Class #23: Friday October 23

The Extratropical or Midlatitude Cyclone

1Class #23: October 23

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 #23: October 23 2

Sank on Day 4

Class #23: October 23 3Fig. 10-1, p. 276

Ore freighter Edmund Fitzgerald

Class #23: October 23 4Fig. 10-2, p. 277

North is at lower right. Ship sank near Whitefish Point

Class #23: October 23 5Fig. 10-4, p. 278

Day 1 and Day 2

Class #23: October 23 6Table 10-1a, p. 279

Table 10-1b, p. 2797Class #23: October 23

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

Class #23: October 23 8Fig. 10-5, p. 280

Birth to NW of Amarillo

Class #23: October 23 9Fig. 10-5a, p. 280

Strong westerlies upstream

Class #23: October 23 10Fig. 10-5b, p. 280

Low humidity, clear skies

Class #23: October 23 11Fig. 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 #23: October 23 12Fig. 10-6, p. 281

Fig. 10-6a, p. 28113Class #23: October 23

Fig. 10-6b, p. 28114Class #23: October 23

Fig. 10-6c, p. 28115Class #23: October 23

Shows Panhandle hook path

Class #23: October 23 16Fig. 10-6d, 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 #23: October 23 17

Cyclonic spin (or vorticity) is important for cyclogenesis.

Class #23: October 23 18Box 10-1, p. 282

Cyclonic spin is generated by flow over mountains

Class #23: October 23 19Box 10-1, p. 283

Day 2: Young adult stage

Class #23: October 23 20Fig. 10-7, p. 285

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

Class #23: October 23 21Fig. 10-7a, p. 285

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

Class #23: October 23 22Fig. 10-7b, p. 285

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 #23: October 23 23

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

Class #23: October 23 24Fig. 10-7c, p. 285

A close-up of last slide

Class #23: October 23 25Fig. 10-8, p. 286

Fig. 10-9, p. 28726Class #23: October 23

Fig. 10-10, p. 28827Class #23: October 23

Fig. 10-10a, p. 28828Class #23: October 23

Fig. 10-10b, p. 28829Class #23: October 23

Fig. 10-10c, p. 28830Class #23: October 23

Fig. 10-11, p. 28931Class #23: October 23

Fig. 10-12, p. 29032Class #23: October 23

Fig. 10-13, p. 29133Class #23: October 23

Fig. 10-14, p. 29234Class #23: October 23

Fig. 10-14a, p. 29235Class #23: October 23

Fig. 10-14b, p. 29236Class #23: October 23

Fig. 10-14c, p. 29237Class #23: October 23

Box 10-2, p. 29338Class #23: October 23

Box 10-2, p. 29339Class #23: October 23

Fig. 10-15, p. 29440Class #23: October 23

Fig. 10-16, p. 29541Class #23: October 23

Fig. 10-17, p. 29542Class #23: October 23

Box 10-3, p. 29743Class #23: October 23

Box 10-3, p. 29744Class #23: October 23

Box 10-3, p. 29845Class #23: October 23

Box 10-3, p. 29846Class #23: October 23

Box 10-3, p. 29947Class #23: October 23

Box 10-3, p. 29948Class #23: October 23

Fig. 10-18, p. 30149Class #23: October 23

Fig. 10-18a, p. 30150Class #23: October 23

Fig. 10-18b, p. 30151Class #23: October 23

Fig. 10-18c, p. 30152Class #23: October 23