Case study and mesoscale modeling of the 28 April to 1 May 1999 heavy rain event over southeast Colorado

From 29 April to 1 May 1999, heavy rain fell over the southeast plains and southeast mountains of Colorado. Rainfall totals of 150 to 310mm fell in a corridor extending from the Spanish Peaks northward to the eastern slopes of the Rampart Range and western portions of Colorado Springs. These totals are 25% to 50% of the average annual precipitation in these locations. On the eastern plains, rainfall totals ranged from 75mm to 150 mm. The heavy rains produced a 25 to 40 year flood on the Arkansas River at La Junta with estimated peak flows around 1130 cubic meters per second.

This presentation has three goals. The first goal is to describe the synoptic situation for this event. This event occurred with an omega block. A deep 500mb cutoff low moved southeastward to northwest Arizona early on 29 April, and it remained nearly stationary for two days before lifting northeast over Colorado on the afternoon of 1 May. The omega block also caused a strong anticyclone over the upper Midwest. Between the cutoff low and anticyclone, strong southeast flow brought air with high equivalent potential temperature (theta-e) from the Gulf of Mexico into southeast Colorado. Wind speeds at 700mb were up to 35 m/s in the tongue of high theta-e air from the Gulf of Mexico. There were three periods of heavy precipitation. The first period occurred when the cutoff low moved into northwest Arizona early on 29 April. The second period of heavy precipitation occurred when a disturbance swung around the cutoff low from late 29 April through the afternoon of 30 April. Over southeast New Mexico an MCS developed which moved north-northwest and weakened. The remnant of this MCS moved into eastern Colorado. The third episode of heavy precipitation occurred as the cutoff weakened and moved over the region on 1 May.

Topography played an important role in focusing the precipitation. Colorado Springs is on the south slope of the Palmer Divide. South to southeast flow is orographically lifted as it flows into this region enhancing precipitation. Heavier precipitation also occurred along the eastern slopes of the mountains in southeast Colorado due to orographic lifting.

The second goal is to determine if a locally run mesoscale model would have improved quantitative precipitation forecasts. The NCEP ETA model forecasted storm total precipitation of around 75 to 100mm in the vicinity of Colorado Springs. While this provided a strong indication of a significant precipitation event in the region, it did not identify the area of 75 to 150mm precipitation totals along and near the east slopes of the southeast mountains. The CSU RAMS was run with nested grids to better simulate the event. Analysis of presently being conducted to determine if a configuration of the model, which can be run in real-time, would have provided better precipitation forecasts.

The third goal is to provide background information for a companion paper on poor precipitation estimation by the NWS WSR-88D at Pueblo, CO.