Monday, September 17, 2012

A Western Blocking Pattern

I can sum up the forecast for the next 10+ days in one word.  Dry.  An upper-level ridge is going to be parked over the western United States, with our only hope of any precipitation being the possibility of a closed low swinging underneath it and tapping into a last gasp of monsoon moisture.

We had a weak upper-level trough brush by last night.  That trough is presently moving across the upper midwest and eastern Rockies, with nothing upstream except a high amplitude ridge.  

IR satellite imagery and 500-mb geopotential height analysis for
1200 UTC 17 September 2012.
The medium range ensembles show nothing but ridging from now until the end of time (that's an exaggeration as they only go out 384 hours in the future, but that is the end of time as far as they are concerned).  Here are a few snippets at forecast hours 120 (valid 0000 UTC Sep 22), 240 (valid 0000 UTC 27 Sep), and 384 (valid 0000 UTC 3 Oct).

Source: Penn State E-Wall
All of the ensemble members keep the jet well to our north throughout the 384 hour forecast period.  A few bring the occasional rogue closed low into the area. 

Forecast skill degrades with time, so one needs to be cautious about surprises as the forecast lead time increases, but it appears we are in for an extended spell of clear fall weather.  Meteorologists call this a blocking pattern because the ridge acts to block or redirect troughs and cyclones away from our area, leaving us relatively high and dry.   


  1. I think it is really interesting that the stronger and more persistent high-pressure ridges frequently contain a well-developed closed low in their core. It seems like the two are mutually reinforcing, in other words, the low could provide a counter-balance that helps the surrounding ridge to maintain strong subsidence. Perhaps similar to the way a hurricane develops sinking air in the eye that helps to compensate for the rising air around it? I have noticed that when the closed low within a ridge begins to dissipate, the ridge itself is usually in a weakening phase. What is your take on this?

    1. David:

      Techy response below. I'm on the road, so apologies.

      The high-over-low block is sometimes referred to as a "Rex Block" following Rex (1950). It is a fairly common pattern during a block. Nearly all blocks are associated with the development of a high-amplitude upper-level ridge. The Rossby-wave breaking that occurs during the formation of some of these ridges often results in the development of a cyclonic PV streamer downstream and equatorward of the ridge, which can lad to the development of a closed low to the south of the ridge. I've never really thought of how the development of the trough might contribute to blocking persistence, but it an interesting idea.

      In this case, however, there is a preexisting subtropical cyclonic PV anomaly that contributes to the Rex Block. Whether or not that's important or pure coincidence is worthy of further investigation. Is the trough along for the ride or an important contributor?

      See Pelly and Hoskins (2003, and references therein and citations of for more information.


    2. Jim, thanks for the link. It seems like the Rex Block is a sort of meteorological dipole, or semi-closed system that is less affected by its surroundings than a ridge on its own. So I tend to think that a closed low within the ridge makes it more difficult to dislodge. Also, that kind of configuration seems to frequently split an incoming trough upstream, which can in turn form a new closed low that may reinforce the blocking pattern. At any rate, hopefully we will see less of it this year.