The loop below shows the IR satellite imagery and 500-mb height analysis for the 2-day period ending this morning. It shows an outstanding example of what meteorologists refer to as a digging trough.
A 500-mb height analysis is essentially a topographic map, except instead of showing the elevation of the terrain, it shows you the elevation of the 500-mb pressure level. Troughs are areas where the 500-mb level is locally low, and ridges are areas where the 500-mb level is locally high. The flow roughly parallels the height contours with lower heights on the left and is stronger in areas where the height contours are close together. Thus, a meteorologist can look at a map like the one above and quickly ascertain the large-scale flow and how it has been evolving (or, see how it will evolve in the future using a model 500-mb height forecast).
A digging trough moves very strongly southward and often amplifies, forming a closed low center. One can see this in the loop above. Note how a subtle short-wave trough along the SE Alaska/SW British Columbia Coast amplifies, forms a closed low, and then "digs" very dramatically southward into northern California.
Typically digging troughs are characterized by a short-wave trough and associated jet streak, an isolated area of strong jet-level flow, moving down the back side of a long-wave trough, as is the case above. Often there is a cyclone forming and acting to build the ridge immediately upstream, as also can be seen above over Alaska.
The ability to forecast events like this is a product of advances in satellite meteorology and computer modeling. We take it for granted today, but reliable forecasts of events like this simply weren't possible until we advanced the diversity and density of satellite observations collected over oceanic regions and developed computer models that could effectively use those observations and produce reliable forecasts of dynamic "nonlinear" weather systems like digging troughs.