This past week, we have been hit another meteorology buzz term of “Lake-effect Snow” (known in the meteorological community as LES). Buffalo, NY was inundated by heavy snow amount with some places receiving > 80” in a 5-day period. It snowed so much that the NFL had to postpone and relocate the Buffalo Bills football game this past Sunday to Monday night in Detroit. Even the governor of New York, Andrew Cuomo, got into the LES talk by blaming the National Weather Service (NWS) about their terrible weather prediction, about which he was totally incorrect. The NWS did a fantastic job on letting people know of the severity of this LES event ahead of time. Since, here in Colorado, we do not get to experience LES events, let’s talk briefly about what they are and what atmospheric processes cause them.

What is Lake-effect snow?
A snowstorm occurring near or downwind from the shore of a lake or large body of water resulting from the warming (destabilization) and moistening of cold air during passage over a relatively warm body of water. Lake-effect snow occurs when mean lake temperatures exceed mean land temperatures; therefore, the lake is not usually frozen for this phenomena to occur.

Overview of the Lake-Effect Process
Lake-effect snow occurs to the lee (east) of the Great Lakes during the beginning of the cool season. When the cold air mass travels across a relatively warmer lake, the air mass acquires heat and moisture, which destabilizes the air mass. Cloud formation is further enhanced by thermal and frictional convergence and upslope along the lee shore.

Lake-Effect Snow Characteristics
Lake-effect snow is considered a mesoscale convective snow phenomena. Destabilization of the cold air mass is sometimes strong enough to produce thundersnow. Although convective in nature, the clouds in LES are very shallow compared to their Cumulonimbus counterparts (see Figure below), but can still pack quite a convective punch. These LES bands can come in a single- or multiband variety (See Figures below).

Lake-effect snow occurs from late fall through winter, though lake-effect rain can occur from late summer through mid-fall. Tremendous snowfall amounts and snowfall gradients are associated with LES events (see Figure below). Lake-effect snowstorms rarely produce multiple fatalities directly, but are very disruptive to commerce and transportation, with the exception of the recent Buffalo event (November 2014), where at least 12 confirmed fatalities have been reported.

While Buffalo is infamous for LES events, other parts of the Great Lakes region and North America and the world do experience this type of weather phenomena. The image below shows where these events can and do occur across North America.

Typically, the rule of thumb is that once the Lakes are frozen over, the majority of the LES events will diminish or end the lake-effect season. This is not always the case. A frozen lake does not necessarily preclude a lake-effect event if ice cover is not continuous. Sensible heat flux can occur through the ice but not as efficiently; therefore, if enough moisture is available, LES bands can still occur.

Key Processes from a Forecasting Point of View

• Localized instability - lapse rate and boundary layer depth
• Fetch – the distance in which the cold air moves across the warmer lake water (the longer the fetch, the higher the snowfall potential)
• Wind direction and shear
• Cloud microphysics – snow crystal habits
• Synoptic (large) scale forcing
• Orography/topography -- lake shape and orientation and lee-shore topography
• Upstream moisture
• Snow/ice cover on the lake

Forecasting Challenges
Lake-effect snowstorms are difficult to observe and forecast due to several factors. First, LES bands are shallow systems (depth often < 3 km), and the lowest elevation radar scans “overshoot” the tops. Secondly, the onset, intensity, orientation, and exact location are very sensitive to wind shear/direction and inversions in the lower troposphere. Third, LES bands are difficult to distinguish from orographic influences in some locations (e.g., Great Salt Lake). Lastly, some operational NWP models do not have sufficient resolution, microphysics, etc. to resolve the scales of lake-effect snow bands quite yet. The HRRRX, the ESRL version of the HRRR, performed well compared to its NCEP counterpart keeping the heaviest snow just of south of Buffalo.

Final Thoughts
Lake-effect snow is weather phenomena that can be difficult to forecast due to the sensitivity of many atmospheric variables, especially upstream fetch and wind direction. These events can last as long as the cold air moves across the “relatively” warm lake without much deviation from the overall wind field. In most cases, LES episodes do not create major societal distribution and multiple fatalities. The November 17-21, 2014 event near Buffalo, NY was an historic event and somewhat atypical as far as LES events go. Snowfall rates for this storm reached 5” per hour over many hours revealing the highly convective nature of the snow band. In the end, the NWS did a fantastic job in forecasting and anticipating societal impacts, but mitigation for an outlier event is nearly impossible.