A climatology of springtime dryline position in the U.S. Great Plains region

A climatology of dryline frequency and location is presented based on 30 yr (1973-2002) of April, May, and June surface observations from the Great Plains region of the United States. Drylines having a horizontal specific humidity gradient greater than or equal to 3 × 10^-8 m^-1 [greater than or equal to 3 g kg^-1 (100 km)^-1] are found to be present on 32% of the days, with the peak frequency occurring in mid- to late May. The most favored longitude of the generally meridionally oriented drylines is near -101°W at 0000 UTC, although the favored longitude tends to shift westward as the April-June period elapses. There is no robust suggestion of a shift in the annual mean dryline position over the period studied. Relationships between dryline position and wind and relative humidity data at mandatory levels (e.g., 850, 700, and 500 mb) also are investigated. Dryline longitude increases with increasing westerly momentum aloft. Dryline longitude also increases with decreasing relative humidity at 850 mb, primarily at stations in the western Great Plains region, west of the climatologically favored dryline position near -101°. Dryline position is not as closely associated with either 850-mb relative humidity east of the climatologically favored dryline position or relative humidity in the middle troposphere.