1. March 17, 2011 Severe Weather Workshop Mike York (Forecaster / Winter Weather Program Leader)

2. How did we do?  Preliminary Verification Statistics: Very good, but is that the whole story? Issued 70 county Winter Storm Warnings False Alarm Ratio: 26 percent Probability of Detection: 80 percent Average Lead Time: 5.1 hours

3. Stats compared to average of past several seasons:  Average number of warnings: 70 vs. 177  Average false alarm ratio:.26 vs..33  Average prob. of detection:.80 vs..88  Average lead time: 5.1 hrs vs. 21 hrs

4. What is lead time?  The time between warning issuance time and the time 4” is on the ground  Lead times are not computed for watches.

5. Why the short lead times?  Snow amounts were under forecast until the storm was in progress.  Why?

6. After the 4 th under forecast snow event, the boss was not happy.  Science team tasked with investigating why  Preliminary results still not complete  What we do know… will follow shortly

7. Four events under review:  Dec. 24 (Christmas Eve – Paducah area)  Jan. 25 (Pennyrile region)  Feb. 7 (Western Kentucky)  Feb. 9 (Tennessee border)

8. Dec. 24…  Heavy snowfall rates for a few hours after dark  Around 1” per hour  Total was around 4” in Paducah and nearby areas

9. Dec. 24 Preliminary Findings  “Split flow” pattern: Moist southern branch of the jet played a greater role than expected  Band of moisture/heavy snow streamed northeast faster than expected  Warm pavement temps were a non-factor due to heavy snowfall rates

10. Jan. 25-26 (late at night)

11. Jan. 25-26 Prelim. Findings  48-72 hours in advance: All models showed system bypassing region to the south  Models then trended slowly north  Within 12 hours, NAM and RUC caught onto a deformation zone but missed the location

12. Feb. 7… struck in morning

13. Feb. 7 preliminary findings  Deformation zone played a key role in heavy snow  Models began picking up on this zone about 12 hours prior  Warm pavement temps again a non-factor

14. Feb. 9: During the day

15. Feb. 9 preliminary findings  30-48 hours prior, forecasters suspected models were too weak based on 2/7 system  Liquid to snow ratios were a concern (dry and powdery vs. wet and heavy)  Banding was not anticipated

16. Common thread:  Mesoscale bands of heavy snowfall  Bands from 4 to 40 miles wide  Sometimes accompanied by thunder

17. Mesoscale Bands:  Difficult to forecast because of their size  Computer models cannot explicitly forecast these bands  Conditions favorable for banded snowfall can be forecast  BUT not precisely!

18. Forecaster options:  At longer time ranges, use the caveat “locally higher amounts possible”  At shorter time ranges, satellite imagery is an excellent tool for first identification Feb. 5, 2004 Near Paducah, KY NWS Photo – Mike York

19. Feb. 7 - Satellite Precip Estimate:

20. Common threads of these events:  Computer models under forecast precipitation amounts  Unforecast “deformation zones” caused intense snowfall rates  Warm pavement temperatures

21. What next?:  Science team is looking at snow to liquid ratios (dry snow vs. heavy wet snow)  Science team is looking at what role banding played and how to anticipate it

22. Summary:  We are still researching “what went wrong”  More than one factor played a role  Computer model limitations were one factor  Forecaster ability to troubleshoot the models may be a factor  Forecasting snow to liquid ratios may be a factor