Wednesday, March 4, 2015

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The Unwelcomed Guest Returns

After a welcomed moderate snowfall this past weekend, our wintertime foe, aka Omegasaurus Rexbloxus, the Ridge of Steel, the Impenetrable Wall, or the Unwelcomed Guest, returns for another extended visit for the unforeseeable future. Figure 1 illustrates the flow blocking demon reestablishing a presence.
Figure 1. 300 mb wind/height illustrating the intensifying ridge off the California coast. Courtesy of Golden Gate Weather Services.
The day/night trend ahead, at least for the next week, will be a familiar one with the following expected:
  • Rebounding daytime temperatures with mainly sunny skies
  • Light north winds becoming easterly with time 
  • Cold mornings in the valleys with moderate inversions and slow mix out
  • Pleasant and milder at higher elevations with south-facing snow softening by mid-late morning
Much attention centers on the root cause of the persistent ridging we've been experiencing throughout the winter. In fact, the last three winters have been pretty exhausting for Sierra snow enthusiasts. An overwhelmingly positive Pacific North American (PNA) pattern with a dominant ridge off the west coast of California and a trough over the eastern two thirds remains a consistent theme. PNA behavior has been linked to all the prominent intraseasonal (within season) modes of variability such as the Arctic Oscillation, North American Oscillation, and not excluding the more slowly changing Madden Julian Oscillation. Interseasonal (between season) variability such as El Nino/La Nina modulates the behavior of the former intraseasonal indices as well. The full extent of the interaction amongst these modes remains unclear despite all the teleconnection studies that have been published. The near-neutral (macroscopic) signal for El Nino/La Nina over the last three winters is the only persistent observation to be gleaned. It sure seems like there's a larger component of this puzzle at work. The elusive nature cries out to lesser understood phenomenon such as a natural external forcing mechanism (e.g., the variation in solar activity), or an internal forcing mechanism as part of the oceans?

For now, put on the sunblock and shades and enjoy our "not too shabby" snowpack while we have it!

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