Weather In Depth: The dark mystery into why storms fire up east of Rochester

Have you ever wondered why, when the forecast calls for afternoon thunderstorms across Western New York, and the Finger Lakes, most of the time those storms either fire up well south or Rochester or well east?

In order to understand why, we need to go back to the basics — that is, the lake breeze. The lake breeze is a circulation between Lake Ontario and the surrounding land. This circulation can set up at any time during the year, but is most pronounced during the spring, summer, and fall. Water takes longer to heat up when compared to air, and that temperature difference between the cooler water and warmer air sets the stage for the circulation to begin. Warm air likes to rise, so the warmer air over the land will rise in the afternoon. This will then create a pressure difference with lower pressure over land and high pressure over the water. Air moves from high pressure to low pressure, which means that the cooler lake air overrides the void left by the rising warmer air. This will cool our surface temperatures near the lake.

With that happening, there are also pressure differences above that surface circulation. About 3,000 or so feet above the surface, the warm air that has risen created an area of high pressure with relatively lower pressure over Lake Ontario. This is what keeps the circulation going. The high pressure aloft will circulate over to the lower pressure above Lake Ontario.

Great, but what does this have to do with thunderstorms firing up away from our region? Well, with the cooler air moving in at the surface a “cap” is formed. A “cap” in meteorology means an area within the atmosphere where air cannot rise (where cooler air is underneath warmer air), and depending on how far inland that cap reaches it stunts thunderstorm growth. The only way to beat the “cap” is with frontal forcing, warming the air below, or mixing the atmosphere to lessen the strength of the “cap.” This area where the “cap” is forms clear skies while areas outside that area of stable air have cumulus clouds bud. Most of the time, that clear patch of sky is over our region. This means we have stable air over Rochester with unstable air east or south of the city, depending on how deep the lake breeze is. This in the summer creates a “lake shadow.” The “shadow” is the region of where skies are clear, and the shadow is what keeps storms east or south of our region.

The lake shadow can help and hurt us. It is helpful for keeping the strongest of the storms away from our region on most occasions, while it can hurt us because when we need a drink of water it can keep us dry from any summertime storms. Another reason why thunderstorm forecasting is challenging in our part of New York state.