Wednesday, December 7, 2011

A little cut-off low in the central Pacific

I've noticed over the past few days that there's been a curious dry spot in the central Pacific water vapor imagery.  It starts at around 8Z on Monday.  You can see the development of a structure sometimes loosely referred to as a "baroclinic leaf" in the area I've circled.  This often marks the initial formation of a surface low pressure center.  Notice the strong gradient between drier air (the yellow and brown colors) and more moist air (the whites, blues and greens).  Drier air tends to be sinking while the moist air tends to be rising.
By late Monday evening, however, the pocket of dry air became nearly circular and somewhat surrounded by the stream of moist air that had originally been flowing to its east.
And it has remained there for the past 48 hours or so, continuing on into this morning.

A loop of the past 72 hours of water vapor imagery (which includes a jump when NOAA changed the satellite that gives us these images from GOES-11 to GOES-15) can be found at:

In that loop, you can see the full evolution of this dry spot.

A glance at the surface map (from our WRF-GFS model analysis this morning) shows no apparent surface low, though there is a trough of lower pressure at the surface in the vicinity.  In the map below, the dry spot is located in the area between the two high pressure centers--one off the coast of the Pacific Northwest and the other in the central Pacific.

So this is not a feature that means much for the surface.  However, looking at the 500mb chart we do indeed see a little cut-off low analyzed in the location of that dry spot.  Overlaid on the image in color is the relative humidity at 500mb.  Red indicates high relative humidity (very moist air) whereas blue indicates low relative humidity (very dry air).  I personally would have reversed the color scale, but it is what it is.  Anyhow, you can see that in the center of the 500mb low, it is indeed very dry.  There is also an area of more moisture shown just to the east and south of the low center, and on this morning's water vapor imagery we also see the same thing.
While this whole feature may not really mean much in the way of our day-to-day weather, I find if kind of fascinating how we can get a little cut-off low like this that just kind of gets "forgotten" by the main flow for several days.  Originally it looked like this would spin up a rather decent cyclone.  However, the jet stream moved north and left this little pocket of low heights spinning around for the past few days.  It looks like the strong front to the north (seen by the band of higher water vapor to the north and the strong temperature gradient on the surface map) is moving south and east, and our little low will probably become absorbed in that over the next few days.

1 comment:

  1. The way it fights against the surrounding flow almost looks beyond natural. Thanks for pointing this feature out.