1. NCEP SREF for the 2010 Christmas East Coast snowstorm Jun Du Mesoscale Modeling Branch/EMC/NCEP

2. (I) Initially under forecast for the east coast snowstorm in general (a mixture of model physics, IC quality and predictability issue)

3. Problem: cyclone track forecasts were too to east of the observed track prior to Dec 25: e.g., 03z, Dec. 24, 2010 SREF mean forecast (84hr, Dan Petersen)

4. Sensitivity of cyclonegenesis to initial condition e.g., 03z, Dec. 24, 2010 SREF individual members (one Eta_KF and one RSM members are very close to the observed)

5. Sensitivity of cyclongenesis to initial condition, e.g., 03z, Dec. 24, 2010 SREF “bad” and “good” members (84hr) Bad Eta_KF memberGood Eta_KF member Bad RSM memberGood RSM member

6. Consequently, the problem for precipitation is that the major precipitation is too far from the coast prior to Dec. 25, 2010: e.g. 03z and 09z, Dec 24, 2010 SREF >1” probability (Rich Grumm) 03z, 84hr 09z, 78hr

7. Sensitivity of precipitation to initial condition: 03z, Dec. 24, 2010 SREF “bad” and “good” members Bad Eta_KF member Good Eta_KF member Bad RSM member Good RSM member

8. (II) Over-forecast for Washington DC area (mainly predictability issue)

9. SREF mean pcp (Geoff Manikin) 03z 12/25 21z 12/2515z 12/25 09z 12/25 All means have > 0.50” in DC; LWX issued WS warning, partially based on this

10. SREF Prob 24hr Tot Pcp > 0.50”, valid 15z 12/27 (mean dashed) (Geoff Manikin) 25/03z 26/09z26/03z25/21z 25/15z25/09z Sharp gradient between 30 and 70% probability between Dulles and Chesapeake Bay

11. Sensitivity of precipitation to IC: e.g. 09z, Dec 25, 2010 SREF – a range of solutions from “good” to “bad” members 0.01-0.1” 0.1-0.25” 0.5-0.75” 1.0-1.5”

12. Summary Predictability is definitely an issue for this storm, which is the reason why the EPS is so important in operation Although physics deficiency and IC quality must play a role in the systematically biased forecasts prior to Dec. 25, 2010, the truth seems still lies within the SREF envelope (with low probability though) After many years of “ensemble” era, people are, unfortunately, still looking for “yes or no” deterministic answers. It’s time to issue probabilistic forecasts such as this case for Washington DC: e.g., we should issue a forecast like this “20% chance of light snow ( 10”)”. We need to change current forecast format