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.