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The Carribean on the Chesapeake


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Autumnal Splendor

The World Series, weekend and Monday night football, eye-pleasing foliage followed all too soon by the familiar sound of leaf raking, the first frosty mornings, the still fresh farmstand vegetables, pumpkins in the otherwise empty fields, the busy stacking of wood, the first fires of the season in the hearth, the unmistakable smell of wood smoke, and lest we forget Indian Summer - all sure signs of fall and of October. It can be a time of great beauty and splendor. The weather tries to cooperate and for the most part does. This is often one of the most tranquil months of the year.

Tropical storms are diminishing though still a concern. Actually, the first part of the month often sees a renewed spurt of activity as the Caribbean once again becomes an active source region. The activity declines steadily thereafter as the oceans, whose warmth drives these heat engines, cool.

When tropical storms develop in the Atlantic in October, they tend to stay further offshore as the westerly steering winds have once again dropped south. Those that develop in the Caribbean or Gulf can prove to be more of a problem as they may track across Georgia and Florida and up along the east coast as did Hazel in 1954 and the Great Hurricane of 1846.

Temperatures are now accelerating downward, in many locations down 10 degrees or more from September levels. The first snowstorms of the season are in the news, mostly now in the higher elevations but all too soon down to where most of us live and work.

Before the winter cold and snows set in for good, we often see in October and even November a period or two of hazy, warm weather. These reliable spells have been called Indian Summer ever since the American Revolution. It was a period which Indians and early settlers took advantage of the fine weather to make final preparations for the hardships of winter.

It usually occurs after a big cool Canadian or mild Pacific high pressure settles into the Ohio Valley and Mid-Atlantic region and stalls. It may produce several days of warm daytime, cool nighttime weather. The sinking of the air in the high pressure and clear skies causes an inversion to develop which traps low level moisture and pollutants which in turns results in haze.

In October, the rainy season begins rather suddenly in the Pacific States as the big Pacific high weakens and retreats south to 33 N latitude (abreast of San Diego). This allows Pacific storms to once again come ashore into the west coast. Sometimes, these early storms can be very intense with damaging winds and coastal flooding.



While the west becomes wetter, most of the rest of the nation dries out. October is actually the driest month of the year in places like Portland (ME), Providence, Philadelphia, Pittsburgh, Atlanta, Little Rock, Birmingham, New Orleans, Nashville, Charleston (WV), and Louisville (KY). Nearly gone are the afternoon "air mass" thunderstorms of summer. The tropical moisture from the Gulf of Mexico begins to get suppressed by the increasing westerlies. Eventually, it will be hard to find this tropical warmth at the surface except in Florida.

The Aleutian low strengthens, fed by Pacific storms and dying typhoons. This feature will be a dominant feature for the next 7 months. The exact position and orientation of the low will have a profound effect on the weather across North America - where it will be cold, warm, wet and dry.

The primary storm track is still in Canada but dropping south. A new storm track appears in the U.S. from the western Central Plains to the Great Lakes and Ontario. Occasionally an early coastal storm will develop near the East Coast. When they form this time of year, they can be very wet as tropical moisture is still readily available. Occasionally these storms interact with tropical storms, which can cause rainfall to be especially heavy and winds and tides to be a problem.

The polar highs are beginning to grow in size and become increasingly cold as sunshine in the northern latitudes source regions vanishes. It is these high pressure systems that drop south and expand to bring for some areas in the deep south the first welcome relief from the heat and humidity of summer since May or June. In October, these highs are often slow moving and as they warm, can bring very pleasant Indian Summer conditions.

Cold polar highs have some difficulty moving over warm water. A prime reason for their high pressure is the density (heaviness) of the air. As the air warms, both the density and pressure diminish. Thus these cold highs often erode away as they try to move offshore thus often appearing to stall.

The thermal low in the southwest weakens and the Great Basin high becomes established. This feature occasionally builds in response to strong ridge building aloft and causes an easterly gradient across the mountains to the coast.

This in California results in the Santa Ana winds, which can bring extreme heat to the coast as the dry air descends and heats due to compression. September and October are actually the two warmest months in places like San Francisco due to this effect. This area recall had a "cool" summer due to the steady onshore winds around the dominant Pacific high.

In the Atlantic, the dynamically caused Bermuda high is also weakening and dropping south and east. This high which helped steer tropical activity westward in the tropical Atlantic now makes way for the eastward tracking non-tropical storms in the Central and North Atlantic.

Temperature extremes for October range from 116 at Sentinel, AZ on October 5, 1917 to -33 degrees in Soda Butte, WY on October 29, 1917.