SUBSEASONAL VARIATIONS IN WARM-SEASON MOISTURE TRANSPORT AND PRECIPITATION OVER THE CENTRAL AND EASTERN UNITED-STATES

Subseasonal variations in warm-season (May-August) precipitation over the central and eastern1 United States are shown to be strongly linked to variations in the moisture entering the continent from the Gulf of Mexico within a longitudinally confined ''channel'' (referred to here as the Texas corridor or TC). These variations reflect the developmen t of low-level southerly wind maxima (or jets) on a number of differen t timescales in association with distinct subcontinental and larger-sc ale phenomena. On the diurnal timescale, the TC moisture flux variatio ns are tied to the development of the Great Plains low-level jet. The composite nighttime anomalies are characterized by a strong southerly moisture flux covering northeast Mexico and the southern Great Plains, and enhanced boundary layer convergence and precipitation over much o f the upper Great Plains. The strongest jets tend to be associated wit h an anomalous surface low over the Great Plains, reflecting a predile ction for periods when midlatitude weather systems are positioned to p roduce enhanced southerly how over this region. On subsynoptic (2-4 da ys) timescales the TC moisture flux variations are associated with the development and evolution of a warm-season lee cyclone. These systems , which are most prevalent during the early part of the warm season (M ay and June), form over the central Great Plains in association with a n upper-level shortwave and enhanced upper-tropospheric cross-mountain westerly flow. A low-level southerly wind maximum or jet develops und erneath and perpendicular to the advancing edge of enhanced midtroposp heric westerlies. The clash of anomalous southerly moisture Aux and a deep intrusion of anomalous northerly low-level winds results in enhan ced precipitation eventually stretching from Texas to the Great Lakes. On synoptic (4-8 days) timescales the TC moisture Bur variations are associated with the propagation and intensification of a warm-season m idlatitude cyclone. This system, which also occurs preferentially duri ng May and June, develops offshore and intensifies as it crosses the R ocky Mountains and taps moisture from the Gulf of Mexico. Low-level so utherly wind anomalies develop parallel to the mid- and upper-level wi nds on the leading edge of the trough. Widespread precipitation anomal ies move with the propagating system with reduced rainfall occurring o ver the anomalous surface high, and enhanced rainfall occurring over t he anomalous surface low. On still longer timescales (8-16 days) the v ariations in the TC moisture transport are tied to slow eastward-movin g systems. The evolution and structure of the mid- and low-level winds are similar to those of the synoptic-scale system with, however, a so mewhat larger zonal scale and spatially more diffuse southerly moistur e flux and precipitation anomalies.