Class #24: Monday October 26

The Extratropical or Midlatitude Cyclone continued

1Class #24: October 26

The story of an extratropical cyclone

• Day 1, Sat., Nov. 8,1975: Low (Panhandle Hook) forms just NE of Amarillo

• Day 2, Sun., Nov. 9: Ship sails at 2 p.m.; Storm has matured to young adult stage

• Day 3, Mon., Nov. 10: The storm is occluding. Ship sinks at 7:20 p.m.

• Day 4, Tues., Nov. 11: The storm is dying • Subtract 7 from date to get Day #

Class #24: October 26 2

Sank on Day 3

Class #24: October 26 3Fig. 10-1, p. 276

Day 1 and Day 2

Class #24: October 26 4Table 10-1a, p. 279

Day 1 weather maps: Low forms NE of Amarillo, TX

Class #24: October 26 5Fig. 10-5, p. 280

Birth to NW of Amarillo

Class #24: October 26 6Fig. 10-5a, p. 280

Strong westerlies upstream

Class #24: October 26 7Fig. 10-5b, p. 280

Low humidity, clear skies

Class #24: October 26 8Fig. 10-5c, p. 280

Typical cyclone paths

• Depend on topography• Depend on position of the polar front• Depend on upper level winds:– Extratropical cyclones move approximately with

the 500mb wind at about half the speed

Class #24: October 26 9Fig. 10-6, p. 281

Ingredients for a cyclone

• Cyclone growth is called cyclogenesis• Cyclogenesis requires 3 ingredients:– Surface temperature gradients: a front– A strong jet stream: helps the low grow and the

fronts intensify– Mountains or other surface boundaries (for example,

a coastline along a warm ocean current• Cyclones get their energy in a process called

baroclinic instability—related to horizontal temperature gradients and vertical wind shear

Class #24: October 26 10

Day 2: Young adult stage

Class #24: October 26 11Fig. 10-7, p. 285

Note fronts, Low, and path. Ship sets sail at 2 p.m.

Class #24: October 26 12Fig. 10-7a, p. 285

Note upper-level short-wave trough to west of surface Low

Class #24: October 26 13Fig. 10-7b, p. 285

Satellite picture from 10:12 a.m.Note the comma shape of cloud

Class #24: October 26 14Fig. 10-7c, p. 285

A close-up of last slide

Class #24: October 26 15Fig. 10-8, p. 286

Back to the story

• The cyclone moves very rapidly, steered by the upper-level winds at 500mb.

• On Day 2 (Nov. 9) gale warnings were issued in the mid-afternoon for the next day (Monday, Day 3, Nov. 10)

• Gale warnings are for winds up to 38 knots (44 mph), not typical, not too unusual

• Ships take the northern, longer route

Class #24: October 26 16

Days 2 and 3 in Madison Wisconsin

Class #24: October 26 17Fig. 10-9, p. 287

Day 2

Class #24: October 26 18Fig. 10-10, p. 288

Fig. 10-10a, p. 28819Class #24: October 26

Fig. 10-10b, p. 28820Class #24: October 26

Fig. 10-10c, p. 28821Class #24: October 26

The story and the forecasts

• At first the storm was expected to pass south of Lake Superior

• A northern course over the lake would set the ships in northeasterly winds north of the low pressure center and a shorter fetch (distance over which the winds would blow) for smaller waves

• By 10:39 p.m. on Day 2 the forecast changed.

Class #24: October 26 22

The new forecast at 10:39 p.m. on Day 2

• Winds of more than 40 knots (46 mph) on Lake Superior

• Winds from the west and southwest• Waves of height 2.5 to 8 meters (8 to 15 feet)

over Lake Superior• Over past 12 hours forecast wave heights

doubled and forecast wind changed 180°• Shortly gale upgraded to storm warning

Class #24: October 26 23

What happened

• A jet stream helped the storm to intensify, that is, for the surface pressure to decrease and the winds to increase

• A jet stream helped to steer the storm to the northeast more quickly

Class #24: October 26 24

Surface pressure fallswhen there is divergenceof the wind in the columnof air above the low

Class #24: October 26 25Fig. 10-11, p. 289

Fig. 10-12, p. 29026Class #24: October 26

Jet stream (300mb) winds at 6p.m. on Day 2

Class #24: October 26 27Fig. 10-13, p. 291

Day 3, Day 4, and later

Class #24: October 26 28Table 10-1b, p. 279

Day 3

Class #24: October 26 29Fig. 10-14, p. 292

Morning of Day 3: occlusion

Class #24: October 26 30Fig. 10-14a, p. 292

Fig. 10-14b, p. 29231Class #24: October 26

Fig. 10-14c, p. 29232Class #24: October 26

Fig. 10-16, p. 29533Class #24: October 26

Fig. 10-17, p. 29534Class #24: October 26

Box 10-3, p. 29735Class #24: October 26

Box 10-3, p. 29736Class #24: October 26

Box 10-3, p. 29837Class #24: October 26

Box 10-3, p. 29838Class #24: October 26

Box 10-3, p. 29939Class #24: October 26

Box 10-3, p. 29940Class #24: October 26

Day 4: A dying storm

Class #24: October 26 41Fig. 10-18, p. 301

Fig. 10-18a, p. 30142Class #24: October 26

Fig. 10-18b, p. 30143Class #24: October 26

Fig. 10-18c, p. 30144Class #24: October 26

Fig. 10-15, p. 29445Class #24: October 26