National Weather Service United States Department of Commerce

by Dan Miller, former General Forecaster at NWS Norman, Oklahoma


A localized, but significant late-season tornado outbreak occurred in parts of west central and central Oklahoma during the late afternoon and evening hours on 9 October 2001. The large scale meteorological features and atmospheric conditions over much of the central and southern plains for this event were quite similar to what might be expected on a severe weather day during the spring months of the year. This emphasizes yet again that if the right atmospheric conditions are present, severe weather can and will occur regardless of the time of year.

By early evening on 9 October, a large scale mid and upper-level trough, which had strengthened significantly since early morning on the 9th, was located over the central Rockies with two smaller scale disturbances (short-wave troughs) embedded within the larger scale flow pattern, one extending from central Nebraska to Oklahoma, and another over the central Rockies.

At jet stream level, a speed maximum of 65 knots was located over Oklahoma and eastern Kansas. At 850 mb (roughly 5,000 feet above ground level), a strong low-level jet was located from Oklahoma northward into eastern Nebraska and western Iowa, with wind speeds in excess of 40 knots. Along the low-level jet axis, deep layer of rich moisture was present, with 850 mb dewpoint temperatures (a measure of the absolute moisture content of the air) in excess of 14 degrees Celsius over a large part of Texas and Oklahoma.

At the surface, the primary area of low pressure at 4 PM CDT was located over southern Nebraska, with a secondary low pressure center over South Dakota. A dry line extended from the low pressure center in Nebraska, through Kansas, along the Texas/Oklahoma border and southward to just west of San Angelo, Texas. A cold front extended from South Dakota to eastern Colorado, with a warm front extending eastward into Iowa from the surface low in Nebraska.

The surface thermal ridge (axis of warmest surface temperatures) was located just to the west of the dry line, where temperatures warmed into the upper 80s from the Texas panhandle northward into western Kansas. To the east of the dry line, surface dewpoints were in the mid 60s to lower 70s over a large part of central and western Oklahoma, with the surface moisture axis extending from near the Dallas/Fort Worth metroplex, to Lawton, to near Hutchinson, Kansas.

There were several smaller scale features at the surface which appeared to be very important for this event. Although not clearly evident in the surface pressure field as a true mesolow, the Oklahoma mesonet showed that a rather strong small scale cyclonic circulation developed along the Texas/Oklahoma border during the afternoon. Strong surface pressure falls greater than 5 millibars in 3 hours also developed over southwest Oklahoma during the late afternoon. In addition, a complex of thunderstorms had affected much of central and eastern Oklahoma during most of the day on October 9, and by mid afternoon, the outflow boundary from those storms was located from near Clinton, to Duncan, to Ardmore. In the vicinity of this outflow boundary, surface dewpoints increased into the lower 70s by around 4 PM.

The combination of these 3 small scale features resulted in rather strong east to southeast surface winds from 15 to 25 mph during the late afternoon across much of the southwest third of Oklahoma. As a result, atmospheric conditions over western Oklahoma were very favorable for the development of severe thunderstorms by mid-afternoon on October 9.

The atmosphere was very unstable with CAPE (Convective Available Potential Energy) values from 2500 to 3500 j/kg (joules per kilogram). The surface to 6 kilometer (roughly 18,000 feet above ground) wind shear was around 50 knots, which is generally strong enough to support supercell thunderstorms.

Given the strength of the instability and deep layer wind shear that was present, there were two other features of particular concern by late afternoon: 1) the presence of a nearly saturated and very unstable boundary layer airmass just east of the dry line where temperatures were in the mid 70s, with dewpoints from 70 to 72 and 2) very strong values of wind shear within the lowest 1 kilometer (roughly 3,000 feet) above ground. Surface to 1km storm relative helicity values were in excess of 400 j/kg for much of the late afternoon and evening across areas of Oklahoma along and south of Interstate 40, and west of Interstate 44, with peak surface to 1 km helicity values in excess of 600 j/kg observed just southwest of the Oklahoma City metro area just after sunset.

Not surprisingly, thunderstorms rapidly developed and intensified about 30 to 50 miles east of the surface dryline during the late afternoon. Most of these thunderstorms became supercells and produced numerous tornadoes as they moved eastward through areas of west central into central Oklahoma where the best combination of instability and wind shear was located.