The forecastability of a cold-air outbreak over eastern North America durin
g January 1985 has been studied with ensemble forecasts from the NCAR Commu
nity Climate Model version 2 run at T42 horizontal resolution. The cold-air
outbreak case was characterized by a pool of very cold air (T < -35 degree
s C at 850 mb) that moved southward into the central United States and inte
nsified. The ensemble's 10 member forecasts were initialized at 0000 UTC 15
January 85, a few days before the cold-air pool began its southward moveme
nt and reached its peak intensity. The ensemble members predicted the south
ward passage of the cold air but faster and weaker than analyzed. The predi
cted weakening of the cold-air pool was consistent with the model's systema
tic error Quasi-Lagrangian diagnosis of the 850-mb temperature tendency bud
get revealed that the analyzed intensification of the cold-air pool was due
to residual rather than adiabatic effects. These residual effect, could ha
ve been diabatic in origin but also attributable to observational errors. S
imilar diagnoses applied to selected ensemble members indicated that diabat
ic cooling, specifically longwave radiative cooling, contributed to the for
ecast cooling of the cold-air pool by one ensemble member but was overwhelm
ed by adiabatic warming in a weakening cold-air pool predicted by another e
nsemble member. These results suggest that the forecast details of a cold-a
ir outbreak may depend upon the subtle balance between diabatic and adiabat
ic processes. Furthermore, forecasts constructed from ensemble predictions
must account for model biases as well as information from the ensembles.