<P> The convective season for the Plains ranges between May and September . Organized systems of thunderstorms known as mesoscale convective systems develop over the region during this period, with a bulk of the activity occurring between midnight and 6 a.m. local time . The time of maximum precipitation during the day gradually varies from late afternoon near the slopes of the Rockies to early morning near the Ohio River valley, in part reflecting the west - to - east propagation of mesoscale convective systems . Mesoscale convective systems bring 30 to 70 percent of the annual warm season rainfall to the Plains . An especially long - lived and well - organized type of mesoscale convective system called a mesoscale convective complex produces on average 8% to 18% of the annual warm season rainfall across the Plains and Midwest . Squall lines account for 30% of the large thunderstorm complexes which move through the region . </P> <P> The subtropical jet stream brings in upper level moisture from the Pacific Ocean during the cold season . Ahead of storm systems, significant moisture becomes drawn in from the Gulf of Mexico, which increases moisture within the atmospheric column and leads to precipitation ahead of extratropical cyclones . During the El Niño portion of ENSO, increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, polar jet stream . Rare ocean effect snows are possible along the coast of eastern Florida . In the area around Memphis, Tennessee and across the state of Mississippi, there are two rainfall maxima in the winter and spring . Across Georgia and South Carolina, the first of the annual precipitation maxima occurs in late winter, during February or March . Alabama has an annual rainfall maximum in winter or spring and a dry summer . </P> <P> During the summer, the subtropical ridge in the Atlantic Ocean strengthens, bringing in increasingly humid air from the warm Atlantic, Caribbean, and Gulf of Mexico . Once precipitable water values exceed 1.25 inches (32 mm), afternoon and evening thunderstorms break out at the western periphery of the subtropical ridge across the Southeast on a daily basis . Summer is the time of the second rainfall maximum during the year across Georgia, and the time of the main rainfall maximum in Florida . Evidence shows that increased air pollution is causing rainfall to reach a maximum in intensity during the middle of the week . During the late summer and fall, tropical cyclones move into the region from the Atlantic and Gulf of Mexico, supplying portions of the area with one - quarter of their annual rainfall, on average . Fall is the time of the rainfall minimum across Louisiana . Sometimes, Gulf moisture sneaks up the Front Range of Rockies as far north as the northern High Plains, bringing higher dewpoint air into states such as Wyoming and Montana . </P> <P> Extratropical cyclones can bring moderate to heavy snowfall during the cold season . On the backside of these systems, particularly those moving through the eastern United States, lake effect snowfall is possible . Low level cold in the winter sweeping in from Canada combine with relatively warmer, unfrozen lakes to produce dramatic lake - effect snow on the eastern and southern shores of the Great Lakes . Lake - effect precipitation produces a significant difference between the snowfall around the Great Lakes, sometimes within small distances . Lake effect snowfall accounts for 30 to 60 percent of the annual snowfall near the coasts of the Great Lakes . Lake Erie has the distinction of being the only great lake capable of completely freezing over during the winter due to its relative shallowness . Once frozen, the resulting ice cover alleviates lake - effect snow downwind of the lake . The influence of the Great Lakes allows the region to lie within a Humid Continental Climate regime . </P>

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