Thursday, May 19, 2011

Perspectives on the Rain

This has been and continues to be a fascinating storm for northern Utah.  I'm about as close to developing seasonal affective disorder as I've been since I left Seattle 16 years ago.

At KSLC, it seems like it has been raining continuously since 1400 UTC (0800 am) yesterday morning, although there were some breaks overnight from 0150-0426 UTC and 0519-0805 UTC.  

An interesting aspect of this event is that it has rained everywhere.  As of 7 am this morning:
  • .87" in Logan
  • 1.7", 1.59", 1.3" in Centerville, Bountiful, and Ogden, respectively
  • 2.21", 1.6", 1.35", 1.0", and .82" in Cottonwood Heights, Taylorsville, Sandy, KSLC airport, and the Great Salt Lake Marina, respectively
  • .32-2.36" in the West Desert, including 1.51" at Aragonite
  • 1.21", .91" and .73" on the Wasatch Back at Weber River near Morgan, PC Golf Course, and Kimball Junction, respectively
  • 2.8" and 2.3" at the Harbscrabble and Farmington SNOTEL stations
  • 2.62" and 1.97" at Brighton Crest and Alta UDOT
This coverage is quite remarkable, exhibits fairly limited orographic enhancement, and is occurring in large-scale easterly flow at upper-levels.  

The precipitation dynamics of this event are quite complex, but I'll try to summarize here about some of the key aspects.  First, we have a situation where we have a trough camped out pretty much right over us.  Further, even though the upper level flow has been out of the east, northeast, and north, it is associated with large-scale warm advection.

For example, at 0000 UTC yesterday afternoon, note the inverted sea level pressure trough and confluent wind shift that extends through Utah and into Idaho (lower right panel).  It sits pretty much right over northern Utah.  Meanwhile, at 700 mb (lower left panel), there is weak warm advection over northern Utah.  Note how the flow is NE–N along the Utah–Idaho border, but there is actually warm air to the north.  

This has created a remarkable situation where we have large-scale ascent in the flow aloft generating large-scale precipitation.  Even though this flow has an easterly component, it is not downsloping on the west side of the Wasatch because there is a shallow layer of stable W–NW flow at low levels.  For example, the KSLC sounding from 0000 UTC yesterday afternoon shows westerly flow below 725 mb, above which the flow veers (i.e., turns clockwise) to NE and then E by 500 mb.  Note also the stable layer at the top of the western flow.  

Source: NWS/SPC
The trough pivoted somewhat overnight, but the general pattern remained largely the same.  The sounding at 1200 UTC this morning shows NW flow at low levels, veering to NE aloft, with a stable layer below about 675 mb.  

The MesoWest surface plot for 1733 UTC this morning shows the situation really well.  Note the NW flow over the West Desert, W flow along much of the Wasatch Front, but E flow over SW Wyoming.  

This is a complex event and there's much going on that I cannot cover, but this analysis shows that the positioning of the trough has created a situation we really don't have downslope occurring on either side of the Wasatch range.  There is no lee side at present, and, because of the high static stability at low levels, the flow aloft isn't really feeling the mountains,  It's just sliding along at the top of the stable layer. This is a contributing factor in the lack of contrast between lowland and mountain precipitation (although it is not the only one).  


  1. Looking at the radar/satellite loops and surface maps across the Rockies and Plains states, it is clear that this system also has a strong tap of moisture from the Gulf of Mexico. How much impact that has on SLC precipitation in a case like this I don't know, but it looks like it is a contributing factor.

  2. Someone could run some Hysplit trajectories to test that hypothesis. It could be possible for the mid-level moisture feed.

  3. I actually did run one of those starting at the surface in southwest Nebraska (40.5 N 101.0 W) 36 hrs ago (06Z 5-18). It did indeed bring much of the trajectory over northern Utah by this morning, between about the 700-400 mb level. I noticed, however, from archived surface plots that the true Gulf moisture plume was later to arrive in the Plains than I had suspected, with dewpoints in Nebraska only in the 40s at the initialization time.