Extratropical cyclones (lows) are horizontally large (thousands of km in diameter) relatively thin (11 km thick) storms in mid latitudes. They undergo an evolution of intensification (cyclogenesis) and weakening (cyclolysis) within a roughly three-day period while they translate eastward and poleward. The storm core is generally cold, which implies that the upper-level Rossby wave trough is generally west of the cyclone.
Cyclones rotate counterclockwise (clockwise) in the Northern (Southern) Hemisphere around a center of low sea-level pressure. Bad weather (clouds, precipitation, strong winds) is often concentrated in narrow frontal regions that extend outward from the low-pressure center. Cyclone strength can be inferred from its vorticity (rotation), ascent (updrafts), and sea-level pressure.
Weather maps are used to study synoptic-scale storm systems such as mid-latitude cyclones and fronts. Many case-study maps were presented in this chapter for the 4 April 2014 cyclone, and a few maps were for a similar cyclone during 23 February 1994.
Mid-latitude cyclones that form over the warm ocean waters east of Japan often intensify while approaching the dateline (180° longitude). But by the time they reach the eastern North Pacific ocean they are often entering the cyclolysis phase of their evolution. Thus, most land-falling cyclones that reach the Pacific Northwest coast of N. America are already occluded and are spinning down.
The cyclone labeled L1 in Fig. 13.54 has just started to occlude. Satellite images of these systems show a characteristic tilted-“T” ( ) shaped cloud structure (grey shaded in Fig. 13.54), with the cold front, occluded front, and a short stub of a warm front.
As the cyclone translates further eastward, its translation speed often slows and the low center turns northward toward the cold waters in the Gulf of Alaska — a cyclone graveyard where lows go to die. In the late occluded phase, satellite images show a characteristic “cinnamon roll” cloud structure, such as sketched with grey shading for cyclone L2.
For cyclone L2, when the cold front progresses over the complex mountainous terrain of the Pacific Northwest (British Columbia, Washington, Oregon), the front becomes much more disorganized and difficult to recognize in satellite and surface weather observations (as indicated with the dashed line over British Columbia).
The remaining portion of L2’s cold front still over the Pacific often continues to progress toward the southeast as a “headless” front (seemingly detached from its parent cyclone L2).
Sometimes there is a strong “pre-frontal jet” of fast low-altitude winds just ahead of the cold front, as shown by the black arrows in Fig. 13.54. If the source region of this jet is in the humid sub-tropical air, then copious amounts of moisture can be advected toward the coast by this atmospheric river. If the source of this jet is near Hawaii, then the conveyor belt of moist air streaming toward N. America is nicknamed the “Pineapple Express”.
When this humid air hits the coast, the air is forced to rise over the mountains. As the rising air cools adiabatically, clouds and orographic precipitation form over the mountains (indicated by “m” in Fig. 13.54). Sometimes the cold front stalls (stops advancing) while the pre-frontal jet continues to pump moisture toward the mountains. This atmosphericriver situation causes extremely heavy precipitation and flooding.
Fig. 13.55 shows an expanded view of the Vancouver-Victoria-Seattle region (corresponding to the lower-right “m” from Fig. 13.54). Low-altitude winds split around the Olympic Mountains, only to converge (thick dashed line) in a region of heavy rain or snow called the Olympic Mountain Convergence Zone (also known as the Puget Sound Convergence Zone in the USA). The windward slopes of mountain ranges often receive heavy orographic precipitation (dotted ovals), while in between is often a rain shadow of clearer skies and less precipitation.