Cutoff highs occur when warm tropical air is advected poleward into the crest of a steep long-wave ridge. This warm air, and the high that it develops, ends up on the poleward side of the strongest westerlies. In this position, it is "cut off" from receiving additional tropical air.

Cutoff lows take on far more importance than their counterpart highs because of the bad weather that accompanies them. In the Northern Hemi-sphere, they occur most frequently along the southwest coastal areas of the United States and northwest coastal areas of north Africa. They form when cold polar air is advected equatorward into the base of a deep long-wave trough. This pool of cold air, and the low it develops, ends up on the equatorward side of the strongest westerlies. Figure 8-3-4 shows the typical stages in the development of a cutoff low. The latter stages are often associated with occluding low-pressure systems at the surface; however, cutoff lows are noted for their production of bad weather without frontal or cyclonic circulations at the surface. In the meridional flow of which they are a part, they are the cold valleys between mountains of warm air. On the 500-mb chart, the warm ridge to the west is very sharply oriented southwest to northeast. To the northeast, an intense warm stationary high or ridge BLOCKS the normal path of migratory systems.

Figure 8-3-4.Development of cutoff low.

BLOCKS Blocking is the obstructing of the normal west to east progress of migratory systems. Like cutoffs, blocks are associated with pronounced meridional flow in the upper levels, occur most frequently in spring and least often in autumn, and tend to form in the same geographical regions. In the Northern Hemisphere, they form most frequently in the eastern North Atlantic and eastern North Pacific oceans.

Blocks are warm long-wave ridges or upper- level highs that take up residence in higher latitudes and move very slowly, if at all. As a block sets up and intensifies, it tends to move west (retrograde). Figure 8-3-5 shows three types of

Figure 8-3-5.Types of blocks.

blocks that occur in the circulation pattern. Blocking highs are either warm-core or dynamic systems, but in either case, they cause a split in the westerlies. The split causes the migratory systems to divert from their normal paths. It is at these times that prolonged periods of good or bad weather occur.

In the winter of 1976-77, a block developed over the western United States. Basically, it lasted from 15 Oct 1976 to 20 Feb 1977. To the west of this block, warm air was transported far north; Alaska basked in balmy weather. Meanwhile to the east of the block, the eastern half of the United States had one of the worst winters on record. Cold arctic air was advected from over the North Pole southward to the Bahamas. The Ohio basin was particularly hard hit and suffered one of its coldest winters on record. The Chesapeake Bay of Virginia and Maryland froze over, and ice breakers were required to keep the shipping lanes open. The citrus industry of Florida suffered severe losses, and the list goes on. In the western half of the United States, drought-like conditions existed, because the block prevented moisture-laden air of the Pacific from reaching the coast. Granted, this block was far from normal, but it serves to show a blocks effect on weather patterns.

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