While we're all familiar with lake effect snow (if not, click here), lake effect rain is a far less common occurrence. The same phenomenon applies, cold air blows over the relatively warmer waters of Lake Ontario, picking up moisture and depositing it upon us. This time, the air is not quite cold enough to support snowfall, so precipitation falls as rain. Just like lake effect snow, the rain falls in finger-like bands. The example below is from this morning. As we can see, a narrow but intense band of rain is situated just south of Watertown.
The rapidly changing wind direction today will lead to periods of rain and low clouds, followed by breaks of sun. As the wind changes to a southerly direction tomorrow, we will see a return to pleasant and sunny conditions. However, as another cold front swings through on Monday, switching the wind to a northwesterly flow, the lake effect rain will return, and we will see the frequently changing conditions throughout the day.
Why don't we see this more often?
As discussed in the lake effect page, lake effect snow and/or rain is fueled by a difference in temperature between the lake water and the air mass above it. With today's high being in the mid 40s, and the lake temperature being about 58ºF, the temperature difference is just enough to allow for lake effect. If the air temperature were 5ºF warmer, there would not be enough of a difference to allow for lake effect. If the water were 10ºF colder, the precipitation would be falling as snow, not rain. Given the relative stability of lake temperatures, air temperatures that can support lake effect rain are generally only range from about 35ºF to 45 ºF. On the other hand, a temperature anywhere from 34ºF to -20ºF, a 54ºF range, can support lake effect snow. To summarize, there is a lot smaller margin of error for lake effect rain conditions than there is for lake effect snow conditions.