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Summer In Full Swing

June is the first full month of meteorological summer. The days continue to get longer until the third week of the month, when the summer solstice occurs (the 20th or 21st).

The high angle of the sun and the long days lead to significant changes across the country. Snow cover disappears everywhere except the highest elevations. The land becomes warmer when compared to the oceans, leading to cooling seabreezes along the coast. The heated land causes surface air to rise, resulting in the inward flow of air from the oceans. Low pressure areas tend to form over land, while high pressures areas form over the oceans. Overall, the surface pressure gradient is much weaker than the previous months, resulting in gentle breezes much of the time.

High pressure areas take up semi-permanent residence off each coasts. The Bermuda High in the western Atlantic pumps warm and humid air northward over the Eastern U.S., resulting in occasional heat waves. The Pacific High extends eastward to the Pacific Northwest. This feature combined with the Southwest U.S. thermal low produces a northwest flow off the ocean, resulting in the comfortably cool and dry summer climate found along coastal regions.

The jet stream and main storm track shift further north, with the axis lying across the northern part of the country. The frequency of severe thunderstorms and tornadoes move northward with the jet stream as well, although the Southern Plains and Southeast certainly still get there share of severe weather. In recent years, June has edged out May as the month with the most average tornadoes. In fact, the all-time monthly record for tornadoes was set in June, 1992 when 399 tornadoes touched down. Tornadoes in June tend to be weaker than those during the spring months, due to weaker temperature contrasts and a weaker jet stream.

June is the first official month of hurricane season, although tropical storms and hurricanes do not occur every year. What storms do form usually do not get too strong, as sea temperatures are still recovering from the winter season, and upper level winds are not quite yet ideal for significant development.

Afternoon showers and thunderstorms become common across the country during the month. A large area of the country from the central Appalachians westward to 100W and from Lake Superior to Arkansas has over 4 inches of rainfall in an average June.

Another 4 inch area extends along coastal regions from Cape Hatteras to Florida and then along the Gulf Coast to northeastern Texas. Southwest Florida averages over 8 inches as the tropical rainy season begins. West of the Rockies, precipitation drops off from spring amounts. In the Southwest, June is one of the driest months of the year.

Average temperatures rise to over 70 F over much of the country, with averages exceeding 80 F over most of the southern U.S. The highest temperature recorded for the month was 129 F at Volcano Springs, CA set on June 23, 1902, and the lowest temperature was 2 F at Tamarack, CA on June 13, 1907.

The Dog Days Of Summer Begin

Summer heat reaches its greatest intensity in July. In all but the northern fringe and mountain areas, the daily temperatures average above 70 F in July. The southern half of the nation averages over 80 and average temperatures of 90 or higher appear over portions of California, Arizona and Nevada.

In addition to the prevalent heat and humidity, July is characterized by relatively gentle winds and frequent showers. The polar region cold air factory is largely shut down so polar highs are weak and small in size but the sub-tropical highs over both the Atlantic and Pacific continue to expand and shift to their northerly summer position. In the Pacific the high called the Pacific High stretches east west along the latitude of Central California. The Atlantic high is centered roughly near the island of Bermuda and is nicknamed the "Bermuda High". The Bermuda high typically stretches out along the latitude of Cape Hatteras, North Carolina.

In the Pacific, the clockwise flow around the sub-tropical Pacific high causes upwelling in the coastal ocean waters, which transports cool water upwards. The northwest wind flow around the high pressure crosses the cool water and provides natural air conditioning to the entire West Coast. However on the eastern side of the continent, the same clockwise flow around the Atlantic Bermuda High means southwesterly winds for the eastern United States. This wind moves across the warm Gulf Stream or Gulf of Mexico waters and as a result is very warm and humid.

The Bermuda high builds inland in July, resulting in the humidity spreading well westward often to the Rockies. The moisture available for precipitation is at its maximum. However, the contrast in temperatures across the nation is near its annual minimum. Except near the cool waters right along the northeast and Pacific coasts, very little cool air is to be had in July. The polar front is at its weakest and near its northernmost position across the northern tier of states. Low pressure centers along this front, which rely on the thermal contrast to develop, remain relatively weak.

The main precipitation mechanism in July is convection. For most of the nation, the precipitation occurs during the hottest part of the day, when the air is most unstable. These "air-mass thunderstorms" tend to be brief and/or localized. The thunderstorms can organize in clusters and become severe when the warm, moist air converges on the weak frontal zones or encounters other boundaries.

Most of the nation east of the Rockies average between 2 and 4 inches of rain in July. A few areas like interior New England and New York and along the Atlantic and Gulf Coast average more than 4 inches. The heaviest rains fall along the Gulf coast from Key West to Louisiana with an average of more than 8 inches. Along the Gulf Coast it is the sea breeze that provide the added convergence and lifting that triggers the almost daily thunderstorm formation and the heavy resulting rains.

Nocturnal thunderstorms also peak in July. Large complexes of thunderstorms, called mesoscale convective complexes or MCCs, develop often near the polar front. They feed off the extreme heat in the tropical air mass well to the south. The hot air rises and then is carried northward by the low-level jet stream to the front where it helps feed developing storms. These large storms are often characterized by drenching rains, powerful straight line and occasionally tornadic winds and hail. They develop at night and move quickly, often traveling hundreds of miles before dissipating during the early daylight hours.

In the tropics, activity begins to increase as the more northerly position of oceanic high pressure causes easterly winds to blow from Africa across the ocean to the West Indies and the Gulf. Disturbances spin of the African coast and move westward in the flow causing squally showers.

Given the right environment (warm enough ocean waters, favorable outflow aloft, no and strong shearing), these disturbances can develop into early season tropical storms and possible hurricanes. These disturbances and storms typically travel around the southern end of the Bermuda high before recurring northward. Depending on how strong and how far westward the high pressure ridge to the north extends, the re-curvature can be harmlessly out into the Atlantic, up along the east coast, into Florida or the Gulf Coast or even straight westward into Mexico or Central America.

Temperature extremes for July: maximum temperature 134 F at Greenland Ranch, California on July 10, 1913. The minimum temperature was 10 F at Painter, Wyoming on July 21, 1911.