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Mississippi River Valley Effects
Because the Mississippi River Valley leads from the Great Plains to
the Gulf, it provides a natural way for migratory pressure
systems to enter the coastal regions. Outbreaks of cP air are
frequent during the winter with occasional cA air reaching
the coastal area. During the spring and fall, diffused and
modified mP air masses are generally the rule. Most frontal
systems that affect the local area must have a deep upper-level
trough of sufficient magnitude to force the Polar Jet deep into
the Four Corners region of the southwestern U.S. or central
Texas. If this troughing is followed by a cA air mass, freezing
temperatures can be expected locally.
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