Organized areas of thunderstorm activity reinforce pre-existing
frontal zones, and they can outrun cold fronts. This outrunning
occurs in a pattern where the upper level jet splits into two
streams. The resultant mesoscale convective system (MCS) forms
at the point of the upper level split in the wind pattern in the
area of best low level inflow. The convection then moves east
and toward the equator into the warm sector, parallel to low-
level thickness lines. When the convection is strong and linear
or curved, the MCS is called a squall line, with the feature
placed at the leading edge of the significant wind shift and
pressure rise. This feature is commonly depicted in the warm
season across the United States on surface analyses, as they lie
within sharp surface troughs. If squall lines form over arid
regions, a duststorm known as a haboob may result from the high
winds in their wake picking up dust from the desert floor.
Squall lines are depicted on National Weather Service surface
analyses as an alternatin