The models underestimate precipitation amounts when a subtropical moisture plume is feeding into a weather system. Underforecasts monsoonal precipitation across the west during the warm season. This can be especially true with tropical storms. Superimposed on the errors in the longwave pattern can be additional significant errors in identifying and timing shortwaves. These type errors are at their maximum in faster than normal zonal flow with short spacing between systems.
Transitions can be troublesome for all models.
Not only are seasonal transitions difficult, but a large scale transition from high to low longwave amplitude, or vice versa, is a major source of errors in all models. Another potential source of large error comes with extremely high amplitude ridges in the Gulf of Alaska and the forecast supergradient flow crossing to the right of the height contours on the east side of the upper ridge. Not digging upper energy far enough S, they are prone to miss closing off systems. The MRF ensemble members can often be helpful in determining when any model, particularly the operational MRF, is an outlier and straying too far from reasonable solutions offered by consensus of all the available models. A West Pacific typhoon, in the process of becoming extratropical as it enters the westerlies, can significantly alter run-to run continuity in all the models.