Outlier Mode: Here We Go!

A remarkably high amplitude upper-level pattern is setting up over the western United States, one that will bring record or near record temperatures aloft and then the possibility of an almost monsoon-like surge of tropical moisture into the southwest and eventually Utah.  Yup, we are heading into outlier mode.  

The loop below shows the latest forecast from the GFS and some of the key large-scale circulation features at play.  Over the next couple of days, a high amplitude upper-level ridge develops along the west coast and progresses eastward.  Nearly concurrently, a closed low forms off the coast of the Gulf of California.  This is a result of what meteorologists call anticyclonic wave breaking and it results in a high-over-low block (a.k.a., Rex block) over the western United States.  Subsequently, the closed low draws tropical moisture northward over the Gulf of California and into the southwest U.S. in a manner that is reminiscent of monsoon surges during the warm season.

0600 UTC 22 Jan 2015 GFS forecast of 500-mb geopotential height (black contours), precipitable water (color contours), outgoing long-wave radiation (i.e., clouds, black-and-white fill), and precipitation (color fill).

The high-over-low pattern is most pronounced at 0000 UTC 26 Jan (late Sunday afternoon MST) with the ridge centered over Nevada and the trough centered west of Cabo.

We're very fortunate that we are starting this period with relatively little pollution and no snow over the valleys of northern Utah as temperatures aloft may be reaching record high values as the ridge moves overhead.  Forecast 700-mb (10,000 ft) temperatures are around 3ºC Sunday afternoon and peak at 6ºC on Monday afternoon.  If 6ºC verifies, and a sounding is taken at the time of the peak, it would be the highest 700-mb temperature in the Salt Lake City sounding record from late-Jan to early March.

Source: SPC

Although we have high confidence of the building ridge and warm temperatures aloft, the forecast after Monday gets a bit dicier.  Let's talk first about the GFS forecast, which by Tuesday has the ridge axis to our east and tropical moisture streaming northward into Utah.  The pattern reminds me a great deal of the monsoon surges we get during the warm season, although the easterly flow in this instance is displaced quite a bit further south of its typical location during the warm season.  

 Nevertheless, if this forecast verifies, the surge of moisture is going to bring some rain into the southwest.  Although the southwest sorely needs precipitation, this is going to be a very warm storm and snow levels will probably be as high as 9000 or 10000 feet over Arizona, so additions to their mountain snowpack will be limited.  The GFS then spreads moisture and showers into northern Utah late Tuesday and Wednesday.

The ECMWF model, however, has different ideas and has the moisture surge further west, so we'll have to see if and how forecasts come together the next few days.  Personally, I'm hoping for a GFS-like solution.  This winter has been so bizarre, we might as well continue the streak of weirdness.