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A Rapid Transition To Summer Weather

Unlike April, when the advance of the springtime warmth was uncertain, uneven and for many of us too slow, in May the rapidly lengthening days and increasingly strong spring sun makes the arrival of spring for all inevitable.

With the exception of the mountains, the snow and ice vanishes from most all of North America. The ice disappears even in the most resisting lakes and ponds in Canada and Alaska by mid-May. With the disappearance of the ice and snow in northern continental areas, the factory of cold polar high pressure systems for the most part shuts down. The chilly waters of Hudson Bay, James Bay and the Great Lakes can still cause relative high pressure to form but these air masses tend to be small and cool instead of cold.

As the season advances towards summer, the action shifts from the Polar Regions to the Tropics and Sub-tropics. The maritime tropical air masses centered over the Atlantic and Pacific become the dominant highs across the summer hemisphere. Because the circulation around these high pressure centers is clockwise in the Northern Hemisphere, the effect on the United States differs east and west. The Pacific high helps keep the West Coast cool and dry while the Atlantic high (known as the Bermuda High) brings the three H's of summer (haze, heat and humidity) to the eastern states.

This is because the circulation around the Pacific high off the West Coast brings north and northwest winds to the coastal areas. This wind flow also induces upwelling of cold water off the West Coast. The flow off the cool water induces a natural air conditioning for the coastal areas, although the dry air and clear skies allow the inland areas to get quite warm, even hot.

Along the Gulf of Mexico and off the East Coast, the flow around the Atlantic Bermuda High brings southerly winds. The flow off the very warm water brings warm, humid air well inland. By mid May, the air conditioners switch on in the south where they will for the most part stay on until late September.

Except along the West Coast and Gulf, temperatures in many areas of the United States warm about 10 F from April average levels. By the end of the month, frost is unlikely for most areas and vegetation across most of the nation is in full bloom.

Over most of the nation east of the Rockies, the sun working on the increasingly warm, moist unstable air causes showers and thunderstorms to become frequent. When fronts are present, more organized activity and often, severe weather occurs. The polar front and storm track lifts north to the northern half of the nation in May. Troughs can still make it inland in the west and cross the mountains and induce storms along this boundary. This scenario brings the biggest severe weather outbreaks.

May and June are the two peak months for tornadoes. May had been the statistically the worst month for a number of years, but in the recent years some very active Junes pushed that month into the lead.

The peak month for deaths continues to be April. This may be due to a number of factors. For one, the dynamics are often strongest early in the season causing more powerful tornadoes. Another reason may be the lack of preparedness as early season storms catch some of the forecasters, media and the public not yet in the swing of the season and prepared for action.

Another reason may be that early in the season, the storms occur in the south where trees and hills may limit distant vision and where tornadoes tend to occur within heavy rain areas, obscuring them from view. Later in the season when action shifts further north, tornadoes can often be seen many miles away over the treeless, flat plains and the funnels are often separated from the rain areas, thus making them more easily discernible.

This flow of tropical moisture into the nation causes precipitation to increase rapidly. Precipitation is both convective and frontal in nature. May and June are the wettest months in Kansas City, MO and Minneapolis, MN.

Meanwhile, along the southern Atlantic coast areas, as the cyclonic activity decreases, rainfall totals actually drop. Later in the warm season, when the tropical waters heat enough to cause tropical activity to increase, these areas will see precipitation once again increase (over double the April and May totals).

In the west, as the polar front shifts north and the cool, dry, stable air from the Pacific high becomes dominant, rainfall in California and southern Oregon become insignificant and further north in places such as Seattle, WA and Portland, OR drops to half the winter month totals.

May temperatures have been as hot as 124 F (Salton, CA on May 27, 1896) and as cold as -15 F (White Mt. CA on May 7, 1964).