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Blocks and tracks:long range weather prediction - 10.17.04
An understanding of block and storm tracks is the fundamental tool of the climatologist. This article describes these features on the North American continent as an aid to understanding the techniques of long range weather prediction.
The climatology of North America is created by two fundamental atmospheric patterns. In the language of climatology one pattern is known as a block and the other is known as a track. Blocks are also known as ridges because they consist of immense mountains of air that sit in one place and do not move for a comparatively long time. Ridges tend to form in predictable places in a given season due to certain pattern in the climate. Storms steer around ridges and the climate patterns on the continent are the result of ridges being placed in particular positions at particular times. When a ridge sits in one place for longer than a few days it is known as a block.
The high altitude river of air that steers storms around blocks is known as the jet stream. The path of the jet stream determines where and when storms will strike a given location. As the blocks shift across the continent the path of storms will also shift accordingly. Since some blocks are regular features of the atmosphere above the United States regular jet stream paths can be predicted if the position of the block is known. These paths are known in the language of climatology as storm tracks or tracks for short. If blocks are imagined as boulders in the streams of the atmosphere, tracks are the paths that the current takes to get around the blocks. Tracks are the typical path of the jet stream in a given season when a seasonal block is dominating the atmosphere over a particular location. The fundamental task of the forecaster is to be able to tell, in advance, the position of the blocking ridges that will steer storms during a particular time frame.
Fig.1
The jet streams are high altitude, rapidly moving rivers of air that circle the whole earth. There are several jet streams in the Northern Hemisphere. Storms are imbedded in the tracks of the jet streams. In the language of climatology the jet streams are known as long waves and a storm that is being carried on the jet stream is called a short wave. These two kinds of waves are illustrated in (fig 1). The short waves are small wave disturbances that are imbedded in the long wave fluctuations of the jet stream.
On a given day the weather map is composed of a number of short wave storm centers moving through large amplitude long wave streams of air that are in turn moving around quasi- stationary ridges of air contained in the blocks. The long wave streams of air follow predictable paths in particular seasons. These predictable paths are known as storm tracks. Since the blocks have a semi-seasonal quality that can be somewhat predicted, the art of predicting climate shifts that lie behind weather events can be developed around correctly predicting the position and onset of a blocking ridge and the long wave track that is streaming by it. When a particular block is in place, the jet stream constitutes the long wave seasonal storm tracks that can be recognized as seasonal features across the North American continent. The blocks are semi-permanent features that arise in certain time frames and block for a period of time and then either migrate to the east or dissolve. The short wave storms that are following the long wave tracks are turbulent eddies in the long wave stream of air. Blocks and tracks are the large-scale forms that climatologists study in order to understand how weather patterns arise.
Fig.2
In the language of climatology, there are five fundamental semi-permanent blocks that can arise over the North American continent. Starting in the west, there are two semi-permanent blocks that can form in the eastern Pacific. The first is the Hawaii High. (fig 2) This block is a large horizontally oriented mound shaped block that often stretches from Hawaii in the west, eastward to the Pacific coast of North America. The trans-Pacific jet stream flows horizontally along this block from Hawaii towards the West Coast. In summer this block expands across the whole eastern Pacific and dominates wind patterns on the West Coast by expanding up into the latitude of Oregon. This brings drought conditions to California and Oregon during the entire summer since storms cannot get past the block to move south into the coast. In winter, this block shrinks back towards a southerly latitude and moves into a modality where it alternately swells and shrinks. When it swells it creates a blocking pattern for the West Coast. When it shrinks it allows the jet stream out of the polar regions to flow farther south in latitude, bringing storms below Mt. Shasta.
Fig.3
The second block in the eastern Pacific is the PNA (Pacific/North America) block. (fig 3) This occurs when a strong ridge builds parallel to the latitudinal, north /south meridian of the West Coast. This ridge often has the form of a finger and when it is present it sits tightly against the coast for a week or more. When this happens the jet stream from Alaska is steered from the eastern Pacific, far to the north and into Canada. This block brings dry weather to the West Coast. Since the storm jet is pushed so far north this block pattern also brings strong cold into the Midwest as the cold Canadian air descends along the eastern side of the ridge into the continent. This type of block usually only occurs in winter and even then, it is often intermittent. However, the PNA block is very important in winter drought cycles on the West Coast and in severe cold outbreaks in the Midwest in winter.
The relationship between the PNA block and the Hawaii High forms a kind of garden gate for the entrance of storms into the North American continent though the Straits of Juan De Fuca. This is a water inlet at the north end of Puget Sound between the Olympic peninsula to the west of Seattle in the south, and Vancouver, Canada, to the north. This jet stream gate determines to a large part the continental winter regimes in the Midwest. When the PNA block forms on the West Coast it is very cold in the Midwest. When the block fades the storm jet moves horizontally across the northern tier of states and brings warm, moist Pacific air into the northern sections of the United States.
Fig.4
Another seasonal block for the continental United States is the Mississippi ridge. (fig 4) This block forms in the early fall of the year straddling the axis of the Mississippi river. When it is strong, late summer thunderstorms are steered northeastward into the Corn Belt threatening the ripening stages of the corn and soybean crops that are the economic backbone of North American agriculture. When the Mississippi ridge is weak the cold fronts that spawn thunderstorms move easily to the east bringing moderate or scattered rains across the Corn Belt usually at ten-day intervals in the late summer. When the Mississippi ridge is very strong or placed far to the west the late summer thunderstorms may bring rains to the High Plains but the eastern Corn Belt is dry. Predicting the accurate placement of the Mississippi ridge during a given growing season gives a forecaster valuable insight into predicting the onset and duration of the critical late summer and early fall rains in the Midwest. Knowing the placement of the Mississippi ridge in advance in a given season is valuable information for soybean and corn farmers in the Corn Belt.
Fig.5
A fourth continental United States block that is linked to the Mississippi ridge is the Bermuda high. (fig 5) This block is a normal part of the climatology of the western Atlantic. It forms over Bermuda in the summer and fall of the year and helps steer hurricanes into the East Coast of the US. When it is not a factor in the western Atlantic in the late summer and early fall the hurricane season is benign with few storms. This block can migrate from a position in the center of the Atlantic to a position near to the coast of Florida. When it is near to the east coast of Florida the clockwise circulation of the winds around the Bermuda High move westward across the southern edge of the block and enter the Gulf of Mexico. Depending on the extend of the high-pressure area the winds around the high can turn north over the Gulf of Mexico and push north into the Gulf States bringing moist warm air into the continent from the south. In the most westerly position for this block, the moist air creates a monsoon condition for the Midwest that figures in abundant rains and flood patterns during the summer by supporting unusual thunderstorm activity northward from the western Gulf of Mexico. Climatologists know this American monsoon pattern as the Low Level Jet (LLJ). The LLJ shunts moisture from the Gulf of Mexico into the Midwest. Whether the Mississippi block dominates the continent in the late summer and early fall, or the Bermuda High dominates determines to a large extent the agricultural potentials for a given year.
Fig.6
A fifth block that strongly influences continental weather patterns is the Greenland block. (fig 6) This is a winter forming ridge over the Maritime Provinces or over Greenland. A ridge here in the winter causes the cold air following the prevailing Westerlies from west to east across Canada, to back up and begin to flow southward through the Midwest or more specifically along the East Coast of the US. When this happens the storm jet brings cold and severe weather and fierce snowstorms to the East Coast. Without this block, the cold air coming across the continent from the west flows off of the continent easily and moves out to sea without causing the storm jet to loop to the south. The absence of a Greenland block generally brings mild winter conditions east of the Mississippi. The presence of a Greenland block brings severe cold and stormy weather to the East Coast and into the Southeast. Most Florida citrus freezes are the result of a Greenland Block.
Fig.7
The placement and intensity of these five blocks creates many variations in the formation of the jet stream loops that create the storm tracks across the North American Continent. These variations can be seen in the composite of (fig 7). Starting in the west again we will describe the storm tracks and their variations that are connected to each block.
In winter when the Hawaii High maintains enough strength to form a vast elongated cushion across the eastern Pacific, but is weak enough to be unable to block storms from the coast, the storm jet can sometimes ride along the northern edge of the cushion and steer cold storms from the Aleutians on a long looping track from west to east at a low latitude. To form this pattern cold arctic storms drop to the south in the longitude of Hawaii and pick up a lot of water as they make their way eastward into the coast. The wet storms then come into the West Coast at a low angle. Strong rains fall up and down the coast and in the Sierras and Utah mountain snows are abundant. These storms are known locally as the Pineapple Express since they travel the Hawaii track. This can sometimes be a pattern during El Nino events.
In contrast to this Hawaii pattern, on the West Coast the PNA ridge can sometimes be found situated farther to the west over the ocean or in the central Gulf of Alaska. In this western placement of the finger-like high pressure ridge the jet stream goes up into Alaska around the western side of the block and brings intense cold into the central valley of California on the eastern side of the block. This type of pattern happens at an approximate ten-year interval destroying orange and lemon trees in the Central Valley of California. The presence of stong cold into the West Coast is the Alaska track.
When the finger of the PNA block is situated right along the West Coast itself then the jet stream rises into British Columbia and then drops into the High Plains instead of the West Coast. From there, the jet stream turns east in a curve extending from the Dakotas through the central states of the Corn Belt to the Mid Atlantic seaboard. This is the central track. In some winters the central track is the prevailing storm track. However, when the PNA block is situated farther east over the Northern High Plains, then the Canadian cold drops into the area east of the Great Lakes. This is the Atlantic track. This is often the source of unusual cold snaps in the Great Lakes area and the Northeast.
On the same track Atlantic track if the cold outbreaks are strong enough they can penetrate into the south and the Gulf Coast threatening the Florida orange crops. When the high-pressure on the PNW is strong and the blocking ridge is placed over the inter-mountain regions, this creates a condition where the storm jet moves up into western Canada before turning south, then ice storms often form along the southeastward leading edge of the descending cold as it meets the warm moist air streaming north out of the Gulf of Mexico. These storms can then curve eastward into the Tennessee Valley and the Smokies. These patterns are primarily winter patterns for the continent. The ability to accurately predict the placement of the blocking ridge on the continent is the most valuable tool for the forecaster.
Looking at these blocks in the long term, it can sometimes be seen that the ridge of the PNA block moves from an eastern position over the Midwest one year into a more westerly position over the Rockies the next year finally shifting over the West Coast the year after. This gradual westward motion of the dominant blocking area is consistent with the yearly east to west progression of the nodal motions that Doc Weather uses to form the basis for the forecasts.
However, climate is not composed only of blocking patterns. One strong variable to these east-to-west semi-annual migratory blocking patterns is the placement of a strong semi-permanent annual low- pressure area over Hudson Bay in the early winter each year. This Hudson Bay Low forms because the warm water of the bay, when compared to the cold land masses around it, supports ongoing low- pressure formation through the fall and early winter. This semi-permanent low migrates eastward or westward around Hudson Bay depending upon the placement of the various blocks just described. The Hudson Bay Low is a major winter weather maker for the Midwestern US and the East Coast. It is linked in tandem to a semi- permanent annual low-pressure area over the Aleutians. When the Hudson Bay Low is strong the Aleutian Low is weak. When the Aleutian Low is strong the Hudson Bay Low is weak.
Taken together these blocks and tracks are the source of endless patterns that form and dissolve during the rotation of the seasons. In Doc Weather the placement of these features is linked to the placement of eclipse influences that move from east to west in 18.6-year cycles. These rhythms based on eclipse positions allow the researcher to follow unusual climatic patterns with accuracy as they move across the face of the American continent in their stately cadences.