Lake-Enhanced Precipitation

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In order for a low-pressure system to become “lake-enhanced”, the low must be passing over a large source of relatively warm moisture, such as the Great Lakes, the Manitoba lakes, or another large and open body of water, and moving onshore.

The lake(s) must be mainly to entirely ice free and have a surface temperature that is warmer than the air around it. This will allow a significant amount of warm, humid air to rise from the water’s surface.

Similarly, lake enhanced precipitation requires an active low-pressure system to be passing over an open body of water, while surface winds below the system move onshore. Similar to lake effect snow, the lake-enhanced precipitation will form when onshore winds take warm, humid air from over the lake and brings it to shore.

This is an example of a low-pressure system path (in white) that would allow for lake enhancement. The low-pressure system here (an Alberta Clipper) is moving towards the Great Lakes, which would be completely or mostly ice free.

Image Source: Environment and Climate Change Canada

When the low moves from land to over the open, warm water source (Step 1), it is fueled from below by the water (Step 2). Since the temperature of the air is cooler than the water, the warm, humid air from the lake is able to rise, adding additional moisture and heat to the low-pressure system. This additional energy source prolongs the life of the low-pressure system and increases the amount of available moisture to turn into precipitation.

Not only does this additional “precipitation fuel” get taken in, but as the low and winds push onshore (Step 3) there is additional localized moisture thrust up into the lowest layers of the low-pressure system, further boosting precipitation. The onshore flow in combination with the low striking the shore results in locally higher precipitation rates at and inland of the downwind coast.

Similarly, lake enhanced precipitation only requires some level of onshore flow within the low-pressure system. Once this modified air passes over the lake and crashes into the shore, it will shoot up into the low-pressure system, rapidly generating additional precipitation to fall from the low-pressure system onshore.

Image Source: Environment and Climate Change Canada

Dissipation

The low-pressure system will continue to be modified by the warm, open waters as long as:

the sea surface temperature remains warmer than the air;
the water remains open (mainly/entirely ice free); and
the low-pressure system is able to draw in that warm, moist air from the water.

Additionally, lake-enhanced precipitation can occur as long as the onshore flow coincides with the synoptic precipitation, usually occurring ahead/north of the low/warm front in easterly flow, or behind/west of the low in northwesterly flow.

Once one of those mechanisms is gone, then the lake enhancement will stop.

Duration

The time over which the low is modified depends on how quickly the low moves over the water source but is usually limited to a few hours.

The heaviest precipitation in the combination of the onshore flow under the enhanced synoptic feature will only last as long as the onshore winds persist, under the organized precipitation. Generally, this will also last only up to a few hours.

As this phenomenon requires open, warm bodies of water, it would be most typical to see this phenomenon in late fall or early winter, where the lake is still warm compared to the air around it, and generally ice-free. Lake enhanced low pressure systems generally need to pass over large open bodies of water, such as the Great Lakes, Manitoba Lakes, over Hudson Bay, or the coasts before too much ice forms. Here is a map of Canada showing water basins. Regions surrounding Manitoba Lakes, Great Lakes as well as coastal regions are susceptible to lake enhanced precipitation.

In the case of lake enhanced precipitation, there are many locations in Canada that can offer this additional onshore-flow precipitation fuel to existing low-pressure systems:

Easterly/north-easterly flow:

Golden Horseshoe (west end of Lake Ontario, including Mississauga Pearson (CYYZ), Billy Bishop (CYTZ), and Hamilton (CYHM) airports)
West end of Lake Superior (including Thunder Bay, ON (CYQT))
Eastern Nova Scotia, New Brunswick, Newfoundland, and Labrador
East coast of the Gaspé peninsula

Westerly/northwesterly flow:

West coast of Newfoundland
Cape Breton Highlands
Annapolis Valley
East/southeast of Lake Huron/Georgian Bay/Lake Superior
South/east Lake Winnipeg/Manitoba
North coast of the Gaspé Peninsula

Image Source: Environment and Climate Change Canada

Numerical guidance can have difficulty with moisture transport in the lowest layers of the atmosphere, so it can be difficult for them to detect the lake-enhanced process, and manually adding in the effect of lake-enhancement in a forecast can be difficult.

Additionally, upstream or nearby observations that are receiving precipitation as part of the same low-pressure system but are not in onshore flow may not be as helpful in predicting visibility and precipitation rates for a station in onshore flow.

Lastly, the area of lake-enhanced precipitation will vary with the winds, so timing wind shifts, and frontal passages are very important.

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Lake-enhanced precipitation is rarely represented in the GFA, but it could be mentioned as a locally higher snowfall rate with blowing snow in onshore flow within a precipitation area.

The TAF will likely be represented by locally lower visibility in snow and possibly blowing snow compared to nearby and upstream TAF sites also experiencing snow.

This model image taken from the 1200Z run of the Regional Deterministic Prediction System (RDPS) on December 12, 2022 is the T+81hr forecast of precipitation rate and type (solid fill; blue for snow, green for rain, pink for ice pellets, red for freezing rain, and yellow for mixed precipitation or wet snow) and mean sea level pressure (black contours, with areas of minimum pressure marked with a red “L” and areas of maximum pressure marked with a blue “H”). From this model prediction, there is a relatively large swath of synoptic precipitation north and east of the low-pressure system and warm front. However, the model is picking up on higher snowfall rates (noted with the darker shades of blue) hugging the western shore of Lake Superior. This is a result of the onshore flow with easterly winds enhancing precipitation rates.

Image Source: Environment and Climate Change Canada

This radar animation shows the presence of lake-effect snow flurries on the west end of Lake Ontario with easterly flow between 1200Z and 1500Z ahead of the large-scale (synoptic) band of precipitation seen over southwestern Ontario. Visibilities in the synoptic snow band (shown in the text boxes overtop of the TAF sites) generally range from 1-3 statute miles (SM) at worst. However, with the lake-enhancement near Toronto, visibilities fall below 1SM at CYYZ and CYTZ when that band goes overhead between ~1530Z and 1800Z. There is then another round of lake-effect snow flurries at CYYZ and CYTZ between ~1800Z and 2100Z.

Image Source: Environment and Climate Change Canada

Given that poor visibilities or significant snow accumulations are not SIGMET criteria, a SIGMET will not be issued for these events. However, if a forecaster notes widespread low visibilities that are not accurately depicted on the GFA, an AIRMET will be issued to notify the aviation industry of the hazard.

Will likely be represented by lower visibilities and higher snowfall rates at TAF sites experiencing onshore flow during a large-scale storm system while other surrounding TAF sites are also experiencing precipitation but at a lower intensity and with higher visibility.

This is a series of observations from CYYZ between 1500Z and 2100Z on November 15, 2022. These observations follow the radar animation shown in the radar section that CYYZ reports snow with visibilities as low as 1/2SM with the lake-enhanced snow band between 1530Z and 1800Z. Thereafter, between ~1800Z and 2100Z, the visibilities become much more variable with the lake-effect flurries.

Image Source: Environment and Climate Change Canada

Special weather statement (SWS) issued by the public forecast office in Ontario for Thunder Bay on December 12, 2022. In this public bulletin, locally higher snowfall amounts due to lake enhancement from easterly flow off of Lake Superior are mentioned. This is from the same low-pressure system as pictured in the model data shown in the Strategic Charts section.

Image Source: Environment and Climate Change Canada

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