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North American Climate Patterns - 07.11.04


The yearly climate patterns in the United States are described in monthly sequences in this article. A seasonal alamanac of the United States is the result.

Most researchers in climate and weather would be surprised to hear that the envelope of the atmosphere is not the only system in which the forces that produce weather and climate can be observed. Doc Weather includes the cosmos as the other system. But before we take off into the stratosphere in search of climate patterns, it is necessary to ground our vision in the accepted truths of climate study.

In meteorological circles, the term climatology refers to the default level of weather for a particular geographical area. This means if all signals fail to give a clear weather picture of a coming time period, the forecaster will refer to the historical climatic mean for that area on that date in order to make a prediction. In weather bulletins in which the forecast is not really evident from an analysis of the data the term climatology will be printed over the chart as an indication of the default. To make forecasts, climatologists take the most prevalent pattern for that date as the basis for seeing into the future. If it rained for the past six years on the first of March it will probably rain on the first of March this year. This is the basis for climatology.

Many long-range forecasts are made using climatic techniques to make the predictions. In such a prediction the climatic mean for a season will be compared statistically to the present season and a probability scheme will be formed which will be the basis for the long range prediction. At the foundation of this are several semi-permanent climatic scenarios that can be predicted in advance based on the climatic records. In this paper the climatology of the continental US will be presented in a series of charts and then a few standard climatic patterns will be presented as examples of the climatology for particular places in particular time frames.


Fig.1


Fig.1

In (FIG 1) the mean pattern for both summer and winter from Hawaii to the east coast of the US is illustrated. It shows a stream of rapidly moving air high aloft, known as the polar jet stream moving from west to east across the continent. The jet stream is being deflected north by the mountains in the western states. Then in the lee of the mountains a trough is formed in which the jet stream dips first south over the Midwest and then back into the northeast as it exits the continental US. This is the most fundamental pattern for North American climatology. It brings cold Canadian air into the US from the north and distributes warm monsoon air from the southwest across the continent. In the west the mountains create a barrier for Pacific air, preventing the easy entrance of Pacific moisture into the western US for most of the year.


Fig.2


Fig.2

In (FIG 2) the lee trough is strong enough to have formed a stationary low pressure area which is placed far to the east of the Rocky Mountains just south of Hudson Bay. This placement and the usual counter- clockwise circulation around the low would bring stormy weather and cold temperatures into the eastern US. In this pattern it would be dry in the high plains and cold would penetrate down into Florida threatening the citrus crops.


Fig.3


Fig.3

In (FIG 3) a weak low-pressure area is placed in the east and the cold air moves horizontally across Canada as warm air flows into the continent from the south in the monsoon jet. In this situation there would most likely be heat waves from the desert rising up into the Midwest. As the cold air meets the warm air over the Great Lakes area it could be expected that strong summer thunderstorms would be the result for the Great Lakes and the northeast. From this we can see that the geography of the US, especially the placement of the mountains on north south axis creates a particular physically driven dynamic for weather on the continental US.

Troughs or low- pressure areas are formed by rising warm air. Winds blow into the center and upwards in a trough as they also circulate counter clockwise. A ridge or high-pressure area is formed by falling cool air. Winds blow down and out from the center around a ridge as they circulate clockwise. Troughs and ridges can sometimes generate semi-permanent lows and highs respectively which fall out of the general circulation and can persist over weeks in the same place without moving to the east. Semi- permanent features are the source of most climate patterns.


Fig.4


Fig.4

In (FIG 4) a strong mid-continent trough is displaced to the west against the Rockies. In this chart the cold comes down into the Great Basin and a low forms in the vicinity of Denver with storms and cold in the Midwest. A pattern like this can often bring cold and snow into the mountains of Utah and even into California, threatening frost sensitive agriculture there. In general this pattern includes a strong low over the Aleutian islands or the Gulf of Alaska and a reciprocal weak high over Hawaii for most of the winter, as well as a strong low over Hudson Bay and a weak fluctuating high over the Great Basin for most of the winter. When the Great Basin high is strong the polar jet does not come south into the basin and range area. When it is weak a trough forms in the lee of the Rockies bringing storms out of the Gulf of Alaska through the Great Basin to the High Plains.


Fig.5


Fig.5

Taken together, these relationships can give rise to several different patterns of climate in any given year. In (FIG 5) we see the annual climatology for April, May and June. In the eastern Pacific there are two general features in the spring and early summer. One is strong high- pressure air between Hawaii and the coast. The high pressure forms a ridge-like air mass of clockwise moving air that effectively blocks the storm jet from the North Pole from dropping south into the mid-latitudes. For this reason every year there is a drought condition on the west coast from May or June until the Hawaii high diminishes in September. This rotating air mass forms a semi-permanent high pressure ridge which lodges against the Sierras and keeps skies clear and a predominating northwest wind blowing into the coast most of the summer. This in turn sends warm surface water to the south to be replaced by cold deep flowing currents in a narrow strip right against the coast of California. It is this cold upwelling which is being driven by the relentless clockwise winds which creates conditions for the famous San Francisco fogs in the summer months. The Hawaii high builds up to summer strength during the months of April, May and June. When this happens the center of storm activity in the Aleutian Islands is reduced in prominence. Polar breakouts of very cold air are much less frequent or severe during the summer months since northern latitudes are being warmed by the midnight sun every day in summer.

As a result, another annual climatic east Pacific pattern is the reciprocal to the growth of the Hawaii high, this being the shrinking of the Aleutian low in March and April. However, as the Hawaii high builds in strength against the Sierras a low in the Great Basin builds in response and begins to create storm centers as the desert heats up. Convection currents set the stage for lines of thunderstorms streaming to the east along a path into the mid-Atlantic seaboard. Moisture from the Sea of Cortez and the Gulf of Mexico feed this storm genesis. At the same time the polar jet coming from the Gulf of Alaska often dips down into the corn belt meeting the warm monsoons from the southwest forming vital early spring rains for the high plains and the plains states. Typically, a dry high-pressure area forms over the coastal states of the Gulf of Mexico during these months. This high forces the storm jet up into the Corn Belt at this time to meet the polar jet coming down from Alaska. Between these areas spring storms arise in the Smokies and the Corn Belt, bringing early rains to support the corn and soybean agricultural base of the upper Midwest. Failed April rains in this sector send the soybean market into a frenzy.


Fig.6


Fig.6

In (FIG 6) we see that by July and August the Hawaii high is firmly established, the Aleutian low is
usually a non-player for the West Coast but the low in the Great Basin has grown considerably. This is often the wettest time for the southwest. This is due to the strong heating taking place here during the summer months. The polar jet moves north, away from the Dakotas bringing a dry period to the northern tier states in June and July. At the same time a ridge of high pressure air usually forms over the Mississippi Valley blocking any fronts coming from the low in the Southwest .

This usually results in a storm line that runs from the Ozarks eastward into the Carolinas In June and July. Along the western side of the blocking ridge strong thunderstorms develop in the south, that sometimes give birth to tornados. Some years the ridge is placed more to the west then the High Plains gets a monsoon summer of storms and lightning. If the ridge is placed more to the east then there can be flooding in the Mississippi valley as lines of thunderstorms dump rain along the sloping line of the blocking high or ridge. As the summer progresses the blocking ridge moves into a north / south placement paralleling the drainage of the Mississippi. The tornados and thunderstorms also move north. To the east of the block the very early onset of hurricane season is usually the most active part of the July / August climatology for the southeast producing local thunderstorms in summer when the hurricanes curve out to sea and heavy rains when they travel up the coast. Early hurricanes can happen in late summer though climatically September and October are the true hurricane months.


Fig.7


Fig.7

In (FIG 7) we see a peculiar American climatic singularity. The September patterns produce a maximum of thunderstorm activity for the Midwest as the Mississippi block grows to a maximum. The block over the Mississippi has pushed the monsoon jet far to the north. This stimulates strong thunderstorms into the Great Lakes area and into Wisconsin and Minnesota. These storms can be accompanied by severe tornados making the middle of the US a world center in lightning strikes and tornado frequency. By the middle of September however, the Sun is making rapid progress to the south, the Hawaii high begins to shrink and the Aleutian low begins to grow. The Sun, rapidly shifting in declination, is causing the heat balance for the northern hemisphere to be upset. The summer monsoon from the Gulf of Mexico collapses. Any residual warm air on the continent flows northward initiating Indian Summer in Wisconsin and Minnesota.


Fig.8


Fig.8

In (FIG 8) we see the situation for October. The thunderstorms in the Midwest have stopped as the Hawaii high shrinks further and the Aleutian low sends the first fall storms into the Pacific Northwest and the continent. The monsoon in the Gulf of Mexico breaks down and the Mississippi block fades. This allows the jet stream from the Pacific to begin to be a stronger element in the weather for the continent. On the whole the Midwest finds this time as the most tranquil of the year. On the East Coast hurricane warnings continue but the tracking usually moves from an early season east to west track from the Cape Verde islands into a south to north track from the Caribbean Sea.


Fig.9


Fig.9

In (FIG 9) the chart shows the mean winter climatology. It should be noted that this is extremely simplified but it is useful for the purpose of illustrating some principles. The Aleutian low is strong while the Hawaii high is much diminished. This lo-w pressure area is the source of most of the storm energies that come into the west coast during the winter months. Cold storms flow directly south out of the Gulf of Alaska bringing snow to the Sierras. Wet storms usually drop from the Aleutians into the Pacific in the latitude of Hawaii before turning east and on into the coast. There is a delicate balance for these storms in the vicinity of Mt Shasta at the top of the central valley in California. The winter polar jet often steers to the north of Mt Shasta when the track straight out of the Gulf of Alaska is active. When the polar jet drops down into the vicinity of Hawaii the storms will track to the south of Mt Shasta bringing rain into California. It is often the case that the strength of a high over the Great Basin determines which way this will go.

During the early winter there is a high in the Great Basin that builds up when the first few cold storms come south out of the Gulf of Alaska leaving cold air in the basin. The Hawaii high and the Great Basin high sometimes connect to each other in winter causing blocks to the south of Mt Shasta on the West Coast.

The Hudson Bay low is the main low- pressure feature of the atmosphere at this time on the continent. This forms when the land surrounding the bay cools much more rapidly than the water. The water then serves as a source of rising warm air and is the site of a semi-permanent low pressure area for most of the early winter. The Bermuda high, which is active during hurricane season, shifts to the east to settle over the Azores, where it dominates the eastern Atlantic for the winter months. This finds a counterpart to the Iceland low which is the source of most of the European storms during the winter. The low forms where the warmth from the Gulf Stream enters into the cold waters of the north Atlantic and forms a pool that generates upward motion of warmth into the cold atmosphere.

Besides these patterns there are many variations that can greatly influence the weather over time frames from weeks to months. Using these fundamentals we can now look at a few recognized climatic patterns which accent one feature above another feature in a given season, giving rise to climatic variations. Climatologists consider that each scenario tends to fluctuate between a positive phase and a negative phase, giving rise to different climate patterns in different years.


Fig.10


Fig.10

The first pattern is illustrated in (FIG 10). The positive PNA pattern (Pacific / North American) is a variation of the interaction between the Aleutian low and the Hawaii high. It is a strong influence on the weather on the west coast and the southwest. In the positive phase the PNA often appears in El Nino years. In this pattern a high develops along a north / south axis on the west coast over the mountains in Canada. The center of the high is usually in Alberta or farther north in British Columbia. Most often it flows to the north and then wedges into the eastern shore of the Gulf of Alaska. From there it acts to drive the polar storm jet to the south by blocking passage into the continent through the Alberta area. If this block is long lived there are often flood conditions created to the south as the jet steers in a long curve or trough over the warm eastern Pacific before making a landfall. Usually in this pattern a cold northern storm is pushed south in the vicinity of Hawaii and it picks up moisture as it crosses the Pacific. This pattern yields wet conditions in California, Utah, the desert Southwest, Denver, the Gulf States and Florida.


Fig.11


Fig.11

In the PNA- pattern (fig 11) a high from Hawaii forms over the eastern Pacific and connects to a high in the Great Basin forming a strong block to the polar storm jet. As a result the jet is pushed to the north on the west coast and then descends into the continent at a strong angle. This placement is often, but not always, the case during La Nina cycles. The PNA- pattern creates severe drought below Mt Shasta on the West Coast. For the continental US in this scenario, the Aleutian low is weak and it is in a reciprocal relationship with the Hudson Bay low that is strong. The counter clockwise circulation around the strong semi-permanent low in eastern Canada dominates the polar jet there and pushes cold and stormy weather down into the continental US, usually in the form of a strong trough on the east coast. Fronts from the Hudson Bay low drop out of Canada on a ten-day interval bringing snow and ice as far south as Florida.

These patterns are climatic variations on the theme of high and low pressure alternating in different ways in different seasons. It is tempting to think that the Pacific jet somehow works independently of the other features of the map. If this were the case forecasting simply would be a case of plotting the statistical mean for a given year and then checking this against the climate record and then making a prediction. However, the placement of the low in the Aleutians is intimately tied to the placement and strength of the low over Hudson Bay in a given year. The placement of the Hudson Bay low in a given year is dependent upon the placement of the Iceland low. This in turn is dependent upon the strength and placement of the high- pressure area in the Azores.

There are three low latitude semi-permanent high- pressure areas in the northern hemisphere which steer the storm jet into its sinuous path through the temperate regions. The first is in Hawaii, the second is over the Azores and the third is, in winter, over Siberia. However during the winter a secondary high-pressure area also forms over the Great Basin as we have previously seen. It is this area that is a very good distant warning area for upcoming weather changes on the west coast of Europe. The reason for this is a pattern known as the NAO (North Atlantic Oscillation). In the NAO a reciprocating relationship similar to the Hawaii high and the Aleutian low exists in the eastern Atlantic. The low is centered in the latitude of Iceland. The reciprocating high is centered in the latitude of the Azores. When the Azores high is strong the Iceland low is weak. Conversely, when the Iceland low is strong the Azores high is weak. The Azores high blocks the polar jet in the east Atlantic from entering the continent by pushing up from the south. When the Iceland low is strong the jet swings to the west and south and storms move into the west coast.


Fig.12


Fig.12

As with the other patterns there is a negative and positive phase to the NAO. In (Fig 1 ) the NAO+ is depicted. In the chart it can be seen that on the American continent there is a situation where the Hudson Bay low is strong and is linked across Greenland to an average Iceland low. This would happen when the Hudson Bay low is placed to the east near the Maritime Provinces. This pattern creates a situation in which Pacific maritime air comes in through Washington State, into Alberta and then continues horizontally across the northern tier, exiting the continent without drawing down strong cold from Canada. This is the positive NAO in the US.

In the Atlantic the polar jet moves off of the American continent and out to sea drawn by the flow around a strong Azores high and a moderate Iceland low which is connected across the whole north Atlantic to Hudson Bay. When the Hudson Bay low is strong and the Great Basin high is weak this type of pattern can arise. The jet moves out to sea and crosses the Atlantic by being shunted along by the circulations around the Azores high and the Iceland low. This makes for strong westerlies across the Atlantic. As the name implies there is often an oscillation to this pattern. The oscillations can occur over years or over weeks or days. When the high in the Great Basin becomes strong it pushes the jet up into western Canada. If the Hudson Bay low were strong this would tend to tweak the polar jet strongly to the south east of the Mississippi creating a trough. If the Azores high is strong and more to the west, the center of the high can sometimes shift farther to the west at a high latitude, and move into the area near Greenland. The Hudson Bay low can, in this instance, also drift to the west allowing high pressure to come into eastern Canada. This puts a blocking high over the Maritimes. This pattern is known as the Greenland block or the negative NAO. In (fig13) we can see this pattern.


Fig.13


Fig.13

There is a flipping of the relationships established in the positive NAO. Such a pattern would bring strong cold and snow into the east coast of the US. It would also warp the polar jet over the Atlantic giving rise to storm cycles since when the high pressure is strong over Greenland in the west, the area near Iceland tends to deepen in low pressure. The period of oscillation between these two phases is very erratic and provocative in its implications. During the 1920's the positive phase dominated. During the early 1930's the negative phase dominated. In the early 60's the negative phase dominated. During the 70's and 90's the positive phase dominated. Correlations between eclipse positions, planetary positions and these dates have been established through the techniques of the planetary flux model.

RECORD NAO PATTERN

The negative NAO pattern is a strong influence on the polar jet over the continental US. It is characterized by an unusually strong high- pressure development over the Hudson Bay/ western Greenland area. This region is usually the site of a persistent low- pressure area in the early winter as was just shown. This pattern in which the Hudson Bay low is strong, is the positive NAO. This is the dominating climatic pattern for east coast weather in November, December and January.

In the negative NAO pattern the low- pressure in Hudson Bay and the Maritime provinces is replaced by a persistent high- pressure ridge. The ridge blocks the normal west to east passage of cold air across Canada and drives it to the south into the Great Plains and to the eastern seaboard. This results in record cold winters in these areas.

A final climate pattern for the continental US is of interest to those who follow corn and soybean crops. A final climate pattern for the continental US is of interest to those who follow corn and soybean crops. This pattern is the low level jet (LLJ). In the summer the grain crops in the Corn Belt require regular thunderstorms to keep growing. The thunderstorms arise from a combination of a monsoon like moist flow of warm air from the Gulf of Mexico interacting with a cold front from the north. When these patterns occur on an average of a week or so apart then the crops do well. If the monsoon fails then the crops fail along with it. The failure of the monsoon, leading to drought and crop failure, or the onset of flood conditions leading to crop failure, are both controlled by the LLJ pattern.

The monsoon mentioned in the beginning of this article is one element of the LLJ. This feature of the atmosphere is controlled by the placement of the Bermuda high, which is a semi permanent high pressure area that oscillates from a position over the central Atlantic in some years to a position against the coast of North America in others. When the high is against the coast the clockwise circulation of winds around it moves into the Gulf of Mexico and drives the moist air from the Gulf into the Midwest. When this happens a low level (around 5,000 ft to 10,000 ft) tropical jet stream establishes itself in June or July and brings moisture northward. The moist flow from this low level tropical jet supports abundant rains. In 1993 this flow created the record floods on the Mississippi and its tributaries. The position of the Bermuda High up against the coast is depicted in figure 14.


Fig.14


Fig.14

Also on figure 14 the polar jet is shown. This jet normally moves far into the north during Northern Hemisphere summer. In 1993 it was positioned to the south of normal during the summer. It resembled a winter jet stream pattern. The blocking high over the western Atlantic prevented the summer's cold air from Canada from leaving the continent as it normally does by flowing off of the coast of New England. As a result the cold flowed southeast from the Northwest and met the enhanced tropical LLJ over the northern Mississippi watershed. The results were torrential rains. This in itself is not so unusual. What was unusual was that the pattern lasted for weeks rather than days.


Fig.15


Fig.15

In figure 15 the Midwest drought conditions are illustrated for the years 1953 and 1988. The pattern here is one in which a strong western Atlantic high is replaced by a consistent East Coast trough, and the strong high pressure area is found over Western North America. We can see that the storm jet for the Gulf of Mexico is effectively neutralized and the dry high- pressure area steers the Pacific jet stream high into western Canada and then down into the east coast to form the rain trough there. Most Midwest droughts feature a ridge like this one that appears every second year for a decade. This type of pattern set the stage for the Dust Bowl of the thirties.

The climate patterns described in this article are the symptoms not the causes of these unusual weather events. Science understands the causes of these patterns only in a dim way. One way to look at the time periods for these events is to compare the placements of blocking highs and troughs to the position of the planets especially, in the context of the position of the eclipses. This approach is known as the planetary flux model. The predictions in the Urban Almanac are done using these methods. If you find them accurate, considering that they are always made a year in advance, then please visit other areas on this site for more information.