Monday, November 25, 2019

Bomb Cyclone to Impact West Coast

OMG, my cup runneth over.  So much weather and so little time.  I know you are interested in mountain snow, but it is necessary this morning to look to the west at the forecast of a truly remarkable cyclogenesis event.  In some respects, this is the storm that "stirs the drink" for the weather that follows.

The GFS forecast loop below shows the development of the cyclone off the Pacific Northwest coast.  It forms along the pre-existing warm front associated with a parent cyclone south of the Aleutian Islands as an upper-level trough (indicated by color fill) approaches from the west.  

GFS forecast loop of sea level pressure (black contours), 925 mb temperature (red contours), and dynamic tropopause pressure (color fill)
Deepening rates are quite remarkable and consistent with "bomb" cyclone classification (i.e., > 24 mb in 14 h or 18 mb in 12 h).  Central pressures fall 25 mb in 12 hours, 35 mb in 18 hours, and 40 mb in 24 hours for the period ending at 0000 UTC 27 November, which is just before landfall. 

The GFS analysis wins no beauty contests, but it shows two key features associated with intense marine cyclones.  The first is a warm-core seclusion — a pocket of warm air surrounded by cooler air that is roughly colocated with the low center.  The second is a bent-back front — an extension of the warm or occluded frontal zone into the cooler airsteam behind the low.  

Paraphrasing a wise Norwegian meteorologist, "beware of the poisonous tail of the bent-back front."  The cyclone reaches maximum intensity right at landfall when it features a very intense inner-core pressure gradient.  This is a scenario for producing very strong winds.  Indeed the Eureka National Weather Service forecast office has hoisted just about every non-convective warning you can possibly imagine for their county warning area.  The Oregon coast will also see impacts from this storm, especially the southwest coast should the low take a northerly track.  

So, pardon my digression from the developing snowstorm over northern Utah.  No promises, but perhaps there will be another post later.  

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