NCEP SREF for the 2010 Christmas East Coast snowstorm

Jun Du

Mesoscale Modeling Branch/EMC/NCEP

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

and predictability issue)

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)

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)

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

Bad Eta_KF member Good Eta_KF member

Bad RSM member Good RSM member

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

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

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

issue)

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

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

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”

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 (<1”), 50% chance of median snow (1-5”) and 30% chance of heavy snow (>10”)”. We need to change current forecast format