Saturday, January 12, 2013

A Real OMG Omega Block over the E. Pacific

When the winds aloft form into the shape of the Greek letter Omega (Ω), they tend to resist changes both aloft and on the surface. This is the type of pattern mariners like to see as we then have more confidence that things tomorrow will be much like they are today. This tends to me more useful in the summer than winter, because we are not likely to be in the North Pacific in the winter... unless of course we have a job that forces us to be there. And in the winter there is much more activity to disrupt the pattern.

Here is what we see today, which is a rather remarkable example of such a pattern. These are the winds at the 500 mb level, at about 18,000 ft.

Here is the huge blocking High below it on the surface."Blocking" in the sense we use in our classes, meaning [1] more or less in the right place (ie due west of San Francisco, on the rhumbline from WA to HI), [2] central pressure higher than 1030 mb, and [3] with at least 2 nearly round isobars encircling it.

Here is the surface 48 hr later, and we still see more or less similar High pressure, but this is the winter, so look at the real butt kicking Low coming at this pattern. It cannot withstand that sort of attack too long.

 Even with this, 96 hr later we still have high pressure dominating the Eastern Pacific


And 96 hr later the omega block starts to give in to the attacks from the west. At this point, we no longer have any idea what the surface will look like the next day.... until the NWS tells us, and even in this case we will have to use all forecasts with caution because it seems a cut off low is in the process of forming which usually means the surface forecasts are more in question. 


Thanks to Starpath instructor Larry Brandt for alerting us to this notable example taking place.

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