Going Negative - Part III

More than the other two teleconnection indices...the Pacific-North American index is a reflection of the LW flow regime/s amplitude over North America. When PNA is negative...there/s troughing in the west and ridging in the E. When PNA is positive...the opposite is true -- there/s ridging in the west and troughing in the E. +PNA...especially if it/s increasing with time...often contributes favorable conditions for the process Sutcliff termed 'self-development' which can produce snowstorms in the E.

The 'self-development' process occurs when the Rossby (AKA 'long wave,' planetary wave') wavelength shortens from warm air advection (WAA) pulling the downstream ridge back to the W and cold air advection (CAA) pushing the upstream trof to the E. This causes the LW to stretch in the N/S direction which increases its amplitude, divergence aloft, and curvature. The increase in curvature produces greater vorticity and the advection of the vorticity by the geostrophic wind (PVA) generates upward vertical motions above the surface cyclone...which in turn generates more WAA / CAA and the cycle repeats itself. But I digress.

In the first two installments of 'Going Negative,' the NAO and AO teleconnection indices for 30 NEWxSFC snowstorms were analyzed to see if the popular idea that 'winter just ain/t happenin' and it ain/t gonna happen until they go negative' was a reasonable conclusion. It turns out...it just isn/t so. If +PNA if conducive to the 'self-development' process of cyclogenesis...what kind of snowfalls can be expected when the PNA goes negative?

Pacific-North American
The PNA was positive for 18 (60%) of the 30 Contest storms and less than or equal to zero for 12 (40%).

Seven storms (39%) had a max station snowfall amount of at least 12" when the PNA was greater than zero. Of those storms...four (57%) had a max station snowfall greater than 20".

Four storms (33%) had a max station snowfall amount of at least 12" when the PNA was less than or equal to zero. Of those storms...two (50%) had a max station snowfall amount of at least 20".

Contest storms were 50% more likely when the PNA was greater than zero; however...when a Contest storm occurred...the storms during a -PNA regime were almost as likely as the +PNA regime storms to produce a max station snowfall about of at least 12" and 20".

In tomorrow/s final installment...we/ll try to answer the musical question: what/s it all mean?