Summary of the Atmospheric Conditions
Associated With the Tornado Outbreak on May 5, 2007

May 4th - The Day Preceding the Event

The general upper level flow pattern that was dominating the continental United States was characterized by a highly amplified longwave trough over the western states with ridging over the eastern states. A surface low was in place over Northeast New Mexico and Southeast Colorado. Essentially, there was broad southerly to southwesterly flow through much of the atmosphere across the Plains. The result was a surge of warm, moist air northward during the day. By the latter part of the afternoon, 60ºF surface dewpoints were found as far north as Sioux Falls, South Dakota.

Thunderstorms developed as early as the noon hour over Central Nebraska, but the majority of thunderstorms over the Plains fired later in the day. Morning upper air soundings revealed a substantial capping inversion across most of the Plains states, which helped inhibit thunderstorm development through much of the day. Between 6 PM and 9 PM, isolated supercells began developing from Southern Nebraska into Western Oklahoma as the cap began to break.

The most significant supercells during the evening and early overnight affected Kansas where a number of strong to violent tornadoes occurred. This includes the tornado that hit Greensburg, Kansas and was rated an EF-5 on the new Enhanced Fujita Scale. The Greensburg tornado was the first F5 tornado since the tornado that hit Moore, Oklahoma on May 3, 1999.

May 5th

Showers and thunderstorms that occurred the previous night had generally lifted into the Northern Plains by daybreak, allowing for clearing skies over much of the Central and Southern Plains. A dryline extended from a surface low pressure in Northwest Kansas, south into the Eastern Oklahoma and Texas Panhandles. Dewpoints over Oklahoma were already in the mid-60s (ºF) and remained that way through the day, indicating a good deal of moisture ahead of the dry line.

18z (1 PM Central Time) Surface Temperature and Dewpoint. This is an archived mesoanalysis map, courtesy of SPC. Click for a larger image.

With clearing skies across much of the area along and ahead of the dryline during the morning hours, the lowest levels of the atmosphere were able to warm considerably, which in turn led to the development of a highly unstable air mass by afternoon. As heating continued into the afternoon hours, and forcing along the dryline increased, isolated thunderstorms began to develop. Strong wind shear across the region allowed the thunderstorms to become organized, with some producing severe wind gusts, hail and even tornadoes. Evidence of the strong wind shear was found on the 7 AM sounding released from Dodge City, Kansas. 850 millibar winds were out of the south at 48 knots, while 300 millibar winds were out of the southwest at 72 knots. The strong wind shear and instability that existed over the region on May 5th was very supportive of supercells.

22z (5 PM Central Time) Surface Based CAPE - an indicator of instability. This is an archived mesoanalysis map, courtesy of SPC. Click for a larger image.

The thunderstorms eventually expanded in coverage as they pushed northeast across the Plains states. These thunderstorms ended up producing widespread severe weather across the region through the afternoon and evening hours. The Storm Prediction Center's Preliminary Storm Reports web page logged 409 severe weather reports on May 5th. Based solely on the number of preliminary storm reports, this would be the 3rd most active severe weather day nationwide in 2007.

For more on how the storms developed, and how the more notable supercells tracked across Western Oklahoma, please follow the link to the "Supercell Evolution and Motion" page.