Wednesday, December 29, 2010

Border to Border Precipitation and the Wasatch Blocking Front

Radar echoes this morning extend from border to border.  It's a busy weather day in the western United States!  Looks like southern California is once again bearing the brunt of the storm, at least so far.

In Utah, we are presently in large-scale southwesterly flow with warm advection ahead of an approaching cold front.  The morning sounding summarizes the situation fairly well.  The atmosphere is absolutely stable above about 775 mb.  Veering winds with height above this level reflect to some degree the ageostrophic flow in the Salt Lake Valley, but also the large-scale warm advection.

Steady precipitation is falling over the entire Wasatch Range this morning.  The persistent development of radar echoes upstream of the Wasatch strongly suggests that orographic processes are essential for precipitation at this time.

One might wonder why the radar echoes start so far upstream of the initial slope of the Wasatch Mountains.  In stable situations, the flow can be strongly blocked, and a blocking front and related ascent can form upstream of the Wasatch Mountains, typically near Antelope Island.  This appears to be the case today.  MesoWest data show along-barrier south-southeasterly flow extending along the Wasatch Front from Salt Lake to Roy.  In contrast, the flow at Hat Island is SSW.

Cox et al. (2005) examined a similar case of precipitation enhancement near Ogden using dual-Doppler analysis and developed a simple conceptual model for events like this.  The blocked along-barrier flow is found beneath the dashed line in the cross section.    

Situations like this are usually big precip producers on Ben Lomond Peak north of Ogden.  From 2 to 7 am this morning, the snow depth at the Ben Lomond SNOTEL site increased 11 inches, with nearly 2 inches of water.  It appears they are getting hammered with high density snow.

That's what is happening this morning.  The storm dynamics will change today, however, with the approach and passage of the cold front.


  1. I noticed some other interesting features this morning... parts of the southwestern Salt Lake Valley such as Riverton and South Jordan area (as well as much of Utah Valley, which is colder) received several inches of snow. The southwestern SL valley areas are downwind of the narrow low-level exit region from Utah Valley and remain in the lower 30s while the rest of SL Valley is near 40 degrees (with this warmer air having downsloped from higher altitudes). It also appears that some of the very deep cold pool over the Uinta Basin might be overflowing to the west, possibly as far as Utah Valley through some of the canyons, keeping temps colder through that region.

  2. From my position near the airport I can see low level scud moving from east to west towards the Oquirrh Mountains. From my perspective here I cant tell if they are moving due west or sw. Mesowest does show a more easterly component of the wind field in the southern part of SLV and northern UTV while central SLV is more southerly.

  3. Based on the Mesowest obs, much of this easterly component flow over Utah Valley extends all the way back to the Strawberry Reservoir/Duchesne/Soldier Summit region and appears to be originating in the cold pool area near Price as well as the Uinta Basin. The Uintah Range causes a massive diversion of the low-level winds and much of this colder air appears to take a westerly route around the range.

  4. This easterly flow component is interesting. SE flow is now occurring over South Mountain, the Traverse Range, and near Summit Park. I've never understood why this happens in some events with southwesterly large-scale geostrophic flow, but not others. I do know it's good for snowfall on the Park City side of the Wasatch Range. Keep an eye on snow totals over there until the front comes through.

  5. Would almost be willing to bet that many cases of this are related to low-level stability and perhaps overflow from these large eastern Utah cold pools. It looks like much of the region including Heber/Park City has easterly component up to at least 7,000 feet. I hear that just south of point of the mountain it has snowed at least 10 inches. Will be interesting to watch this more closely in the future.