Formation, movement and decay of the dryline during the May 1999 Oklahoma tornado episode

A large number of tornadoes developed to the east of a dryline over Oklahoma on the afternoon of 3 May 1999. This dry line propagated eastward, and significant rainfall amounts were reported to the east of it. Radiosonde observations are employed to study the formation, movement and decay of this dryline, and to investigate the convectively unstable conditions that developed during the tornadic activity. It is shown that the dryline under examination existed between surface and 700 mb for nearly 48 h. The dryline s western dry boundary developed with the penetration of dry westerlies to low levels from aloft, and its eastern moist boundary formed with the extension upward of the moist low level southerlies. The dryline moved eastward and it decayed when the dry westerlies replaced the moist southerlies in the upper part of the dryline. Strong convective instability developed in the region where the dry line initially grew in a shallow lower layer with dry air aloft; tornadoes developed on the moist side of the dry line in this region. Numerical short range forecast over the United States is provided by the Eta model at the National Centers for Environmental Prediction. It is shown that Eta analyzed structure of the drylne is close to the observed, except that some observed small scale vertical variations are not captured in the analysis. Conditions favoring severe storm formation seen in 3 May analyzed fields include a strong westerly wind maximum in the middle troposphere, a 500 mb trough and a negative absolute vorticity area in the upper troposphere in the vicinity of Oklahoma. Examination of Eta 48 h forecast from 1200 UTC 3 May shows that the dry westerlies (moist southerlies) to the west (east) of the dryline were stronger (weaker) in the forecast than in the observations; consequently the predicted moisture gradients were weaker and the dryline propagated more rapidly to the east in the forecast. Heavy rainfall amounts werepredicted to the southeast of the observed.