1. REGIONAL SCALE ENSEMBLE FORECAST OF THE LAKE EFFECT SNOW EVENT OF 7 FEBRUARY 2007 Justin Arnott and Michael Evans NOAA/NWS Binghamton, NY Richard Grumm NOAA/NWS State College, PA

2. Introduction Today’s computing power allows rapid simulation times for mesoscale models Today’s computing power allows rapid simulation times for mesoscale models –Suggests the possibility of ensembling mesoscale simulations (e.g. Jones et al. 2007, Eckel and Mass 2005) Each NWS office runs an in-house 12 km workstation WRF simulation Each NWS office runs an in-house 12 km workstation WRF simulation –Can a mesoscale ensemble improve short term operational forecasts of high-impact weather?

3. Introduction Eight northeastern US NWS offices have begun an ensemble of 12 km workstation WRF runs Eight northeastern US NWS offices have begun an ensemble of 12 km workstation WRF runs First Goal: Timing/location/movement of lake effect snow bands First Goal: Timing/location/movement of lake effect snow bands –Begins to come within resolvable range of a 12 km simulation –Is not explicitly captured by current ensembles (SREF, MREF, etc.)

4. The Ensemble

5. The Ensemble – 07FEB2007 OfficeCoreBCsMicroCPS #Z Lev OperationalNMMNAMFerrierBMJ60 BGMARWNAMLinKF31 CLEARWGFSLinKF40 CTP-1NMMNAMLinBMJ31 CTP-2ARWNAMLinBMJ31 BTVNMMGFSFerrierBMJ31

6. Case Day: 07FEB2007 Part of a ~10-day prolific lake effect snow event east of Lake Ontario Part of a ~10-day prolific lake effect snow event east of Lake Ontario Band moved significantly throughout the day Band moved significantly throughout the day –Excellent test for the ensemble

7. 07FEB2007 – Synoptic Setup

8. T LAKE : +4C

9. 07FEB2007 – Synoptic Setup

10. Radar Loop

11. Operational NAM Performance

12. Captures basic band evolution Captures basic band evolution –A little slow with initial southward band movement Problems with inland extent of the band Problems with inland extent of the band –Frequently too far inland Can the ensemble outperform this simulation? Can the ensemble outperform this simulation?

13. Ensemble Performance

14. All members able to simulate a band at some point during the day All members able to simulate a band at some point during the day Like NAM, successfully captures general band evolution Like NAM, successfully captures general band evolution Probability plots indicate operational run an outlier with inland extent (06-15 UTC) Probability plots indicate operational run an outlier with inland extent (06-15 UTC) –Provides added value in this case

15. Summary Ensemble provides added value to the operational NAM forecast of band location and movement Ensemble provides added value to the operational NAM forecast of band location and movement –Would have dramatically improved forecast east of Rome, NY on 07 Feb 2007

16. Conclusions Case study suggests ensemble approach to LES may be valuable. Case study suggests ensemble approach to LES may be valuable. –Hone in on high-probability impact areas –Highlight outlier (low-probability) outcomes

17. Future Work Ensemble will be fully operational for the 2007- 2008 season Ensemble will be fully operational for the 2007- 2008 season Complete more LES case studies Complete more LES case studies –Examine impact of ensemble on forecast operations Expand to other cool season/warm season forecast challenges Expand to other cool season/warm season forecast challenges –Flash flooding, convection, mesoscale banding, etc.

18. Acknowledgements Ensemble Participants for agreeing on a common domain/sharing data Ensemble Participants for agreeing on a common domain/sharing data Ron Murphy, ITO BGM for gathering case study data Ron Murphy, ITO BGM for gathering case study dataReferences Eckel, A. F., and C. F. Mass, 2005: Aspects of effective mesoscale, short-range ensemble forecasting. Wea. Forecasting., 20, 328-350. Eckel, A. F., and C. F. Mass, 2005: Aspects of effective mesoscale, short-range ensemble forecasting. Wea. Forecasting., 20, 328-350. Jones, M. S., B. A. Colle, and J. S. Tongue, 2007: Evaluation of a mesoscale short-range ensemble forecast system over the northeast United States. Wea. Forecasting., 22, 36-55. Jones, M. S., B. A. Colle, and J. S. Tongue, 2007: Evaluation of a mesoscale short-range ensemble forecast system over the northeast United States. Wea. Forecasting., 22, 36-55.