1. SPC Input – EMC GFS Satellite Data Denial Experiment 14-16 April 2011 Tornado Outbreak Examination of Day 7 and Day 4 Guidance for SPC Severe Weather Outlooks

2. SPC Assessment of GFS Control and GFS Data Denial Runs Thanks to Fanglin Yang for providing comparative output fields from the two model runs 162h (Day7) and 90h (Day4) forecasts valid at 18z on each of the three days were examined, corresponding to the time of strongest convective initiation. The following parameters were compared to assess basic severe storm potential for each GFS model run: – 500 mb height (synoptic pattern; short-wave troughs) – 500 mb wind (mid-level jet related to deep layer vertical shear) – 850 mb wind (low-Level jet related to heat/moisture transport and low-level shear) – PMSL/precipitation (synoptic pattern; fronts; potential convection) – MUCAPE (instability) – 0-3 km Storm-Relative Helicity (environment supportive of updraft rotation and severe storm potential)

3. 14 April 2011 Severe Weather Case 4 Tornado Fatalities in Oklahoma (2) and Arkansas (2) Wind Gusts to 85 mph and Hail to Baseball & Softball Size

4. 500 mb Height Forecasts Valid 18z 14 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Deeper trough in prposdd2 run at D7 would have suggested stronger system and enhanced severe potential. Both runs provided similar pattern solutions by D4 as prposdd2 run trended toward the less amplified control solution.

5. 500 mb Wind Forecasts Valid 18z 14 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast D7 deeper trough in prposdd2 run was associated with a much stronger mid-level jet, although both forecasts indicated sufficient vertical shear for a severe threat. By D4, both solutions were similar as prposdd2 trended toward the control solution.

6. 850 mb Wind Forecasts Valid 18z 14 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Although prposdd2 run predicted stronger 850 mb winds at D7, both runs indicated a pronounced (unfavorable) westerly component. By D4, both runs predicted more favorable southerly winds in OK/AR.

7. PMSL/Precipitation Forecasts Valid 18z 14 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 prposdd2 run indicated a secondary surface low over western AR and more favorable pattern for severe storms in AR. By D4 patterns were more similar.

8. MUCAPE Forecasts Valid 18z 14 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 the MUCAPE gradient in the proposdd2 run was slightly farther northwest compared to the control run. By D4 the control run indicated instability farther north into southeast KS, which would have more strongly suggested a potential severe threat into KS.

9. 0-3 km Helicity Forecasts Valid 18z 14 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 and D4, the prposdd2 runs both predicted larger values/areas of SRH over KS/OK suggesting greater potential for severe storms given thunderstorm development, compared to the control runs.

10. 15 April 2011 Severe Weather Case 8 Tornado Fatalities in Alabama (7) and Mississippi (1) Wind Gusts to 85 mph and Hail to Baseball & Softball Size

11. 500 mb Height Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Similar to previous day, the deeper trough in prposdd2 run at D7 would have suggested a stronger system and enhanced severe potential. Again, however, both runs provided similar pattern solutions by D4 as theprposdd2 run trended toward the less amplified control solution.

12. 500 mb Wind Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Similar to the previous day, the D7 deeper trough in the prposdd2 run was associated with a stronger mid-level jet, although both forecasts indicated sufficient vertical shear for a severe threat. By D4, both solutions were similar as the prposdd2 trended toward the control solution.

13. 850 mb Wind Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 stronger, more southerly 850 mb winds in the proposdd2 run indicated a more pronounced Gulf moisture return ahead of the system. By D4, however, the prposdd2 run trended toward the control solution with both forecasts being very similar.

14. PMSL/Precipitation Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Consistent with the deeper 500 mb trough at D7, the prposdd2 run developed a deeper surface low farther to the south with attendant higher precipitation. This would have suggested greater potential for a severe risk compared to control solution. By D4 both surface solutions were very similar with occluded low in IA.

15. MUCAPE Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 the instability axis in the control run extends much farther to the north, consistent with the more northward position of surface low. Overall, the control run produced a larger areal extent of instability.

16. 0-3 km Helicity Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7, the prposdd2 run predicted larger values of SRH over a broader area of the TN Valley compared to the control run, consistent with its stronger low level wind fields. By D4 both solutions were similar.

17. 16 April 2011 Severe Weather Case 29 Tornado Fatalities in North Carolina (27) and Virginia (2) Wind Gusts to 79 mph and Hail to Baseball & Softball Size

18. 500 mb Height Forecasts Valid 18z 16 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 the control run predicted an upper low that was deeper and farther south compared to prposdd2 run, which would have suggested a greater severe potential, although both indicated a strong trough moving into the east. By D4 both runs produced a similar solution that closer to the D7 prposdd2 forecast.

19. 500 mb Wind Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Despite the control run forecasting a deeper trough over the east on D7, consistent height gradients resulted in similar 500 mb jet speeds. By D4 both runs strengthened the mid-level flow east of the Appalachians with strong deep layer shear over the area.

20. 850 mb Wind Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 the control run predicted a stronger low-level jet near the Atlantic coast. By D4 the solutions appeared similar with strong southwesterly flow from GA into NC and VA; both were favorable for severe weather.

21. PMSL/Precipitation Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast Although surface pressures were forecast by the control run to be lower over the outbreak region on D7, the apparent front were indicated to be farther inland over the Piedmont in the prposdd2 solution. By D4 both solutions were very similar.

22. MUCAPE Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast At D7 the faster eastward frontal movement in the control run resulted in the instability axis being offshore except across eastern/northern VA. Conversely, the prposdd2 run placed the instability axis into central NC. By D4 both forecasts were similar.

23. 0-3 km Helicity Forecasts Valid 18z 15 April 2011 Day 7: 162h ForecastDay 4: 90h Forecast On D7 the prposdd2 run indicated considerably larger SRH over the outbreak area compared to the control run. This again reflects the faster eastward movement of the cold front in the control run. By D4 both forecasts were similar with the SRH axis, although the prposdd2 appeared to extend larger values a bit farther southward across eastern NC.

24. Conclusions for April 14-16, 2011 Case For this case, on D7 the prposdd2 run showed a tendency to predict (overpredict?) stronger synoptic scale systems compared to the control run. – These stronger systems provided a more pronounced signal for possible severe weather a week in advance. However, by D4 the two runs tended to provide similar severe weather guidance, with the prposd22 runs trending toward the control solutions. Using basic severe weather forecasting pattern and parameter guidance, the results for this case are inconclusive regarding the data denial impact on GFS guidance for severe weather 4- 7 days in advance.