NWC REU 2010
May 25 - July 30



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Updraft Helicity as a Forecast Parameter

Stacey Hitchcock, Patrick Marsh, Harold Brooks, and Charles Doswell III


What is already known:

  • Updraft-helicity (UH) in a new-generation of numerical weather predicition (NWP) model guidance offered to severe weather forecasters.
  • Most NWP models calculate UH over 2-5km layer AGL.
  • No studies have focused on examining UH over different layers.
  • Hourly Maximum (MAXUH) is preferred to UH calculated only at the top of the hour.

What this study adds:

  • First study to compare UH calculated over different layers.
  • Qualitatively demonstrates that the improvement in forecast performance from smallest top of the hour UH integration layer is smaller than the improvement from the deepest top of the hour integration layer to maximum over the hour integration.
  • Lays the ground work for quantitative comparison between MAXUH and top of the hour UH.


Improved science and technology has created the opportunity to explore the impacts of different model diagnostic fields as indicators of convection developed in highresolution numerical models. Indication of the success of different diagnostic fields has been discussed (Kain et al. 2008, Sobash et al. 2008). Updraft helicity (UH) has shown a particular ability to identify supercell-like structure in convection allowing model observed locations. UH will be examined to determine the best integration layer over which to calculate UH.


Output of updraft helicity over different layers from the convection allowing 4-km National Severe Storms Laboratory- Weather Research and Forecasting Radar (NSSLWRF) Advanced Research WRF (ARW) from the Spring Experiment 2008 was compared to Storm Prediction Center (SPC) storm reports using contingency tables. Verification measures (Probability of Detection, False Alarm Ratio, Critical Success Index, bias) were calculated from the contingency tables and used to create several visual comparisons. These include Relative Operating Characteristic curves (ROC) (Mason 1982), and Performance Diagrams (Roebber 2008), as a comparison of different depth’s success as a forecast parameter.

Full Paper [PDF]