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Summers Grand Finale

August is the last of the true summer months, meteorologically speaking. The contrast of temperatures across the nation is at its annual minimum. The sun's intensity is diminishing (down on average 15% from its late June peak). And during the month, temperatures begin to drop slightly especially across the northern states. But the air is still very warm and in most places humid. Pressure gradients are at their annual minimum and winds as a result also tend to be light. This month is often thought of as the "lazy, hazy days" of summer.

The jet stream and storm track lift northwards to their most poleward position. The jet on average enters the West Coast near Seattle, WA and tracks across southern Canada to Newfoundland. The storm track shifts north into Canada. Their effect on the states is usually limited to the fronts that they drag across the northern areas. Important large scale, low pressure storm systems are rare across the United States. Cyclone frequency across the Great Lakes for example is less than 1/3 that during the winter months. The only exception is the southwestern thermal low, a semi-permanent reflection of the intense heat in the desert regions.

Instead, the states are dominated mostly by anticyclones (high pressure). In the east, frequently this is the extension of the warm Bermuda high which now occupies a position across the entire Atlantic with the center of highest pressure averaging about 35 degrees north.

In the Pacific, the high is also at its greatest expanse and strength in August. The axis is about 38 degrees north and usually extends north to British Columbia and the panhandle of Alaska.

Polar highs are also at minimum as Hudson Bay too has warmed and no longer provides a source region. Those that do form instead usually track from the Beaufort Sea across the Prairie Provinces and then quickly out into the Atlantic where they merge with the Bermuda High.

Monthly precipitation totals increase over July in the north and along the Gulf and Atlantic coasts but tend to diminish slightly in the interior. The primary weather problems are caused by tropical storms/hurricanes and nocturnal thunderstorm clusters called mesoscale convective complexes or MCCs, The first rapidly increasing in frequency and the second just beginning to diminish from its mid-summer peak. Also a major contributor for the rainfall totals are the localized afternoon thunderstorms, known as air mass thunderstorms.

Tropical storms develop further east in the Atlantic as waters warm, and track further north as the Bermuda high shifts north. The frequency more than triples that of July and the percentage of these storms that reach hurricane strength also increases to its annual maximum of 70%. In 1995, nine storms churned the waters of the Atlantic during August.

The two principle tracks are west from the Antilles into the Gulf of Mexico or further north towards Florida or the Atlantic coast. Fortunately, most years, most storms will recurve out to sea before ever hitting land. However, when one does make landfall, the result can be devastating. Because the polar front is at its northernmost position and the oceans further north are nearing their warmest temperatures, these storms become a threat even to the more northern states. The threat there increases rapidly late in the month.

The nocturnal thunderstorm complexes that bring flooding rains in summer continue in August. There effect is greatest in the northern states but they can spread deep into the south under the right conditions.

Air mass thunderstorms are still common but they too tend not to be as severe as during the more intense heat and instability of early and mid-summer. In the southwestern states and Rockies, the monsoon effect continues in full swing. Places like Phoenix, AZ and Santa Fe, NM are at or near their annual precipitation maximum as thunderstorms fire up as moisture has been drawn into the thermal low from both the Pacific and the Gulf of Mexico.

Temperature extremes for August range from 127 at Greenland Ranch (Death Valley), CA on August 12, 1933 to 5 degrees in Bowen, MT on August 25, 1910.