Cold Occlusions. 

Occlusions occur when the faster moving cold front 
overtakes the slower warm front.  When it occurs, one of 
two things happens: the cold air forces the warm air 
upward, or the cold air rides up over the warm air.  The 
first case, the warm air riding up, is a cold occlusion.  
A cold occlusion is nothing more than a warm front aloft.  In analysis, if the air behind the occlusion is colder than the air ahead of the warm front, then it is a cold occlusion. 

The polar front jet has two configurations with 
occlusions. In the Type A configuration, the PFJ crosses 
the frontal system at or just north of the triple point.

In the Type B configuration, the PFJ becomes more 
diffuse, "fanning out" behind the occluded front and 
reforming north of the warm front. 

On the surface, temperatures are fairly cool ahead of 
the front, dropping to much colder following passage.  
Surface dew points will fall, since cold air holds less 
moisture than warm air.  Visibility and weather is 
usually poor before and during passage and improving 
after passage.  Ahead of the front, pressures will fall 
with the lowest pressure occurring right during passage.  Following passage pressures rise.  Surface winds can be gusty, usually from the south to southeast before passage, rotating clockwise to the west - northwest after passage (veering winds).