Lake Effect Snow

• Very cold air moving over a relatively warmer water body can produce snow; lake effect snow (e.g., Great Lakes, Great Salt Lake); ocean effect snow (e.g., eastern New England); gulf effect (e.g., Gulf of Mexico); Figure 12.1-3

• Climatology of Lake Effect Snow: most lake-effect snow falls within 80 km (50 miles) from the lakeshore to the lee side of the lake; orographic lift due to local topography will greatly enhance snowfall; Figures 12.4-8

• Lake effect snowfall amounts are often measured in feet, with an extreme lake effect event in the 3-5 foot range

• Most lake effect occurs in late Autumn and early Winter, when the lakes are still relatively warm and ice free

• Temperature difference between lake water and 850 mb should be 13 C or more

• Temperature difference between lake water and 700 mb should be 20 C or more

• There should be an inversion between 2,000 and 10,000 feet, with the heaviest lake effect snows occurring with an inversion present at around 6,000 feet

• For the first 12 hours of lake effect snow, there should be cold air advection (CAA) at least through 850 mb, and possibly 700 mb

• After the initial 12 hours or lake effect snow, some snow will continue even with slight warm air advection (WAA) at 850 mb

• Never forecast lake effect snow with moderate or strong WAA at any level

• Wind speeds near the surface should be 10 kts or more, with most lake effect snows occurring with speeds of about 10-20 kts

• Wind speeds over 20 kts advect the air too quickly over the lake and also displace the snow band farther inland

• The longer the lower-level wind fetch over the lake, the better chance of lake effect snow

• With very large fetches over water, and very large water to air temperature differences, very heavy lake effect snow squalls may occur

• Pay attention to meso-scale features, such as vorticies and convergence zones either at the surface or aloft for possible enhancement of any snow potential

• Decision Tree for Lake Effect Snow

Lake Effect Snow (continued)

• Wind-Parallel Rolls: are created when strong winds flow with a component across the short axis of a lake; rising and sinking motions (due to heat transfer) create rolls, creating lake effect snow bands that are parallel to the wind flow; Figures 12.9-11

• Shore-Parallel Bands: are created when winds are weak and heat from the lake causes rising air, drawing in air from both sides, creating one long band of lake effect snow; places under such a band can received tremendous amounts of snow, while place only miles away will receive very little, if any, snow; Figure 12.12

• Vorticies: sometimes develop during a lake effect event and may be related to many variables, such as: horizontal wind shear, the magnitude of the wind, effects of the variation in shoreline topography, atmospheric stability, and lake-air temperature differences; tend to dissipate over land; have a radar signature not unlike that of a hurricane and have been known to develop an "eye" feature (without the strong winds); the snowbands surrounding this "eye" can drop a quick foot of snow, if is located over the shore; Figures 12.13-14