NSSL SWAT Case Study - Largest Recorded October Tornado Outbreak in the U.S. : Oklahoma

The October 4th, 1998, tornado outbreak in Oklahoma was uniquely impressive.  Not only was it a record setter for the month, but this tornado outbreak even ranks highly among the more typical springtime events.  A variety of storm morphologies and resulting tornadoes were observed.  From HP to LP; wedge to tubular; day to night, this event seemed to cover all bases, and will serve researchers as a valuable case study.

Presented here are various WSR-88D radar images from the Norman, OK (KOUN) radar. Included in some of the images is output from NSSL's Mesocyclone Detection Algorithm (MDA) and Tornado Detection Algorithm (TDA).  A yellow circle represents a mesocyclone as detected by the MDA.  The red-in-yellow circle represents a mesocyclone whose base is at the lowest radar scan (where it is a more likely tornado threat).  A red triangle indicates the location of a tornado as detected by the TDA.

Description:  In the early afternoon hours activity initiated in northwest Oklahoma where a dryline first became well defined.  The storms quickly assumed HP character, their circulating updrafts surrounded by precipitation.  This character resulted from a combination of abundant moisture, with dewpoints near 70 F, and wind fields aloft.  The early storms occurred well ahead of an approaching jet streak.  Stronger winds in that jet streak influenced the character of storms later in the day, working to carry their precipitation cores away from their updraft bases.
  Description:  In the late afternoon hours the weakly capped atmosphere gave way to the development of numerous updrafts.  All were in a deeply sheared environment, and all contained mesocyclones.  One storm in particular was able to stand up against the shear well enough to produce two highly visible, well documented tornadoes.  The first took place near Watonga, and the second near Dover.  The storm may well have continued to produce tornadoes if it had not been cut off by another storm to its south.  The Dover reflectivity loop reveals a storm which is west of Okarche (near the bottom of the screen) at the beginning of the loop.  This left moving thunderstorm collides with the Dover storm near the end of the loop, and the tornadic circulation quickly dissipates.
  Description:  The late evening storms were very impressive and larger than the earlier storms, as can be seen from the Chickasha images.  The Chickasha storm and subsequent storms were located at the southern end of the outbreak.  These storms maintained and actually increased their tornadic character after dark which suggests that this was a highly dynamic event.

The Moore tornado resulted in F2 damage in the city of Moore.  Interestingly, along the same rear flank which produced the Moore tornado, there existed several noteworthy circulations.  At least one is known to have produced a brief F0 tornado.

The same scenario, multiple circulations along a single flank, developed later that evening just east of the KTLX radar site.  This particular storm took on an incredible appearance with a very large, rain-wrapped hook.  Three tornadoes touched down east of Meeker, OK.

Yet another tornadic storm developed in the Shawnee, OK, area.  The 0250 UTC images show that the forward flank of the Shawnee storm overlapped the rear flank of the Meeker storm.  As a result, the Meeker hook echo is obscured in the reflectivity product, but the circulation stands out extremely well in the storm relative velocity product.

The National Weather Service has conducted damage surveys of both the Meeker and Shawnee storms.

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