Rochester and the Genesee valley experience a fairly humid, continental type climate, which is strongly modified by the proximity of the Great Lakes. Precipitation is rather evenly distributed throughout the year in quantity, but frequency is much higher in the cloudy winter months than in the sunny summer ones. Snowfall is heavy, but is highly variable over short distances.
Winters are generally cold, cloudy and snowy across the region...but are changeable and include frequent thaws and rain as well. Snow covers the ground more often than not from Christmas into early March, but periods of bare ground are not uncommon. About half of the annual snowfall comes from the "lake effect" process and is very localized. This feature develops when cold air crosses the warmer lake waters and becomes saturated, creating clouds and precipitation downwind. The exact location of these snowbands is determined by the direction of the wind. Areas east of Rochester receive the most snow from this process, as northwest winds have a longer "fetch" off lakeOntario, while areas south of the city get somewhat less. Lake Erie can even contribute some snow from this process if a west or southwest wind is strong enough. Since Lake Ontario does not freeze in most winters, this Lake Effect machine can remain active throughout the winter.The Rochester area is also subject to occasional general or "synoptic" snowfalls...but the worst effects usually pass the east. Total season snowfall ranges from 70 inches south of the city to about 90 inches in Rochester to over 120 inches along the lake shore east of the city. About 5O inches of this total results from general snows, the rest is due to the lake effect machine.The Lake does modify the extreme cold, the mercury falls below zero on only about six nights in an average winter, with anything below -l0 quite uncommon.
Spring comes slowly to the region.The last frosts usually occur by April 30 near the Lake...but as late as mid-May south of the Thruway. The spring months are actually our driest months statistically, due in part to the stabilizing effect of the Lakes, although soils are wet. Sunshine increases markedly in May.
Summers are warm and sunny across the region. The average temperature is in the 70 to 72 degree range. Rain can be expected every third or fourth day, almost always in the form of showers and thunderstorms. This activity is more common inland than near the lake. Completely overcast days in summer are rare. Severe weather is not common, but a few cases of damaging winds and small tornadoes occur each year. The greatest risk of this type of activity is south of the Thruway. There usually are several periods of uncomfortably warm and muggy weather in an average summer, with nine days reaching the 90-degree mark. Still, the area usually experiences some of the most delightful summer weather in the East.
Autumn is pleasant, but rather brief. Mild and dry conditions predominate through September and much of October, but colder airmasses cross the Great Lakes with increasing frequency starting in late October, and result in a drastic increase in cloud cover across the region in late October and early November. Although the first frosts may not occur until late October along the lakeshores, the first lake effect snows of the season follow soon thereafter. ..usually by mid November. These early snows melt off quickly, with a general snow cover seldom established before mid-December. The growing seassn is relatively long for the latitude, averaging about 180 days.The long growing season, combined with ample spring moisture and abundant summer sunshine...is beneficial for the many fruit orchards and wineries, especially near the Lake Ontario shore and the Finger Lakes.
Steve McLaughlin, Service Hydrologist
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