In March  2010, I was asked to participate in the CASA (Collaborative Adaptive Sensing of the Atmosphere) portion of the 2010 Spring Experimental Warning Program (EWP) in the Hazardous Weather Testbed (HWT) at the National Weather Center (NWC) in Norman, OK (Figure 1).  CASA operates a dense radar network of four X-band 3cm radars between Oklahoma City and Lawton, OK.  These radars only have a 30nm effective range but overlap to provide multiple-radar analyses of reflectivity and velocity. For more information on CASA, see http://www.casa.umass.edu/ or the CASA IP1 Wiki at http://casa.forwarn.org/wiki/. The purpose of the CASA EWP experiment is have experienced forecasters evaluate real-time and case studies of CASA radar data.

Beginning on the week before my arrival at the NWC, I became increasingly excited because of consistent model forecasts of severe weather in Oklahoma.  I even stayed up the night before my departure to view the SPC outlook for May 10 – High Risk of severe thunderstorms and large tornadoes in Oklahoma!  When I arrived at OKC airport at noon on Monday, I immediately began coordinating my arrival with Jerry Brotzge and Brenda Philips (CASA Principal Investigators) via phone and text messages.  Brenda’s flight had also landed at OKC around noon, so we drove down together.  We had just enough time to grab a quick lunch to go and arrived at the HWT around 130pm where we immediately began reviewing the latest information.  Central Oklahoma was still under the gun and storms were developing along the dryline to the northwest of the CASA testbed area.  I don’t think I had finished my lunch yet when Brenda told me it was time to make a forecast!  We used twitter and NWSChat as our primary mediums for disseminating our forecasts, warnings, and updates.  After pegging a time of 5pm for activity to reach the testbed area, I watched the event unfold with one supercell after another developing along and ahead of the dryline.  Unfortunately, all of them seemed to develop just outside of the testbed area.  Around 515pm, one left-moving supercell storm split off to the NE and moved inside the network.  This storm contained an unusually strong anticyclonic mesocyclone (mesoanticyclone?) and hook configuration (Figure 2).  When asked if I would issue a tornado warning on that storm, I replied “No, because anticyclonic mesocyclones rarely produce tornadoes.”  At 525pm, a Tornado Warning was issued by WFO OUN for this storm.  It turned out that a six mile long tracked EF1 anticyclonic tornado had touched down at 518pm near Bray, OK and another pair of tornadoes (one anticyclonic and the other cyclonic) near I-35 and Wayne, OK.  Around this time, two LP-like supercells were approaching Moore and Norman.   As the Norman storm approached, I saw SPC forecasters run to the west windows. Being a conscientious, safety-minded NWS meteorologist, I also ran to the window and observed a rapidly rotating funnel nearly over the National Weather Center (Figure 3).  The tornado grew in size as it tracked east along Highway 9 and even damaged a few of the NWC employees’ homes.  As storms moved east and away from the network that evening, we closed operations for the day.  Between 2 and 8 pm, 31 tornadoes were confirmed across the state [http://www.srh.noaa.gov/oun/?n=events-20100510-tornadotable].  An exciting start to the experiment to say the least!  In the following days, there were several close calls to observing severe storms within the network during and after operations, but none as significant as the May 10 event.

The orientation planned for Monday took place on Tuesday.  During the rest of the week, I went through several displaced real-time simulations using 88D data only, then repeated using 88D and CASA radar data, multi-radar wind analyses and high-resolution model forecasts. The simulations included the Anadarko, OK tornado of 14 May 2009 and the Rush Springs, OK tornado during the early morning of 2 April 2010.  Without knowing any details of either case, I was challenged trying to issue timely warnings based on CASA radar data.  Without going into detail, CASA radars use adaptive scanning strategies that depend on the coverage and intensity of storms.  Data at any one elevation angle can be as frequent as every 30 seconds.  Trying to mentally process data from four CASA radars in the same way we do one 88D data was an exercise in futility.  I do not believe manual interrogation of such high-resolution radar data is a realistic option for warning forecasters of the future.

The Rush Springs, OK tornado case was very eye-opening and showed the tremendous potential of CASA radar technology to detect smaller tornadoes.  Figure 4 shows a side by side comparison of KTLX and the KRSP CASA reflectivity at the time of the tornado.  Many areas east of the Mississippi river are prone to these smaller tornadoes that develop more rapidly than those from supercells.  Trapp and Weismann (2005) more recently showed how tornadoes spin-up in the comma head portion and along the leading edge of quasi-linear convective systems (QLCS).  Tornado warning lead time and accuracy is lower for both QLCS and tropical cyclone storms than that of supercells.  A local study done in the Peachtree City WFO in 2009 showed that 13 out of 16 unwarned F2 or greater tornado events resulted from QLCS storms.  A Hollings Scholar is also studying  QLCS tornado climatology and warning accuracy this summer in the Peachtree City WFO.  So far we have determined that the initial lead time of tornadoes from QLCS storms across the mid-south and southeast averages about 25% of those from supercells (3-5 minutes vs 20 minutes).

During the week, I was able to pick the brains of some scientists. I shared presentations and concerns from the 14-15 March 2008 tornado and 21 September 2009 flash flood events in north Georgia.  I discussed research on unwarned tornadoes recently published in WAF with Jerry Brotzge, who showed how such missed events can be correlated to smaller tornadoes (as just mentioned).  I plan on collaborating further in the future.

I will certainly remember the experience I had at the EWP this year and look forward to the day when technology like this is deployed operationally.

Steve Nelson (Science and Operations Officer, NWS Peachtree City/Atlanta GA – 2010 Week 5 Evaluator)