METEOROLOGIST JEFF HABY
To retrograde means to move in the opposite direction of what is climatologically typical. In the middle latitudes, pressure systems
tend to move generally from the west toward the east. This is the same general direction that the jet stream winds move. A retrograding
low is most likely to occur when the upper level steering winds are weak (jet stream weak and displaced closer to the pole). This
lack of steering can not only contribute to retrograde motion but it can also result in pressure systems that move slowly or stall
for several days. When pressure systems stall or nearly stall it is referred to as a blocking pattern. Both high and low pressure
systems can retrograde or stall. The example that will be focused on below is a retrograding low.
This example shows an upper level low (in 12 hour progged increments) that moves from the Northeast U.S. to the Southwest U.S. On these
500 mb progs, the jet stream is weak and displaced far to the north. The low is moving along in retrograde motion steered by the high
pressure system in the Northeast U.S. This type of steering is similar to what is experienced in the tropics. When the jet stream
steering is far to the north, steering in the mid-latitudes can become weak and it can also be influenced by the tropical easterly
wind pattern. This is most likely to occur in the summer months. Watch for these interesting examples of blocking and retrograde
motion on the forecast models.