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).|
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.
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.