Wednesday, January 2, 2013

Anatomy of Split Flow

Over the next several days, the jet stream will be taking a vacation from the Intermountain West, leaving us high and dry and, for those who live in the Salt Lake Valley, mired in pollution.

During this period, pronounced split flow will develop over the western United States.  Split flow features two jet streams, one to the north called the polar jet and one to the south called the subtropical jet, as can be seen in the GFS forecast for 1200 UTC (0500 MST) 8 January.  

0600 UTC 2 Jan 2012 GFS 250-mb (jet stream) wind vector, wind speed (contours),
and 6-h accumulated precipitation forecast valid 1200 UTC 8 Jan 2012
The split flow reflects a large-scale pattern known as a Rex Block, which features an upper-level ridge "over" an upper-level trough.  Note in the image above how Utah is sandwiched between an upper-level ridge to our north and an upper-level trough to our south.  If this forecast verifies, it will be a quick flight for those traveling from Denver to Salt Lake on the Jan 8th as the jet-stream level flow is completely reversed and easterly over western Colorado and Utah.  


  1. I am the guy who posted the Vermont forecaster's Rex Block prediction a couple of weeks ago (noting my concerns because he compared it to January 2007, which I remember to be a dry month in Utah). What is the typical duration of such an event? Is it something that would typically persist for a month? Are they usually stationary, or can it retrograde to the west, allowing one of the jets to again take aim at Utah?

  2. Events sometimes last for a couple of days, but can last for longer periods. There is no standard event duration. Most of the GEFS ensembles call for the large-scale pattern to evolving into something with a ridge over the eastern Pacific and a deep trough over the continental US around the 10th. What happens for Utah depends on the gory details. We'll see what happens.