1. Basic Convergence and Divergence
This presentation goes over basic concept concerning upper-level divergence and convergence.
It would be best if shift members were comfortable with a 500mb chart before going over this PowerPoint.
If not, show them the basics of a 500mb map before preceding.

2. Definitions/Concepts
Convergence is air coming together
Combination of confluence (air moving together) and speed convergence (air slowing down over a distance)
Surface convergence associated with low pressure and potential for rain, snow, thunderstorms, etc.
Upper-level convergence associated with sinking air and therefore surface high pressure
Fair weather
Divergence is air spreading apart
Combination of diffluence (air moving apart) and speed divergence (air speeding up over a distance)
Surface divergence associated with high pressure and fair weather
Upper-level divergence associated with rising air and therefore surface low pressure
Basic definitions and concepts.
Convergence is a combination of confluence and speed convergence.
Confluence is simply air moving together.
Speed convergence occurs when air slows down over a distance.
A traffic jam analogy can illustrate this. Cars slow down and converge when coming into a traffic jam (or in this case air slows down and converges as it approaches a trough axis).
The following slide reinforces the traffic jam analogy.
Similarly, divergence is a combination of diffluence (air moving apart) and speed divergence (air speeding up over a distance).

3. Traffic Jam Analogy Trough axis is the traffic jam.
Cars slow down approaching the traffic jam (convergence)
Cars speed up exiting the traffic jam (divergence)
The traffic jam analogy.
Emphasize that the trough axis is the traffic jam and air moving in the atmosphere is the cars.
Divergence usually occurs downstream of the trough axis (traffic jam) and convergence generally occurs upstream of the trough axis (traffic jam).

4. Recognize on a Upper Air Chart
Two figures that illustrate where one might expect to find convergence and divergence on an upper level map.
It also shows what surface pressure features might be expected.
Upper level divergence generally leads to rising motion and therefore surface low pressure while upper level convergence leads to sinking motion and therefore surface high pressure.

5. Intro to the 500mb Map http://www.spc.noaa.gov/obswx/maps/
Now that the basic definitions and concepts of convergence and divergence have been presented, it is a good time to introduce the basics of the 500mb map since this is a good map to find troughs and ridges and several 500mb maps will be used in the next few slides.
Be sure to go over all of the pieces of a 500mb chart.
Isoheights, (black lines) isotherms, (red dashed lines) and wind barbs (be sure to cover direction and speed).
Each station has a observed height, (purple number) temperature, (red number) and dew point (green number).
There is a clear ridge located over the western U.S. and a clear trough located over the eastern U.S.
Can use the topographic map analogy to make this clearer if needed (ridges are “mountains” and troughs are “valleys”)
Since we can find troughs and ridges on a 500mb map we can discuss convergence and divergence (see later slides).
The heights represent the height of the 500mb pressure surface at a particular station. This is essentially what height on this map shows. The 500mb pressure surface is at different heights (higher in ridges and lower in troughs) across the nation and the heights show this. If someone wants to know what the pressure is at 500mb, its 500mb everywhere. The picture may help explain this some:

6. Convergence Divergence Trough Axis
Example of a 500mb chart with a trough annotated.
It is important to note that one cannot say there is divergence or convergence simply by looking at a map like this. These are mathematical quantities.
For example there may be weak diffluence (which can be seen on a map like this) but strong speed convergence. The net result would be convergence even though it looks like the winds are spreading apart.
However, you can still stress that most of the time, divergence is occurring downstream of a trough axis and convergence is occurring upstream of a trough axis.
Note: Divergence and convergence can’t actually be found on a map like this as they are mathematical quantities, but in general, divergence occurs downstream of a trough axis and convergence occurs upstream of a trough axis.

7. What does this mean for forecasting?
Basically: fair weather behind a trough axis and clouds/precip possible ahead of a trough axis
Implications on forecasting.
Upper level divergence is associated with rising motion, surface low pressure, and therefore potentially clouds and precip.
Other factors such as moisture, fronts, etc. also need to be considered with forecasting precip and clouds, but this is one useful factor.
Upper level convergence is associated with sinking motion, surface high pressure and therefore clear skies.
Remember that as air descends it warms adiabatically and without an influx of moisture, it will dry.  generally cloudless skies.
Therefore, when a location is downstream of a trough axis, clouds and precip are possible. When a location is upstream of a trough axis, mostly clear skies and low precipitation chances are likely.

8. Precip or no Precip for Oklahoma?
The next four slides present an interactive activity for the shift members.
Feel free to print out the last four slides for each shift member.
Have each shift member locate the primary trough axis and then predict whether or not there will be precip in Oklahoma.
In this case, the trough axis is located over the eastern U.S. and there would be no precip across OK.

9. Trough axis located across the western U. S
Trough axis located across the western U.S. (generally along and just to the east of the Rocky Mountains)
Precip would be expected in Oklahoma since it is downstream of the trough axis  upper level divergence and rising motion.
Of course other factors such as moisture need to be considered but based on this lesson alone we would forecast precip.

10. Trough axis over the east coast.
No precip for OK.

11. The trough that could affect OK weather is located over the southwest U.S.
Based on this map alone, precip would be expected in Oklahoma.
This is the morning of the Jan 28-29, 2010 ice storm and snow event in Oklahoma.