QUASI-GEOSTROPHIC DIAGNOSIS OF CYCLOGENESIS ASSOCIATED WITH A CUTOFF EXTRATROPICAL CYCLONE - THE CHRISTMAS 1987 STORM

A major snowstorm in Colorado is considered in order to demonstrate th e utility of a quasigeostrophic (QG) diagnostic scheme that is capable of separating from the total QG forcing field the cross-isentrope, ag eostrophic circulations associated with jet-streak dynamics. The storm did not develop as a consequence of typical baroclinic wave developme nt but instead developed in association with a previously cutoff cyclo ne. It posed a perplexing forecast problem to Denver area forecasters. It is discovered that at least 12 h before the onset of cyclogenesis there existed QG signatures (computed from rawinsonde data) of the the rmally direct-indirect circulations associated with a jet-level wind m aximum. These circulations are known to be associated with tropopause folding and the descent of stratospheric potential vorticity into the midtroposphere. It is verified that indeed such a process took place b y tracking maxima of potential vorticity on an isentropic surface (295 K) that extended into the midtroposphere. Using analyses of lapse rat e and mixing ratio near a ''dry slot'' in satellite water vapor imager y, our interpretation of the QG signatures are confirmed. The diagnost ic scheme can be of value to forecasters who daily must adapt their kn owledge of conceptual cyclone models to ascertain the dynamic potentia l of threatening storms.