Lake-effect rain

Lake-effect rain is the liquid equivalent of the lake-effect snow where the rising air results in a transfer of warm air and moisture from a lake into the predominant colder air, which ends up in a fast buildup of clouds and precipitation downwind of a lake. The phenomena is made up of accumulated rain bands that can cause localized flash flooding, thunder, lightning and even waterspouts in extreme events. Although the phenomenon is associated with the North American Great Lakes, it occurs downwind of any large lake that can hold its summer heat well into the cooler days of autumn and early winter.

Formation

Lake-effect rain formation is fundamentally the same as lake-effect snow; cold air moves across the relatively warmer waters of lakes, thereby creating a sharp drop in temperature from the lake surface through the first several thousand feet in the atmosphere (a measure of the that temperature drop with height pertains to the lapse rate), and then it precipitates the moisture over the lake or on the downwind shore, depending on how much cold air and the lift. Only difference is the water and the air temperatures are several degrees warmer. The air is still cold enough to carry on the process, but warm enough in the lower layers so that the precipitation falling on the ground is rain rather than snowfall. The boundary layer's temperature must be higher than 0 °C (32 °F) through an adequate depth to melt the snow to liquid precipitation. Generally, a temperature difference of 10 degrees Celsius between the air and a waterbody at around 850 millibars, a lake effect can occur.

When the lake water is cooler than the air temperature, cloud development is hindered. Waterspouts can develop in the rapid cooling of the air with height in the downwind zone. A study of lake-effect rainfall for Lake Erie by Pennsylvania State University meteorologists Todd J. Miner and J. M. Fritsch found out that, unlike many lake-effect snow events, the conditionally volatile layer for lake-effect rain events was more thicker, therefore allowing for higher convective activity and frequent thunderstorms – That is why the highest amount of of lake-effect days with thunder along Lake Erie develop from late September to mid-October (since the sheet of the unstable air is deeper).

Occurrence

In the United States, lake-effect rain showers first begin to form in September east of Lake Erie, through to November, which becomes a transition period where, afterwards, virtually all lake-effect precipitation falls as snow. In the seven years studied in the 1990s, a total of 32 lake-effect rain events were counted, in contrast the 20 snow events, in areas downwind of Lake Erie. In Australia, Port Phillip is often warmer than the surrounding oceans and/or the land mass, particularly in spring and autumn; this can set up a "bay effect", where showers are intensified leeward of the bay (particularly in Melbourne's eastern suburbs), while the rest of Melbourne and surrounds stays dry.