Movement of Fronts
Predicting the movement of fronts
can be more complex than forecasting the baroclinic low. Fronts may extend
for hundreds of miles and not all portions of the front will move at the
Continuity/Extrapolation. The best method to use
to predict frontal movement is called the Continuity/ Extrapolation method,
which predicts a trend based on the entire history of past movements of
the front. The forecast based on this data is that the front will continue
to move in the same direction at a steady speed. However, the front will
stall or accelerate if the low stalls or acclerates.
Persistence. Persistence is similar to Extrapolation but
is less reliable because it only looks at the front's activity in the last
two-three hours, rather than the entire history. Because of this, it is
good only for short-term forecasting.
Warm Fronts. Warm fronts most often extend from
the northeast through southeast quadrant of a baroclinic low. They generally
move toward the northeast at an average speed of 10 knots. Movement
of warm fronts may be difficult to predict because they speed up during
the day due to heating and slow down at night due to cooling. In
the day, mixing occurs on both sides of the front, which causes the front
to move forward. At night, radiational cooling creates cool dense surface
air behind the front. This inhibits both lifting and forward progress.
Cold Fronts. Cold fronts most often extend from
the southwest quadrant of a low, but may extend from the west or northwest
quadrant. They generally move toward the southeast. However,
depending on the length of the cold front, portions of the front may move
toward the east, while other portions move south. Some sections of
the front may move faster than others.
Active cold fronts (slow moving) average 15 knots.
Inactive cold fronts (fast moving) have an average speed of 25 knots.
Although cold fronts generally do not move north, the portion nearest the
low may be elongated to move northeast with the baroclinic low’s movement.
Occluded Fronts. A newly formed occlusion will initially
move at the same speed as the cold front that overtakes the warm front.
Eventually, the occluded front "wraps around" the baroclinic low as the
low moves off of the frontal boundary back into the colder air. The
low usually moves to the northwest, on the equatorward side of the occluded
front. The position and counterclockwise flow of the low causes the occluded
front to "wrap around" the low. Satellite imagery is the best tool
to use for both occlusion identification and any movement of the occlusion.