Wednesday, March 7, 2012

Anatomy of a Shafting

How in the world does a front move into Utah, dump 8" on Tooele, and leave Alta high and dry?

Those of us in Utah tend to suffer from what I'll call "mountain myopia." The mountains usually play such a dominant role in generating precipitation that outlier events where they play little or no role seem quite perplexing.  Sometimes, however, the large-scale atmospheric dynamics dominate over local topographic effects.  

Indeed that was the case last night.  A cold front pushed into northwest Utah last night and served as the locus for precipitation development.  Normally, we would expect that cold front to progress downstream and give the Wasatch a good pasting.  That wasn't the case, however, because as the cold front moved into northern Utah, a deep cyclone formed over southern Nevada.

As a result, the front stalled over northern Utah, with the precipitation band just west of the Wasatch Mountains. The radar loop below is a bit jumpy, and the flow evolution is quite complicated, but note how the flow transitions to easterly with time as the cyclone intensifies to the south.  

Tooele got the goods because they were underneath the most intense precipitation associated with the frontal band for an extended period of time.  Alta got the shaft because the frontal band stalled and never progressed eastward into the central Wasatch Mountains.  If the front had stalled 30 km further to the east, the story for the central Wasatch would be much different.  


  1. We looking at a return to storms in about a week? You seeing what I'm seeing?

  2. Respect to the NAM forecast yesterday - nailed it (though you gave it no respect and courted its prettier friend - the GFS.)

    1. The problem with the NAM is at longer forecast lead times (>36–48 hours). It has consistently produced bad large-scale forecasts that are inferior to the GFS, as was the case for this event ( Thus, I still give it no respect at those lead times :-).

      At shorter forecast lead times, when the NAM has a better handle in the large-scale, it does better. Indeed, you are correct that the 0–18 hour forecast produced by the NAM yesterday nailed it.