Aspects of 6 June, 2007: a null "moderate risk" of severe weather
Jonathan Thomas Kurtz, University of Nebraska, Lincoln, NE
On 6 June 2007, a moderate risk for severe weather was predicted for the north central Great Plains by the NWS Storm Prediction Center (SPC) and mentioned in forecasts from the Omaha/Valley, NE (OAX) NWS forecast office. In review of the resulting weather of 6 June, the event was considered a null moderate risk event in accordance with the definition of a moderate risk by the SPC. The outcome of this event has raised questions in how numerical weather prediction (NWP) guidance performed in predicting the event and, more importantly, how the NWP guidance predicted precipitation for the event. This project investigates how the different numerical models predicted the pattern and amount of precipitation during the event over the moderate risk area. The project particularly looked at archived BUFKIT soundings, NWP guidance predicted shear values, moisture flux divergence, divergent Q vectors and NWP guidance moisture predictions. Also studied was WRF model runs of base reflectivity compared to observed reflectivity from the event.
This project will also discuss to what extent the event was null, given that some pockets of severe weather did occur in central South Dakota. Another unique component of this event was the anomalously strong gradient winds that occurred throughout the plains during this period. The winds exceeded high wind warning criteria, occurred in the month of June, and were out of the south-southwest; all of which are climatologically quite rare. The project also reviewed the forecasts produced from both the SPC and the OAX NWS office beginning with the first mentions of severe weather potential for 6 June 2007 though the actual null event. Since this case was a null event, special attention will be given to aspects of the NWP guidance that may have negatively influenced human forecasters.
After researching this event, it is evident that early model predictions of high dew point temperatures resulted in high predictions of CAPE and CIN, misleading the forecasters. In addition, any vertical forcing over the high plains was hampered by the nose of the jet stream causing subsidence over much of central Nebraska. Shear values were acceptable for well structured storms, however, due to the lack of strong vertical forcing the cap could not be overcome. In the end all the forecasts both computer and human produced pointed to a potential outbreak of severe weather across much of the northern Great Plains, but small details unforeseen resulted in a null event given some parameters were still sufficient for the development of severe weather.
Poster Session 1, Student Conference General Poster Session
Sunday, 20 January 2008, 5:30 PM-7:00 PM, Exhibit Hall B
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