Another remarkable aspect of these storms, in addition to their rapid development, is
the extent of snowfall associated with them. These heavy snowfalls are made possible
by the collision of very warm, moist, maritime air with very cold and dry air of conti-
nental origin. The coastal storms usually develop after a very intense cold front has
crossed the eastern United States leaving cold Canadian air along the eastern
seaboard. This cold dense air can be trapped, or “dammed,” between the coast and
the Appalachian Mountains to the west and remain in place for several days. The
coastal storm, which often begins as a wave along the remnants of the original cold
front, moves up the coast parallel to the offshore Gulf Stream. Because the wind field
around the storm is counterclockwise (cyclonic), warm, moist Gulf Stream air is driven
into and, being less dense, over the cold air dammed along the coast. The moisture
wrung out of the ascending air passes through the colder layer below and creates the
mixture of rain, snow, sleet and freezing rain that is typical of these storms. Along the
coast, the cold air layer is thinner, if present at all, and the precipitation falls as rain.
Further inland, the cold air may be thick enough to freeze the precipitation as it hits
the ground (freezing rain) or as it falls toward the ground (sleet). As the ground rises
into the Appalachians, the precipitation will be mainly snow.
The forecasting of these coastal storms has improved over the last several years with
advances in computing power and improvements in weather forecast models. As a
rough guide, any time a strong Canadian cold front crosses the East Coast during the
winter months and becomes stationary across the northern Gulf of Mexico, there is a
possibility for this coastal storm development.
During the spring months, the focus for severe and unusual weather shifts to the cen-
tral United States. This is the season of tornadoes in the Great Plains. Again there is a
clash of air masses. In this case, warm air from the Gulf of Mexico advances northward
where it collides with southward moving cold polar air. While the systems that produce
severe weather in this season are generally variations on the classic comma cloud
cyclonic disturbance, there are also smaller scale (mesoscale) systems that produce
heavy rainfall and severe weather. These systems, called Mesoscale Convective
Systems(MCS), come in various shapes—from the familiar line squall to the nearly cir-
cular Mesoscale Convective Complex(MCC). The latter system has a unique satellite
signature and is very common over the Great Plains in the spring and early summer.
During the Flood of 1993, a considerable number of MCSs occurred in the Midwest.
Several MCCs are shown In figures 40a and b (page 58). The MCC is an organized
group of thunderstorms that initiates late in the afternoon from a localized area and
develops throughout the night before dissipating late the next morning. In figure a,
taken at 0100 CDT, two mesoscale systems are present over Iowa and Nebraska. Note
the size of the cloud shield associated with each system. These systems initiated late
the previous afternoon from a small group of thunderstorms. These MCC’s persist
throughout the early morning or even through the next night and (figure b) can
migrate considerable distances before dissipating.
These systems often recur over several days and can account for significant local rain-
fall. During the flood of 1993, mesoscale systems occurred frequently during the sum-
mer months. A great deal of research continues regarding the organization and devel-
opment of these systems. For example, figure 40c (page 59) shows another large MCC
occurring the night after the storms in figures 40a and b.