SHW Ch. 9 - Extratropical Cyclones Forming East of the Rocky Mountains

Extratropical Cyclones

• Form along the jetstream between about 30° and 70° latitude.

• Entire life cycle can span several days to well over a week, but can reach maximum intensity (i.e., lowest central pressure) in as little as 36-48 hours of formation.

• Responsible for much of the severe weather in the mid-latitudes.

• Needed to balance the temperature differences between the poles and equator, and between the cold upper troposphere and the warm lower troposphere.

• A fully mature extratropical cyclone's cloud pattern looks like a large "comma"; Figure 9.1

  • severe weather occurs along the "tail" of the "comma"

  • heavy rain, snow, sleet, mixed precip occur within the "head" of the "comma"

  • blizzards occur (in winter) in the northwest quadrant of the "head" of the "comma"

Cyclone Development

• In North America, there are five primary locations where cyclones develop:

  • just east of the Rockies in the central U.S. (particularly in eastern CO)

  • just east of the Canadian Rockies (particularly near southern Alberta); "Alberta Clipper": a fast moving, generally moisture-starved, weak low pressure; the most common storm track.

  • just off the TX-LA coastline along the Gulf Coast of the U.S.

  • along the East Coast of the U.S. (particularly near NC)

  • over the Bering Sea and Gulf of Alaska in the Pacific Ocean

• "Four Air Mass Model": air mass set up for development (Figure 9.2):

  • a cold, dry airmass (cP or cA) over Canada and the northern U.S.

  • a warm, humid air mass (mT) over the southern and eastern U.S.

  • a warm, dry air mass (cT, or modified mP) over the higher elevations of the western U.S.

  • a cool, moist air mass (mP) over the northeast U.S. and Canada

  • a 5^th "air mass" is found aloft and is related to the "upper-level front"

• Long ("Rossby") waves exist in the jetstream all around the hemisphere, due to unequal heating of land and oceans, and due to the air flowing over large mountain ranges (e.g., Himalayas, Alps, Rockies).

• Embedded jetsreaks and changes in curvature within the jetstream create upper-level divergence.

• Lee side development east of the Rockies occurs when a low pressure trough over the western U.S. Is accompanied by a jetstreak moving into the base of the trough; Figure 9.3

• As a surface low pressure circulation develops in response to the approaching upper-level disturbance, the various air masses begin to move; warm air northward (warm front) east of the low (called "overrunning"); cold air southward (cold front) west of the low; Figure 9.4.

• A dry line often develops as surface air from the desert southwest and off the Mexican Plateau moves east and descends, colliding with the warm and moist air (mT) moving northward out of the Gulf of Mexico.

• Sometimes an upper-level front forms when dry, descending air meets rising, moist air, somewhere aloft, but no lower than 700 mb (~10,000 feet AGL); dry air behind this feature creates a "dry slot."

Early Weather Along the Fronts

Figure 9.4

• East & Northeast of the Low: marked by large cloud shield, as warm, moist air "overrides" the colder, drier air at the surface; warm front slope 1:200; clouds (heading north): deep nimbostratus, stratus without precip, altostratus, high cirrostratus, wispy high cirrus; depending on vertical temperature structure and stability of air mass, precip can transition from showers (T-storms), to freezing rain, ice pellets and snow.

• South of the Low: there can be as many as three airmasses at the surface and another aloft; one of three possible scenarios typically occur:

  • Upper-level front leads dry line and cold front (Figure 9.6.A):

    • Conditionally unstable moist air: can result in showers and T-storms due to both upper-level front and dry line, if inversion over dry line is "broken" by daytime heating, otherwise no precip along dry line; Figure 9.7

    • Stable or weakly conditionally unstable with strong inversion: just showers along upper-level front

  • Upper-level front and dry line aligned (Figure 9.6.B): this scenario happens when an old Pacific cold front moves across the Rockies and descends on the leeward side and can coincide with the upper-level front

    • Conditionally unstable, moist air: can result in showers and T-storms ahead of the dry line, and the trailing cold front ends up lifting dry air and produces no precip.

  • Cold front only (Figure 9.6.C): the Canadian cold front moves rapidly southward and overtakes the dry line

    • Conditionally unstable, moist air: showers and T-storms develop ahead of the cold front

• Northwest of the Low: as air flows westward (Figure 9.4), it rises nearly 1 mile before even reaching the Rockies just due to the slope of the Great Plains, and it is forced to rise another 1-2 miles when it encounters the Rockies; this "upslope flow" often produces heavy snow and blizzard conditions along the eastern slopes of the Rockies, eastward onto the Great Plains.

Storm Intensification

Figure 9.8

• "Feedback Process": as warm air moves northward on the east side of a low, the altitude of the pressure surfaces aloft increases with time (i.e., the downstream ridge "amplifies"); as cold air moves southward on the west side of the low, the altitude of the pressure surfaces aloft decreases with time (i.e., the upstream trough "deepens"), which cause the intensification of the surface low because

  • the change in curvature between the trough and the ridge becomes more dramatic, leading to an intensification of the upper-level divergence associated with the curvature effect;

  • the pressure gradient incenses at the base of the trough, leading to stronger winds and an enhanced jetstreak, which also adds to intensification of the upper-level divergence associated with the left-exit region of the jetstreak

  • the intensification of the upper-level divergence causes the low pressure to intensify (i.e., pressure drops and surface winds increase), the transport of warm and cold air becomes more rapid, causing the pressure surfaces to rise more rapidly east of the low and fall more rapidly to the west of the low, which in turn, leads to a further intensification of the upper-level divergence and an increase in the low-level winds.

  • Note that the release of latent heat in the precipitation process adds to the sensible warming of the air to the east and northeast of the low, causing further amplification of the downstream ridge.

  • this spin-up "feedback process" can last from a few hours to as much as 24-36 hours

The Mature Cyclone

• There are typically two areas where strong low-level pressure gradients lead to a band of strong winds at and just above the surface:

  • Ahead of the upper-level front or cold front (ahead of the "comma tail"), where the "warm" severe weather will be found (e.g., T-storms, tornadoes, etc.); called the "low level jet";

  • Just northwest of the low pressure center (under the "comma head"), where the "cold" severe weather will be found (e.g., blizzards, ice storms, etc.), called a "trowal" for "trough of warm air aloft", where "trough" in this case is referring to a "thermal trough" in the potential temperature field; the "trowal" will actually appear as a "ridge" in the height field; sometimes referred to as "wrap-around precip."

  • "Trowal": when the upper-level front leads the cold front and "occludes" the warm front, thus trapping a band of warm, moist air between the advancing dry air to the south and the warm front to the north; Figures 9.9A & B and Figure 9.11A

  • "Cold Occlusion:" occurs when the advancing cold front "occludes" the warm front; in this case the air behind the cold front is colder than the air preceding the warm front; the warm, moist air ends up wedged between two colder air masses; most typical type of occlusion; Figure 9.10 and Figure 9.11B

  • "Warm Occlusion:" occurs when the advancing cold front "occludes" the warm front; in this case the air behind the cold front is not as cold as the the air preceding the warm front; this is a more typical type of occlusion in the U.S. Pacific Northwest; Figure 9.11C

  • "Cut-off Low:" as the system continues to "occlude", a pocket of cold air becomes completely "cut off" from the main flow; the storm will "fill" (i.e., weaken); the "trowal" will continue to produce steady precip, as it is progressively narrowed, stretched and wound between the colder & drier air masses to its north and south; Figure 9.10

The Dissipating Cyclone

• Once the "cut-off low" has formed, there is no longer a change in the flow curvature, thus the upper-level divergence associated with the curvature becomes insignificant.

• The jetstreak moves to the east side of the "cut off low" and weakens dramatically as it moves into the ridge to the east, thus the upper-level divergence associated with the jetstreak becomes insignificant.

• The vort max either moves to the east side of the low or directly under it and eliminates the CVA, thus the upper-level divergence associated with the vort max becomes insignificant.

• The isotherms become more concentric under the "cut off low" and eliminates any WAA, thus the upper-level divergence associated with the WAA becomes insignificant.

• At this point, the upper low drifts directly over the surface low and together with insignificant upper-level divergence and surface friction, the surface low weakens rapidly, becoming a deep vortex with a cold center.

• A "cut off low" can take days to as much as a week to "spin down"

Final Thoughts

• Extratropical storms (and tropical storms in the "warm season") are the "air conditioners" of the planet.

• If there were no storms, the average temperature of Earth would be closer to 60 C (140 F)! The actual average temperature is about 16 C (61 F), much kinder for life.

• Without precipitation, Earth would become a "fog planet" in a matter of days!