As daylight broke on May 3rd of 1999, across Oklahoma, skies were generally cloudy with a deck of low stratus clouds spread over much of the state. The only exception was along and west of a dryline that stretched from near Gage, Oklahoma to near Childress, Texas at dawn. The stratus clouds were the result of significant moisture return that occurred overnight, as morning dewpoints were in the low to mid 60s. Not surprisingly, the air was quite humid at the surface, with temperatures only in the mid to upper 60s.
Meanwhile, the threat for severe weather was being closely monitored by the Storm Prediction Center and the National Weather Service forecast office in Norman, Oklahoma. The Storm Prediction Center had posted a "slight risk" (image courtesy of SPC publication) on their early morning outlooks for most of Kansas, Oklahoma, and north Texas. The Slight Risk was indicative of the general consensus by forecasters that there was a severe weather threat over the Southern Plains, including the risk of some supercells. However, the precise location of the greatest threat area was a bit uncertain that early in the morning.
The forecasters at the forecast office in Norman agreed about the threat of severe weather, and mentioned the risk in a 6:30 AM CDT Thunderstorm Outlook. It mentioned the increasing low-level moisture, dryline and approaching upper-level low pressure trough would combine to cause a threat of large hail, damaging winds and tornadoes. It also cautioned emergency managers and spotter groups to be prepared for possible activation in the afternoon. In fact, the threat of severe thunderstorms was mentioned in most of the zone forecasts for the counties in Oklahoma at the 4:30 AM CDT issuance.
Through the morning hours, forecasters became progressively more certain of a significant severe weather episode. The low cloud cover began to break up in advance of the dryline as daytime heating commenced. Although high cirrus clouds would overspread the area later, into the afternoon hours, at least some filtered sunshine was observed in most spots for most of the day. The sunshine and heating, combined with abundant low-level moisture, would combine to produce a very unstable air mass. As forecasters modified 12Z (7:00AM CDT) upper air balloon sounding data with expected afternoon conditions, they noticed the potential for a weakening cap and CAPE values potentially exceeding 4000 j/kg. Such instability levels are climatologically favorable for severe supercells and tornadoes when combined with the strong directional shear that was also forecast.
Accordingly, the risk of severe thunderstorms was upgraded to moderate (graphic: courtesy SPC) in a new outlook at 11:15 AM CDT. Here is an excerpt of the text from that outlook: "...will provide sufficient shear for a few strong or violent tornadic supercells given the abundant low level moisture and the high instability." Forecasters at the NWS forecast office in Norman mentioned the increasing threat of a severe weather outbreak from late in the afternoon and into the evening in a Thunderstorm Outlook at 12:30 PM CDT. The threat for tornadoes was again mentioned.
As the early afternoon wore on, the potential for a significant severe weather outbreak appeared more and more likely. Special weather balloon launches around 1 PM suggested wind profiles highly conducive for tornado development - speeding up with height, and changing directions with height. They also showed a very unstable atmosphere. As this special article on the Norman Weather Partners website points out, "it became more obvious something major was looming." Therefore, at 3:49 PM CDT, the risk of severe weather was upgraded to a high risk (graphic) for all of the NWS Norman forecast area.
The first cumulus towers developed, but quickly dissipated between 3:00PM and 3:15PM CDT in northwestern Texas. However, renewed cumulus development occurred between 3:30PM and 4:00PM CDT over southwest Oklahoma. A few of these cumulus towers developed into significant supercells by 5:00PM CDT before moving northeast towards the Oklahoma City metropolitan area. The first Severe Thunderstorm Warning was issued at 4:15PM CDT for Comanche County, and five minutes later quarter sized hail was falling in Lawton.
At 4:47PM CDT, the first Tornado Warning was issued for Comanche, Caddo and Grady Counties. Four minutes later the first tornado report was received at the NWS office in Norman, from a spotter 7 miles east-northeast of Medicine Park. Just beyond the 5 o'clock hour, it became obvious that a significant outbreak was unfolding, and that tornadoes would threaten the Oklahoma City metropolitan area. Forecasters at the NWS office in Norman noticed that the thunderstorms continued to gain strength in a very unstable and moist air mass.
At 5:41PM CDT, the NWS in Norman issued a Short Term Forecast that discussed the potential for tornadic supercells to track northeastward into the Oklahoma City metropolitan area. It cautioned people to remain alert through the evening hours. By 6:30PM CDT, the tornadic supercells were expected to threaten the outer reaches of the Oklahoma City metropolitan area within the next hour. A new Short Term Forecast discussed the ongoing supercells and forecasted that the threat of tornadoes "will continue to be high through the evening".
The tornadic supercells then continued to move northeast into the Oklahoma City metro area, and across Central Oklahoma. For information about individual storms, click on the links provided at the top of this writeup. Here is a map of the supercell tracks across the NWS Norman county warning area, as adapted from radar data and the SPC publication previously referenced: