridge: Cambridge University Press.

Salmon R (1998) Lectures on Geophysical Fluid Dynamics.Oxford: Oxford University Press.

Shepherd TG (1990) Symmetries, conservation laws andhamiltonian structure in geophysical fluid dynamics.Advances in Geophysics 32: 287–338.

Shutts GJ, Cullen MJP, and Chynoweth S (1988) Geometricmodels of balanced semi-geostrophic flow. AnnalesGeophysicae 6: 493–500.

Staniforth A and Cote J (1991) Semi-lagrangian integrationschemes for atmospheric models: a review. MonthlyWeather Review 119: 2206–2223.

LAKE-EFFECT STORMS

PJ Sousounis, Michigan State University, Ann Arbor,MI, USA

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Introduction

Each winter, lake-effect storms develop on the down-wind shores of the North American Great Lakes, asarctic winds blow across the relatively warm water.The associated clouds and snow (or rain) showers tendto organize in narrow bands, usually only a fewkilometers wide but sometimes over 200 km long.There may be one band, or there may be asmany as 10or 20, each separated from the next by only a fewkilometers of clear sky. These bands may remainstationary over a region or they may oscillate insnakelike fashion. They may produce nothing morethan one or two centimeters of snow, or they maydump over 120 cm of snow in a single storm. Theselake-effect storms are primarily a product of relativelysimple air mass modification by warm water, compli-cated lakeshore geometry, and the prevailing synopticsituation.

Lake-effect storms develop in other parts of theUnited States, Canada, and the world, but nowhereelse do they occur as frequently or with such intensityas they do in the Great Lakes region. The reasons forthe unique weather in the Great Lakes region can betraced to several geographic aspects. The fact that the

Great Lakes are the largest single source of fresh waterin theworld (except for the polar ice caps), the fact thatthe Great Lakes are situated approximately halfwaybetween the Equator and the North Pole, the fact thatGreat Lakes are located in the interior of a largecontinent, the fact that each of the lakes is approxi-mately the size of a small inland sea, and the fact thatthere are several lakes – separated from each other bydistances less than their own size, make for some veryunique weather in the region. These characteristicssuggest that the lakes rarely freeze over completely,even in the coldest ofwinters, and thus remain a nearlycontinuous and very large source of heat andmoisturefor the atmosphere. Lake-effect storms continue to bea forecast challenge despite improvements in numer-ical mesoscale models because of their meso-g/meso-bscale size.

Climatology

Lake-effect snow accounts for 25–50% of the totalannual snowfall in many lakeshore regions (Figure 1).The snowbelts (areas of heavier snow) that shoulderthe southern and eastern shores of the Great Lakesreflect the direction of the prevailing north-westerlyflow relative to the orientation of the lakes, the sharpcontrast in surface friction between the relativelysmooth lake surface and the rough land, and terrain

1104 LAKE-EFFECT STORMS

effects. The largest snowfall totals exist acrossthe upper peninsula of Michigan, where north-west-erly flow across Lake Superior is forced upwardabruptly over steep terrain upon reaching the northerncoast of the upper peninsula – especially aroundthe Keewanaw Peninsula, and across the Tug HillPlateau in western New York, where west-south-westerly flow across the lower lakes provides a longfetch and ample opportunity for the air to bemoistened and destabilized. In both of these locations,long fetches and orography are key aspects. Terraincan enhance individual snowstorm totals by about5 cm for every 100m of rise. Additionally, portions ofthe lakeshore with enhanced concavity promote con-vergence zones that can further enhance snowfalltotals. Heavier lake-effect amounts fall typicallyduring cold winters, when the lake–air temperaturedifferences are enhanced.

Lake-effect snow falls almost exclusively during theunstable season – that portion of the year when thelakes are climatologicallywarmer than the ambient airand thus provide heat and moisture to the loweratmosphere to destabilize it. Enhanced cloudiness andprecipitation exist across much of the lake shoreregions and far inland as well. The percentage ofcloudy days peaks inNovember formany places of theGreat Lakes region – owing in part to significant lake-enhanced cloudiness. Precipitation during the unsta-ble season begins typically with episodes of nocturnalrain showers during cool nights in late August. As themean air temperature drops through the fall months,lake-effect rain showers change to lake-effect snowshowers. Much of the lake-effect snow falls typicallybetween November and February, which constitutesthe heart of the unstable season, when lake-airtemperature differences tend to be greatest.

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Figure 1 Average 1951–1980 Great Lakes seasonal snowfall total. (From Figure 2 in Norton DC and Bolsenga SJ (1993)

Spatiotemporal trends in lake effect and continental snowfall in the LaurentianGreat Lakes, 1951–1980. Journal ofClimate6: 1943–1956.

Adapted with permission from the American Meteorological Society.)

LAKE-EFFECT STORMS 1105

Climatological lake–air temperature differencesmay be around 7–81C, but may exceed 301C duringintense cold-air outbreaks. Coupled with windssometimes in excess of 20m s� 1, combined surfacesensible and latent heat fluxes can typically exceed1000Wm� 2 – comparable to that found in a cat-egory-1 hurricane (see Hurricanes).

Lake-effect clouds and snow can occur locally onB30–40% of the days in winter under a variety ofsynoptic patterns – whenever there is an onshore fetchand the lake–air temperature difference allows thelowest layers of the air to destabilize.However, certainsynoptic patterns are more favorable than others forallowing lake-effect snow to develop. A typicalsequence of events begins with a synoptic-scale lowmoving across theGreat Lakes region from south-westto north-east (Figure 2). Additionally, in late autumnespecially, these lows are deepening as they cross theregion because of baroclinic forcing and aggregateheating from all the lakes. Strong north-westerlywinds on the back side of the low bring progressivelycolder polar or arctic air across the warm lakes.Subfreezing temperaturesmay reach as far south as theGulf Coast and northern Florida, with � 201C read-ings just north of the lakes. The strong winds and coldair generate strong surface fluxes over the lakes thatmoisten and destabilize the air, leading to snowshowers along the downwind lakeshores of the GreatLakes. The deepening of the low and the destabiliza-tion both allow stronger winds from above to mixdown to the surface and further increase the heat andmoisture fluxes.

Depending on thewind speed, the orientation of thewind flow relative to the long lake axis, stability,moisture, and upper-level forcing, different types oflake-effect storms can develop. Basically, when theprevailing flow ismore parallel to the short axis than tothe long axis of a lake (i.e., there are strong short-axiswinds), multiple wind parallel bands (Type II) develop(Figure 3, middle panel). These bands are typically2–4 km wide and spaced 5–8 km apart. Snowfall isusually spread over a large area of the downwindlakeshore, and amounts are usually light (o4mmliquid precipitation per day). When the prevailingshort-axis winds are weak, midlake (Type I) or shoreparallel (Type IV) bands can develop – even when thelong-axis prevailing winds are strong (Figure 3 upperpanel). These bands can be 10–20 km wide andgenerate copious amounts of snow. If the short-axiswind is essentially not present then the band will belocated near the middle of the lake. If the short axisprevailingwind is present butweak, then the bandwillbe located closer to the downwind shore. Sometimes,especially over Lake Erie or Lake Ontario, a midlakebandwill develop at an obtuse angle to the long axis of

the lake. These bands (Type III) actually develop asmidlake bands in north-westerly flow over LakeHuron, farther upwind (Figure 3, lower panel). Theymay lose their visible cloud characteristics oversouthern Ontario, but maintain the convergencezone so that they redevelop once they reach the lowerlakes. If the wind is very light against areas ofenhanced concavity, then lake vortices (Type V) maydevelop (Figure 3, upper panel). Phenomena thoughtto play roles in this type of lake-effect storm arestretching and tilting of vorticity from low-levelconvergence and vertical wind shear; differentialdiabatic heating; and synoptic scale vorticity andtemperature advection. Table 1 summarizes some ofthe differences between various aspects of multiple-and single-band lake-effect snowstorms in westernMichigan (lower peninsula).

The most common type of lake-effect event over thenorthern lakes (Superior, Huron, andMichigan) is themultiple-band variety, which occurs 50% of the time.The next most common is the shore-parallel ormidlake variety, which occurs 25%of the time. Effectsoccurring during the remaining 25% include meso-scale vortices, hybrid combinations, and undetermi-nable forms. The most common type of lake-effectevent over the southern lakes (Erie and Ontario) is theshore-parallel or midlake variety, because of thedifferent orientation of these lakes (Figure 4). Ingeneral, across the region, the frequency of multiplebands decreases fromwest to east and the frequency ofshore-parallel and midlake bands increases from westto east.

Boundary Layer Dynamics

When cold air flows across the warm waters of theGreat Lakes, strong sensible and latent heat fluxeswarm and moisten the air closest to the surface first,causing the lowest levels of the atmosphere to desta-bilize. Strong turbulent motions mix upward thewarmed and moistened air in convective fashion.Steam fog typically develops and steam devils may bevisible near the surface, especially within a few tensof kilometers of fetch (Figure 5). A very unstableconvective internal boundary layer (CIBL) forms nearthe surface and grows rapidly upward in the down-wind direction (Figure 6). The upwind temperature,humidity, and flow characteristics of the air before itreaches the lake determine how the air will bemodified. As the air crosses the downwind lakeshore,frictional convergence enhances ascent.

Convective updrafts can exceed 4–5m s� 1 in nar-row cores 100m wide. After only a few kilometers offetch the depth of the internal boundary layer may


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Figure2 Surface and upper air analyses depicting typical synoptic setting for lake-effect snow in theGreat Lakes. (A) 700hPa heights (solid, dm) and temperatures (dashed, 1C); (B) 850 hPaheights (solid, dm)and temperatures (dashed, 1C); (C) sea-level pressure (solid, hPa) and locations of highs, lows, and fronts for times shown. ((A) FromFigure6and (B,C) fromFigure 5 inNiziol

TA, Snyder WR and Waldstreicher JS (1995) Winter weather forecasting throughout the eastern United States. Part IV: lake effect snow. Weather and Forecasting 10: 61–77. Adapted with

permission from the American Meteorological Society.)

LAKE-E

FFECTSTORMS

1107

reach or exceed the depth of the planetary boundarylayer. At some point the air within the thermal internalboundary layer becomes moist enough and deepenough for the lifting condensation level to be belowthe top of the boundary layer and cloud to form (Type Iboundary layer). Subsequent latent heat release andradiative heat transfer become important and increasethe rate of entrainment. The clouds develop initially as

two-dimensional bands, but in time, with sufficientfetch and heating, these bands can develop into chainsof three-dimensional cells and may eventuallyevolve into a regular array of three-dimensionalmesoscale cellular convection. Maximum cloud dropconcentration and liquid water content (e.g.,0.25 gm�3) occur near cloud-top and increase overthe first two-thirds of fetch. Near the downwindlakeshore, snow particle production increases (e.g.,5 L

� 1) to reduce drop concentration and increase theheight of cloud base.

The most common type of convection associatedwith cold air outbreaks over the Great Lakes isthe longitudinal roll. This occurs in other parts of theworld – wherever cold air crosses warm water. Overthe Great Lakes, the rolls are usually oriented parallelto the wind at the base of the inversion and also tothe low-level wind shear. Classical rolls are typically2–5 km wide and 0.5–2 km deep, so theaspect ratio is roughly 2 to 5. Rolls over theGreat Lakes are 10–20 km wide and 1–2 km deep, sothe aspect ratio is about 10. Some roll circulationsexhibit a multiscale configuration so that the cloudstreets are best developed when the wavelengths ofthe convection and rolls are in phase. Such multiscalephasing can generate cloud streets with aspect ratiosof 10–20.

Linear analytic studies indicate that three differenttypes of instabilitymechanismsmay be responsible forrolls. First, inflection point instability theory suggeststhat a dynamical instability can develop in a (rotating)Ekman layer that is neutrally stratified if the cross-geostrophic wind component exhibits an inflectionpoint. The most unstable wind profiles lead to rollswith an aspect ratio of 3. Rolls are best developedwhen they are oriented about 141 to the left of thegeostrophic wind (in neutral conditions). Second,parallel instability theory involves the curvature ofwind speed profiles parallel to the roll axes and theboundary layer mean wind shear. This type of insta-bility mechanism generates rolls with aspect ratiosthat are twice as large as those from inflection pointinstability and closer to those observed during lake-effect conditions. Third, convective instability (i.e., anunstable boundary layer) in the presence ofwind shearalso leads to the formation of rolls that are parallel tothe mean wind shear but with aspect ratios smallerthan those from inflection point instability (e.g.,around 2).

Understanding roll development during lake-effectsituations is further complicated by the fact that large-eddy numerical simulations suggest that only convec-tive instability is capable of generating rolls. Gravitywaves generated within the stable air above theconvective boundary layer by wind shear near the

Figure3 Lake-effect snowbandstructures.Upper panel showsa

lake vortex (Type V) over Lake Huron (C); midlake band (Type I)

over LakeErie (A); shore parallel band (Type IV) over LakeOntario

(B).Middle panel:multiple bands (Type II) over LakeHuron (A) and

shore parallel band (Type IV) over Lake Ontario (B). Lower panel:

midlake band (Type I) over Lake Huron (A); hybrid band (Type III)

over Lake Erie (B and C). (From Figure 2 in Braham, RR Jr (1983)

The midwest snow storm of 8–11 December 1977. Monthly

Weather Review 111: 253–272. Adapted with permission from

the American Meteorological Society.)


Table 1 Typical values for multiple and shore-parallel band lake-effect storms in western Michigan (lower peninsula)

Characteristics Multiple Single

Mesoscale features

Location Inland western shore Along western shore

Band orientation NW–SE bands N–S band

Band length 30 to 90 km 90 to 180 km

Band width 2 to 5 km 10 to 20 km

Band (boundary layer) depth 1 to 2 km 2 to 3 km

Aspect ratio 5 to 10 km 10 to 20

Lake temperature 0 to 101C 0 to 21C1000 air temperature 0 to �101C � 10 to � 201C850 air temperature � 5 to� 151C � 10 to � 301C700 air temperature � 15 to �251C � 20 to � 401C1000 winds W–NW above 10ms�1 NW–N at 0 to 10ms� 1

1000-850 shear dir/speed 301/0–10ms�1 101/0–5ms� 1

1000-700 shear dir/speed 601/0–20ms�1 201/0–10ms� 1

Cloud/precip characteristics

Precipitation Moderate snow (o 10 cm) Heavy snow (4 10 cm)

Cell movement With prevailing wind With prevailing wind

Echo persistence Short (o1 h) Long (41 h)

Cloud base 1 km o1 km

Cloud top 2 km 3km

Updrafts o1ms�1 o2ms�1

Liquid water content 0.2 to 1 gm� 3 0.5 to 2 gm�3

Cloud droplet radius 2mm 5 mmCloud droplet concentration 100 to 300 cm� 3 300 to 500 cm� 3

Natural ice nucleus 1 to 10L� 1 1 to 10 L�1

Ice-to-water ratio 10� 6 to 10� 4 FIce crystal size 1 to 4mm FIce crystal concentration 1 to 10L� 1 F

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Figure 4 Percentages of all days categorized as lake-effect (dashed), wind parallel bands (solid), and shore parallel bands (dotted).

Results based on visible satellite imagery from 1988–93 (October–March). (From Figure 2 in Kristovich DAR, Steve III, RA (1995)

Asatellite studyof cloud-band frequencies over theGreat Lakes.Journal ofAppliedMeteorology34:2083–2090.Adaptedwith permission

from the American Meteorological Society.)


boundary layer top have also been suggested as apossible mechanism. If the wind shear at the top of theboundary layer is roughly perpendicular to the bound-ary layer wind direction, then bands of convectionparallel to the mean boundary layer wind can beinduced by gravity waves. Latent heat release, cloudmicrophysical processes and low-level wind shear(e.g., below 200m) may also influence the develop-ment of rolls. The partial agreement between obser-vations and theoretical studies thus far suggests that a

combination of mechanisms may be responsible fordifferent aspects of roll development or during certainconditions.

Sensitivity to Synoptic Conditions

The thermal modification of air over relatively coldland as it crosses a relatively warm lake results in ahorizontal temperature gradient across the lakeshore,

Figure 5 Example of steam fog and roll development during cold air outbreak over Lake Michigan. Photo taken at 14.25 UTC on

13 January 1998 during the Lake-Induced Convection Experiment. (Photo from David C. Rogers at NCAR taken over Lake Michigan

during the Lake-ICE Experiment conducted during December 1997 to January 1998.)

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Figure 6 Vertical cross-section of equivalent potential temperature (solid 1K contour interval) and cloud frequency (dashed 10%

contour interval) across southern Lake Michigan (from Sheboygan, WI, to Benton Harbor, MI) during a cold air outbreak (on 20 January

1984). Presence of cloud determined by cloud droplet concentrations greater than 10 cm� 3. Wisconsin and Michigan land surfaces are

indicated by thick horizontal gray bars. (From Figure 5 in Chang SS, Braham RR Jr (1991) Observational study of a convective internal

boundary layer over Lake Michigan. Journal of the Atmospheric Sciences 48: 2265–2279. Adapted with permission from the American

Meteorological Society.)


which can generate a thermally direct solenoidalcirculation similar to that of a sea or land breeze. Inthat sense, it can be seen that the lake–air temperaturedifference, speed and direction of the prevailing wind,and height and strength of the capping inversion areperhaps the most crucial parameters for determiningnot only howmuch lake-effect snowwill fall, but howit will fall. For example, for conditions character-ized by moderate lake–air temperature differences(4101C), and strong winds (410m s�1) blowingacross the short axis of a lake, multiple snowbandswill usually develop. If the winds across the short axisare lighter and/or the temperature difference is greaterthen some form of shore-parallel band will develop. Ifthe wind is very light against areas of enhancedconcavity then lake vortices can develop.

The impact of the wind speed on the lake-effectresponse characteristics can be analyzed two-dimen-sionally in terms of the Froude number Fr ¼ pU=NH,whereU is themeanwind speed across the short axis ofthe lake, andN andH are the Brunt–Vaisala frequencyand depth of the planetary boundary layer respective-ly. The values for N and H depend on the lake–airtemperature difference and stability of the pre-lake-modified air. The Froude number may be interpretedas the ratio of the mean wind speed U to the gravitywave speed cg ¼ NH=p for a boundary layer of depthH. Three regimes are important to consider. WhenFro1 then the gravity wave speed exceeds the meanflow speed ðcg > UÞ. Opposing sea-breeze type circu-lations develop with respect to the short axis of thelake and the heaviest precipitation falls over the lake(Figure 7A). This regime corresponds to the midlakeband type of event. When Fr >1, the gravity wavespeed is less than the mean flow speed ðcgoUÞ. Theresponse is characterized by alternating regions ofascent and descent that propagate downwind from theleeward shore, and precipitation is diffuse and weak.This regime corresponds to the multiple-band type ofevent (Figure 7B). When Fr ¼ 1, the gravity wavespeed equals the mean flow speed ðcg ¼ UÞ. Aresonance condition develops where gravity wavesgenerated at the downwind shore cannot propagateupwind. This regime corresponds to the shore-parallelband type of event that can generate significantprecipitation at the downwind lakeshore (Figure 7C).Setting Fr ¼ 1, and using typical values of N ¼10�2 s�1 and H ¼ 2 km, suggests that a value of U �6m s�1 maximizes heavy snow along the downwindlakeshore, which is consistent with observations.

The impact of fetch on lake-effect storm develop-ment has been known since the early 1900s. Longfetches usually result in heavy snowfalls. Short fetchesalso may produce significant snowfalls if the pre-lake-modified air is relatively unstable, if the lake shore

geometry enhances radial convergence, or if thenearby orography enhances lifting. These observedimpacts of fetch have been confirmed only recentlyusing analytic and numerical models. For example, ithas been shown analytically for Lake Michigan thatthree convergence centers develop near the easternshore when a westerly wind prevails, two cells orsnowbands develop when a north-westerly windprevails, and one midlake band develops when anortherly wind prevails.

The vertical structure of the environmental windalso affects lake-effect storms. For example, when theprevailingwind is parallel to the long axis of LakeErie,moderate directional shear (e.g., between 301 and 601)from the surface to 700 hPa causes weakly to-moder-ately precipitating multiple snowbands rather than asingle intensely precipitating snowband to occur.Stronger shear (e.g., greater than 601) over Lake Eriecauses the breakdown of precipitating snowbandsaltogether – allowing only instead the development ofa non-precipitating stratocumulus deck. While windshear can understandably inhibit the organization ofrolls orwider bands, it is not clear whether some of theobserved effects from shear are simply amanifestationof a shallow boundary layer.

The height and strength of the (capping) inversionare significant limiting factors to cloud depth andtherefore to precipitation. Typically, the boundarylayer must have a depth greater than 1 km in order forlake-effect snow to develop. The most convectivelyactive lake-effect storms have inversion heights ex-ceeding 3 km. Sometimes the capping inversion isentirely absent. During such cases, thunder andlightning typically accompany copious snowfall ratese.g., these exceeding 10 cm h� 1.

The vertical temperature andmoisture distributionswithin the boundary layer also play a role. Forexample, it has long been known that a minimumtemperature difference of 131C between the lakesurface and the upstream airflow at 850 hPa isrequired for lake-effect storms to develop. This tem-perature difference criterion means that the lapse rateshould be unstable with respect to unsaturated ascent.A dry boundary layer is less conducive for lake-effectsnow than a moist one, although a long fetch cancompensate for very dry boundary layers. The impactofmoisture is greater for low-stability profiles than forhigh-stability ones.

The presence of large-scale forcing can also influ-ence lake-effect storm development. Typically, thecoldest air passes over the lakes as high pressure at thesurface moves eastward across them, accompanied bynegative vorticity advection at upper levels and coldadvection near the surface. Thus, the impactsof synoptic-scale forcing typically act to suppress


lake-effect storm development (Figure 2). There arehowever instances when cold air, positive vorticityadvection, and even warm advection exist simultane-ously over the region. Such situations usually come inthe form of Alberta Clippers (short waves) thatdevelop in cold air masses and move south-eastward

across the region. The synoptic forcing coupled withthe cold air that is already established over the regioncan combine to generate intense snowfall.

Sensible and latent heating from all the Great Lakes(e.g., the lake aggregate) can also influence lake-effectstorms over individual lakes. Basically, if warming

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Figure 7 Gravity wave interpretation of lake-effect morphology dependence on wind speed. (A) Weak windspeeds create subcritical

ðFro1Þ regime, which allows gravity waves to propagate upwind and downwind and a midlake band andmoderate snow to develop over

the lake; (B) strongwindspeeds create supercritical ðFr > 1Þ regime,which allowsgravitywaves to propagateonly downwindandmultiple

bands and light snow to develop beyond the downwind lakeshore; (C) moderate windspeeds create near-critical ðFr � 1Þ regime, which

allows gravity waves to travel downwind but traps gravity waves trying to propagate upwind – resulting in an intense shore-parallel band

and heavy snow to develop along the downwind lakeshore. Heavy arrows indicate windspeed and wavy arrows indicate gravity wave

propagation. Plus (1) signs and shaded columns indicate ascent; minus (–) signs and open ovals indicate descent. Asterisks indicate

snowfall.


(andmoistening) occurs over all the Great Lakes for atleast a day then surface pressures and stability candrop over a broad region and cause a perturbationaggregate-scale, low-level cyclonic circulation to de-velop. The position, the size, and the warmth andmoisture from this aggregate circulation can modifylake-effect precipitation throughout the region. Spe-cifically, when the synoptic-scale flow is north-west-erly, aggregate effects can augment snowfall along thenorth-western shores of lower Michigan, and reducesnowfall along the south-western shores (Figure 8).Shore-parallel bands located offshore can migrateeastward (e.g., onshore) or evolve intomultiple bands.These aggregate affects over Lake Michigan includeenhanced westerly flow, increased heat and moisture,and lower stability. The lake aggregate can alsoinfluence lake-effect precipitation in the lower lakesregion. For example, as the lake-aggregate-inducedplume of heat and moisture extends south-eastward,surface winds across Lake Ontario (north of theaggregate induced plume) can becomemore northerly.

In contrast, surface winds across Lake Erie (south ofthe aggregate-induced plume) can become morewesterly. The aggregate-altered winds can cause alonger fetch across Lake Erie and a shorter fetch acrossLake Ontario, and can shift the regions of lake-effectconvective bands, so that less (intense) lake-effectprecipitation can fall along the lakeshores downwind(east) of Lake Ontario and more lake-effect precipi-tation can fall along the eastern shores of Lake Erie(Figure 8).

Forecasting

The mesoscale nature of lake-effect storms, theirintensity, and the short development times continueto challenge forecasters. Highly variable snow-to-liquid ratios (10:1 to 50:1) and terrain effects, espe-cially near Lakes Erie and Ontario, can enhance theinherently large spatial variability of lake-effect snowand hence the forecast challenge. While the problems

Figure 8 Illustration of lake aggregate effect on prevailingwinds and lake-effect snowstorms.On the south side of the developingwarm

plume (shaded oval), north-west winds respond in sea-breeze fashion to become south-west winds with increased fetch and heavy snow

acrossLakeErie. Lakeeffect snowsacrossportions of westernMichigan (lower peninsula)mayormaynot change characteristics.On the

north side, north-west winds respond to become north-north-westerly winds with reduced fetch and light snow across LakesSuperior and

Ontario and increased fetch heavy snow across Lake Huron.


of forecasting when lake-effect snow is going to occurhave essentially been solved, the equally significantproblems of exactly where lake-effect snowwill occur,what form(s) it will take, how intense it will be, andhow long it will last remain outstanding forecastissues. A combination of high-resolution numericalweather prediction models, statistical methods, Dop-pler radar, and forecaster savvy are the basic forecasttools.

Numerical models have come a long way since theuse of the limited fine mesh (LFM) model. Thehorizontal grid spacing (180 km) and the exclusionof the lakes in terms of their heat, moisture, andmomentum characteristics in that model precludedany explicit model development of lake-effect precip-itation. Regardless, operational forecasters relied on

this model because of its ability to forecast the large-scale conditions to which lake-effect snowstormdevelopment is very sensitive. The LFM model, inconjunction with forecaster decision trees based onkey large-scale parameters, and experience, allowedforecasters to at least be able to issue general forecastsof when lake-effect snow was going to occur.

Two operational models currently being used in-clude the nested grid model (NGM), with 48 kmhorizontal grid spacing, and the Eta model. Severaldifferent versions of the Eta model are run at severaldifferent resolutions and times including one run at12 km horizontal grid spacing four times daily. Theincreased resolution in the Eta model has beenespecially helpful for identifying areas where lake-shore enhanced snowbands may develop. Recently,

Conditional Moderate Extreme

15 20 25 30 35 40

Temp (lake) − Temp (700 hPa) (°C)

12 h extreme instability...030 degree shear from700 hPa to SFC...and 090-mile fetch at 850 hPa.

24 h moderate instability...020 degree shear from700 hPa to SFC...and 070-mile fetch at 850 hPa.



10

20

30

40

Tem

p (la

ke)

− T

emp

(850

hP

a) (

°C) Output from NGM, 12Z 18 Feb 93:Lake-effect guidance: Ontario

Forecast parameters

Variable 12 h 24 h 36 h 48 h

Wind direction (degrees)

700 hPa 260 310 290 270

850 hPa 290 300 290 240

B.L. 290 290 290 180

Change in wind direction with height

850/700 030 010 000 030

B.L./700 030 020 000 090

Fetch (miles)

850 hPa 090 070 090 150

Temp (°C)

700 hPa −30 −28 −21 −18

850 hPa −21 −23 −17 −14

B.L. −10 −14 −10 −09

Lake 02 02 02 02

TS/T3 layer inversion intensity

TS-T3 −08 −08 −03 −05

Vertical velocity (microbar/s)

700 hPa +02 −02 −02 +00

4836

2412

Figure 9 Lake-effect snow guidance product for 1200 UTC on 18 February 1993 generated for Lake Ontario at WSFOBuffalo. TS and

T3 correspond to the surface and 900hPa temperatures respectively andBL corresponds to the boundary layer. (FromFigure 10 in Niziol

TA, SnyderWRandWaldstreicher JS (1995)Winter weather forecasting throughout the easternUnited States. Part IV: Lake effect snow.

Weather and Forecasting 10: 61–77. Adapted with permission from the American Meteorological Society.)


several forecast offices have experimented with run-ning locally high-resolution mesoscale models. TheForecast Office in Buffalo, New York, has beenrunning a 10 km version of the PSU/NCAR modelMM5 since 1996. The Forecast Office in Detroit,Michigan, has been running a 6 km version of the Etamodel since 1998. Both of these offices have reportedthe ability to provide more specific and more accurateforecasts.

Despite significant and continuing improvements innumerical weather prediction, lake-effect snow con-tinues to challenge the abilities of even the mostsophisticated numerical models, because of severalinadequacies. These inadequacies include horizontalresolution that is still too coarse for resolving the 2–4 kmwide bands, convective schemes tuned originallyfor deep (tropical) convection that are inappropriateto simulate intense shallow precipitating convection,and boundary layer schemes that are too simplistic todevelop the low-level temperature, moisture, andcloud-microphysical structures that exist withinlake-effect snow environments near the surface.

To address some of these inadequacies, forecasterscurrently use various statistical methods. These meth-odswere used almost exclusively prior to the existenceof numerical models. As early as the middle of lastcentury, various investigators had outlined conditionsnecessary for prolonged lake-effect storms to occur atthe eastern end of Lake Erie. Afterwards, moresophisticated statistical models, based on multiplediscriminant analysis, the perfect prog (PP) method,model output statistics (MOS), and classification andregression trees (CART), were developed for many ofthe lake-effect snow belts. Currently, the use ofnumerical model output in terms of larger-scalefeatures, coupled with highly tuned, sophisticatedstatistical models, has proven a very effective forecastmethod (Figure 9).

Remaining challenges for numerical lake-effectsnow forecasting include resolving the lakeshoregeometry and nearby terrain, simulating more accu-rately the evolution of the boundary layer – including

heat, momentum, and moisture fluxes – and simula-ting more accurately the convective precipitation. Afourth challenge is initializingmore accurately the lakesurface temperatures, which are specified currentlyusing AVHRR satellite data that represent a multidayaverage and may have gaps because of persi-stent cloudiness. Finally, the simulation of sub-sequent changes in lake surface temperatures, whichmay also improve forecast accuracy, has yet to beincluded.

See also

Air–Sea Interaction: Storm Surges. Boundary Layers:Convective Boundary Layer; Modeling and Parameterizat-ion; Overview. Climate: Overview. Convective Storms:Convective Initiation; Overview.Hurricanes.MesoscaleMeteorology: Mesoscale Convective Systems; Models.Numerical Models: Methods. Synoptic Meteorology:Forecasting. Weather Prediction: Regional PredictionModels.

Further Reading

Braham RR (1995) The midwest snow storm of 8–11December 1977. Monthly Weather Review 111:253–271.

Chang SS and Braham RR (1991) Observational study of aconvective internal boundary layer over Lake Michigan.Monthly Weather Review 48: 2265–2279.

Kristovich DAR and Steve RA (1995) A satellite study ofcloud band frequencies over the Great Lakes. Journal ofApplied Meteorology 34: 2083–2090.

Niziol TA, Snyder WR and Waldstreicher JS (1995) Winterweather forecasting throughout the United States,part IV. Lake-effect snow. Weather and Forecasting 10:61–77.

Norton DC and Bolsenga SJ (1993) Spatiotemporal trendsin lake-effect and continental snowfall in the LaurentianGreat Lakes, 1951–1980. Journal of Climate 6:1943–1956.

Sousounis PJ and Mann GE (2000) Lake-aggregate meso-scale disturbances, part V. Impacts on lake-effect precip-itation. Monthly Weather Review 128: 728–743.


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