Stories from the Sonoran Desert

1. Stories from the Sonoran Desert Severe Weather Technology Workshop 10 July 2007 Doug Green

2. Warning decisionmaking shortcomings • Tools…or lack thereof…to help forecasters discriminate between microburst and non-microburst storms. Three and four dimensional radar data displays (such as GR2 Analyst) are very useful to forecasters. • Basin flash flood guidance does NOT take important basin characteristics into account (e.g., Greg Smith’s FFPI information is not integrated into FFMP.) Office typically uses “forced FFG” in order to increase usefulness of FFMP. • The question “Where’s the line between urban/small stream flooding and flash flooding?” is tricky. Rainfall rate and amount guidelines are used to help delineate between the two…is this “good enough”?

3. Warning decisionmaking shortcomings • Precipitation estimates often leave much to be desired, and are not adjusted for environmental conditions (ex: 24 Aug 2006 flash flood over and near Indian Bend Wash in Phoenix metro). • Max hail size predictions from SCAN never seem to be observed • Lightning data, including # of c-g strikes, is not incorporated into the SCAN Cell Table. • TDWR data: available, but not heavily used. It’d be great if TDWR and nearby WSR-88D data could be combined to generate dual-Doppler radar displays.

4. Warning decisionmaking shortcomings • Current verification scheme discourages issuance of warnings in low-population areas, since it’s unlikely a severe event will be reported • When WFOs warn for what they deem to be severe events, they should not be penalized; however, that’s not the case today • High impact vs significant events • Don’t want to miss any high impact events, so tendency is to warn for borderline severe events over high population areas and nowcast same over low population areas…is this what we should be doing?

5. More shortcomings… • WES is a great training tool, and data upload/download has been streamlined…but it’s still cumbersome. We need the ability to store several months’ worth of data on the system at one time…and that doesn’t exist attm. • Examine numerous cases…to improve capability to provide useful warning guidelines for various synoptic/mesoscale regimes • Have ability to examine a case from time to time without worrying about unloading/reloading the data • Anything that makes working with WES easier for our forecasters “greases the skids” and encourages its use!!

6. How should we address the shortcomings? • AWIPS needs to have GR2Analyst-type capabilities! • Reliable flash flood guidance needs to be created and provided over the entire CWA • Rain gauge data definitely needs to be incorporated into any rainfall estimation scheme. • Microburst potential algorithm needs to be provided to, and evaluated by, field forecasters. • Vertical numerical profiles of cell-based reflectivity and radial velocity convergence/divergence need to be provided to forecasters. • Increase data storage capacity for case review purposes • TDWR help! Dual Doppler output? Cell table? Microburst prediction output for forecasters?

7. And now…for something completely different…here’s a brief overview of a couple of radar issues at WFO Phoenix • Unusually large reflectivity discrepancy observed between KIWA and KYUX radars • Apparent KIWA velocity data quality problems

8. Reflectivity Discrepancy between KIWA and KYUX • During September 2006, forecasters began to notice an unusual discrepancy regarding storm intensity and precipitation estimates for storms nearly equidistant from KIWA and KIWA RDAs • KIWA returns much stronger • KIWA precipitation estimates much greater • Storms were tall, and beam blockage was not “the issue” • Sent a few examples of what we observed to Tony Ray at the ROC

9. ROC determined that… • KYUX was running “cold” • KESX was running “hot” • KIWA was running a tad “hot” • KEMX (Tucson) was running a little “hot” Bottom line: enough discrepancy in transmitted power existed amongst neighboring radars to lead to noticeable differences in dBZ displays, irrespective of sampling issues!

16. What happened? • ROC determined that, when ORDA build 8 was installed, radar calibration changed (the old calibration information was not retained). • Some radars began to run “hot” or “cold” compared to their pre-ORDA build 8 performance. • ROC decided each WSR-88D radar needed to be checked out by local ETs…and directed field personnel to complete this task.

17. KIWA Velocity Data Quality Problems • For the past couple of years, forecasters and the SOO noted several problems with data quality regarding low-level radial velocity images from KIWA • Spikes or localized areas of moderate to strong outbound/ inbound velocities were often observed, especially over the eastern half of the Greater Phoenix metropolitan area. • Very problematic, especially when mesoscale and microscale outflows existed • Areas of unrepresentative velocity data observed in the vicinity of hills/small mountains • Not totally unexpected, but forecasters thought bypass map would filter out unrepresentative radar return • WFO contacted ROC…

18. ROC analysis of the data revealed… • Commanded elevation angle for every lowest cut is 0.48 deg. • Most data collected at cuts much lower than 0.48 deg • Consequently, automobiles and terrain features have a greater influence on the returned power than what was expected or desired

19. Lowest level velocity data collection cut via VCP 11 for KIWA on 4/16/07

20. Doppler Data Collection: 4/17/2007 • Initially, Doppler cut data collection begins at an elevation of 0.35 deg (255-305 degree azimuth). • Data collected at approximately 0.4 deg in the 300-360 degree range • After that, data were once again collected at a slightly lower elevation. • 88D “hunts” for the assigned elevation slice of 0.48 deg, which explains the upward and downward movements of the antenna

21. What happened? • Lowest elevation slice has been raised about 0.1 degree. Still see occasional problems with velocity data, but the problems are not as widespread nor as frequent.