Three-dimensional airflow and precipitation structure in an nondeepening cyclone

During the period 1200 hr UTC on March 3, to 1200 hr UTC on March 5, 1985, a cyclone located over eastern Colorado tracked northeastward across the Great Plains to southern Ontario. Although the low-pressure system decayed considerably during the latter 24 hr of the period, notable amounts of precipitation fell, particularly over the New England states. The time of detailed study encompasses 1200 hr UTC on March 4, to 1200 hr UTC on March 5, 1985. Inspection of the observed 6-hr precipitation totals during this period indicate three distinct maxima. One is to the north and northwest of the low center, another to the east of the low center, and a third along the surface cold front extending southwestward from the low center. To aid in the explanation of these observed patterns, isentropic trajectories were used to illustrate the three-dimensional airflow around the nondeepening cyclone. Low-level easterly flow was found to the north of the cyclone center. Meanwhile, a flow of warm, moist air being transported northward from the Gulf of Mexico between 700 and 500 mb bifurcated east of the cyclone center. A branch of this moisture curved cyclonically and flowed to the north and west of the low center, while the other branch curved anticyclonically and flowed to the east of the low center. These two streams of warm, moist air were forced to ascend over the layer of cold air in the low-level easterly flow, causing clouds and precipitation to result. These processes help explain the cloud patterns and precipitation structure associated with the nondeepening cyclone. The Sutcliffe-Petterssen development equation is used to calculate the surface vorticity tendency field. From the calculations, it is seen that warm advection in the lower troposphere, rather than midtropospheric vorticity advection, is responsible for the persistence of the precipitation over the New England states.