IntroductionMajor lake-effect snow events do occur in Duluth in spite of some limiting influences discussed in following sections. A good example is heavy lake-effect snow that dumped a foot or more on central into west Duluth on December 4, 2007. Snowfall rates of 2 to 3 inches per hour fell under the core of the strong snow band. Most lake-effect snow events in Duluth are in reality lake-enhanced events since snow from the lake supplements greater amounts of snow produced by a synoptic (or large scale) weather system.
Limitations on Lake-Effect or Lake EnhancementFavorable wind directions for lake-effect snow are limited due to western Lake Superior's narrow shape. A general east-northeast wind (one from 70 to 80 degrees) is most favorable to get a long fetch off Lake Superior into Duluth. As winds turn more from the east around a surface high pressure system departing to the east, cold air has often begun to moderate. As a result, the air is less likely to be cold enough to produce unstable lapse rates over the lake. Dry air has also become well established at that time. Lake Superior is obviously on the wrong side of the city. On a positive note, the lake is very deep and takes a while to freeze over making the lake available for most of the snow season.
Lake Enhancement vs Lake-EffectFor most lake enhanced events, conditions for lake-effect, such as instability and moistening of the air by the lake, are marginal. Snow falling from higher clouds helps saturate the air. Snow from higher clouds also seeds the lake clouds to give them a head start on ice crystal production. In cases where cloud temperatures are not cold enough for ice crystals to form (T > -8 oC), cloud seeding is a must. Divergence associated with an upper-level short wave (upper disturbance) can induce lifting and cooling of the air so as to increase the depth and strength of instability over the lake.
Sometimes a better developed storm system can hinder lake-effect. Warm advection associated with the storm's approach reduces lapse rates over the lake. The air becomes more stable and potential for lake snow diminishes.
Two Views of Lake EnhancementAs hinted in previous text above, lake enhancement can be thought of in two ways:
1. Conditions for convective cloud development and precipitation over the lake exist but are marginal. A synoptic weather system provides vertical motion to help destabilize the atmosphere or drops snow to help moisten the air over the lake. Sometimes instability is stronger and the lake snow becomes the major contributor, such as the lake-effect snow event on December 4, 2007.
2. Conditions for convection over the lake are not satisfied but air warms and picks up extra moisture off the lake. In the case of Lake Superior, a combination of orographic lift up the terrain and convergence at the narrowing western tip of Lake Superior combine to produce low level cloud formation. The clouds become a source of water droplets for additional growth of ice crystals falling from higher clouds.
An Example of Lake EnhancementA good example of lake enhancement occurred in a storm from January 25 to 27, 2004. A persistent east wind off Lake Superior resulted in numerous bands of snow moving inland off the lake during the duration of the storm. Toward the end of the storm a broad dominant band produced several inches of snow as it nearly stalled over Duluth for most of the evening on the 26th. Overall 20 or more inches of snow fell along and inland of the ridge crest in central and east Duluth. A persistent east to east-northeast wind provided a favorable trajectory for snow bands to move into the higher terrain.
Trailing Lake-Effect Snow BandsIn some instances, as with the January 25 to 27, 2004 storm, as a low pressure system or trough passes south or southeast of Duluth, a strong dominant band of lake-effect snow develops. The band may be located just along, just inside, or just behind the back edge of the larger area of snow. The band typically produces and additional ½ to 2 inches of snow depending upon the bands width, intensity, and persistence over the city. The band usually forms mid-lake, extending from near or beyond the tip of the Bayfield Peninsula of Wisconsin to the lake shore northeast of Duluth. The band then drifts south through Duluth as winds behind the low pressure system gradually shift from east to northeast and then north.
Colder air, moving over the lake behind the low pressure system, increases instability over the lake. Conditions often become favorable for actual lake-effect rather than just lake enhancement. If winds decrease behind the storm, the residence time of the air over the lake increases, allowing more moisture to be evaporated into the air. Stronger winds that may have occurred earlier leave behind a rough lake surface. The rougher lake surface increases the surface area of the lake from which moisture is transported into the air. Decreasing winds are less disruptive to the formation of stronger or larger bands.
A List of Conditions Favorable for Lake-Effect and Lake Enhancement1. Unstable temperature lapse rate from the surface of the water to the top of a capping inversion: A Lapse rate of 13 oC or greater from the water surface to 850 mb is equivalent to being at or stronger than dry adiabatic. The surface to 850 mb lapse rate provides a good initial check but an unstable layer can exist below 850 mb and needs to be check. Evaporation of moisture into the air near the water contributes to the development of instability.
2. Capping inversion height of 1 km or greater: generally 1 to 4 km is favorable, which translates to approximately 3000 to 13000 feet: The capping inversion puts a limit on how deep the convection can grow. Sometimes the inversion may be weak or hardly existent. For western Lake Superior, with an east wind, anything over 7000 feet is probably high enough for strong convection. An area forecast discussion issued by National Weather Service forecasters in Duluth, MN for the event on December 4, 2007 noted radar echo tops up to 7500 feet. Snow accumulated locally in excess of 12 inches in 12 hours.
3. Long fetch: A longer distance traveled over the water allows more time for evaporation at the water surface and moisture transport vertically into the air. Basic forecaster guidelines list minimum fetches of 80 to 100 miles as necessary for heavier lake-effect snow. An strait east as opposed to slightly north of east wind is a shorter fetch off the lake into Duluth. Snow can still develop with a short fetch given strong lapse rates and other optimum conditions such as pre-moistened air from snow already falling.
4. Low directional wind shear with height
5. Low speed wind shear with height
6. Long residence time of air over the lake: Slower wind speeds and longer fetch increase residence times. Stronger lapse rates and pre-existing moisture in the air can overcome the disadvantage of short residence times.
7. Roughness of the lake surface: Higher waves result in greater surface area from which water can evaporate into the air.
8. Conditioning of the Air Upstream from the Lake: Air passing over other lakes farther upstream warms and adds moisture to the air near the surface. For western Lake Superior, favorable trajectories rarely originate from other Great Lakes since the other lakes are farther south. Precipitation and air flowing from mid-lake are the primary upstream conditioners.
9. Cloud seeding: Snow falling from higher clouds function as additional ice nuclei for snow to form lower clouds off the lake. The seeding process is necessary if lake cloud temperatures are not cold enough to activate ice forming nuclei. The temperatures need to be generally below -8 oC for ice to form if seeding is not occurring.
10. Orographic lift
11. Frictional convergence at the water/land interface
12. Thermal convergence due to land breezes
13. For Lake Superior, high elevations on both sides of the lake help to funnel and converge winds into the head of the lake. The converging winds can help focus snow into stronger bands.
14. An ice covered lake is not favorable, but breaks in the ice may allow enough evaporation. Other lake-effect conditions would need to be optimum.
15. Clearing away of broken or fractured ice by strong winds to open up more water surface.
ReferencesThe Meteorology and Education (MetEd) website contains a good education module called "Topics in Lake Effect Snow Forecasting" which is worth reviewing. You need to create a password to use education materials on the site.