Agenda:

1. Review schematic diagram of divergence ñ how surface low pressure is formed (Reader P. 306)
2. Weather system that produced tornado(s) on Halloween
   1. Watch vs. warning
   2. Super-cell thunderstorm
   3. Hook echo
   4. CAPE
   5. Stability
   6. Upward motion and divergence computed by computers

Keys from the schematic diagram representing divergence:

• Divergence tends to occur on the EASTERN side of upper air troughs due to the fact that air laterally spread apart. (Wind vectors in the diagram are spreading apart on eastern side of troughs).
• Convergence tends to occur on the EASTERN side of upper air ridges.

The rest of the lecture on Nov. 02 2000 was devoted to relating the above concept (as well as others) to the Halloween tornado.

Weather system that produced severe weather and tornado(s) on Halloween:

Note: each concept appears in the order in which it was discussed in class.

Watch: an advisory to severe weather

• Click here for an example of the watch.

  Some of the concepts seen in the watch and warning map for U.S.

  • 2 red boxes on the map of U.S. indicate the area thatís very likely experience severe weather (thunderstorm).
  • Super-cell: the highest level for thunderstorm

Surface weather map for 0 UTC November 1 2000

From the weather map, we can see the wave cyclone with fronts properly drawn. One new front-like line is also seen from the analyzed map (represented by the brown-dashed line with warm front simple). A non-analyzed map is also accessible here.

• Dry line: represented in the weather map by the brow-dashed line with unfilled warm front simple (i.e. a dashed brown line with blank semicircles); this line representing the "boundary" between dry and moist air.

The green "stuff" on the weather map was radar echo.

Dew point chart and CAPE chart for 0 UTC November 1 2000

CAPE is an indirect way of measuring the latent heat release after water vapor is condensed. The higher this CAPE, the higher the latent heat release after condensation, and the higher the chance for the air to become unstable.

The dew-point is very high over the Midwest in the dew-point chart. With the first guess, we can infer that this is the area with the highest CAPE value è unstable. Then viewing the CAPE chart, we can see that itís generally true.

500-mb chart (~18000 ft.)

• Winds are spreading apart on the eastern side of trough. Thus, divergence is resulted there.
• The ground would have a low-pressure system formed beneath area of divergence aloft.

Divergence plot

Divergence pretty much occurs on the eastern side of the upper air trough (from the 500mb chart). It occurs on region where winds are spreading apart aloft, extending from Texas - New Mexico border to Nebraska

Upward motion plot

Upward motion is greatest over Texas ñ New Mexico border, Kansas, Nebraska, and Wyoming. The upward motion does not perfectly matches with the position of divergence aloft and induced surface low pressure, but it still shows that:

-Divergence aloft causes the induced surface low pressure

-then both the divergence aloft and surface low pressure causes upward motion.

Radar

• Hook echo: tornado signature; it looks like a hook on the southern portion of the echo. Note if the radar image from the above link is zoomed in enough, you can see a hook on to the south of Mc Cook, where the echo intensity falls within the red and pink region.

Warning: notice to the public that severe weather is happening right now; severe weather is happening somewhere else. Those who are in the path of the occurring severe storm should seek shelter or some sort.

The difference between watch and warning:

a) Watch ñ not yet happened, just an advisory for people to look out for severe weather in the watch area.

b) Warning ñ happening, a notice for people to seek shelter if they are in the path of the severe storm

Stability: the tendency for an air parcel to return to its original position after being displaced vertically.

• Stable: the state in which an air parcel finds itself colder than the surrounding (at the same level); the air parcel then sinks. (first state in the linked image above)
• Unstable: the state in which an air parcel finds itself warmer than the surrounding (at the same level); the air parcel is then buoyant, and it rises freely. (second state from the linked-image above)
• Conditionally unstable: the stability of the air parcel now depends on the moisture content. If the air parcel is dry, it cools at the dry adiabatic rate, and may find itself colder than the surrounding; then it sinks. However, if the air parcel is moist, it cools at the moist adiabatic rate, and finds itself warmer than the surrounding; then it rises freely. (third state from the linked image above)