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 the utility of a quasigeostrophic (QG) diagnostic scheme that is capable of separating from the total QG forcing field the cross-isentrope, ageostrophic circulations associated with jet-streak dynamics. The storm did not develop as a consequence of typical baroclinic wave development but instead developed in association with a previously cutoff cyclone. 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 thermally direct-indirect circulations associated with a jet-level wind maximum. 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 by tracking maxima of potential vorticity on an isentropic surface (295 K) that extended into the midtroposphere. Using analyses of lapse rate and mixing ratio near a ''dry slot'' in satellite water vapor imagery, our interpretation of the QG signatures are confirmed. The diagnostic scheme can be of value to forecasters who daily must adapt their knowledge of conceptual cyclone models to ascertain the dynamic potential of threatening storms.