Wednesday, December 20, 2017

Lessons in Cold-Frontal Precipitation

If you have studied introductory meteorological text books, you've probably seen schematics of frontal precipitation based on the one below, which is from a seminar paper published in 1922 that synthesized existing knowledge on frontal cyclone evolution into a coherent depiction known as the Norwegian Cyclone Model due to it's development in Bergen, Norway.

Source: Bjerknes and Solberg (1922)
The cold front is depicted on the left, with a narrow band of precipitation forming where the leading edge of colder air is intruding into the warm airmass.  This band of precipitation is known today as a narrow cold-frontal rainband (or alternatively, snowband if cold enough).  

Although, conceptually simple, not all cold fronts behave in this manner.  Sometimes a wide cold-frontal rainband exists upstream of the surface front, as depicted below. 
Source: Matejka et al. (1980)
In some instances, the wide cold-frontal rainband exists in isolation, with no narrow cold-frontal rainband present.

Which brings us to today's frontal passage.  At 1400 UTC (7 AM MST) there was a clear separation between the surface trough, which was draped across central Nevada and northwest Utah and a wide cold-frontal rainband (really a snowband) that was further upstream and northwest.  

The 1300 UTC initialized HRRR forecast valid 1900 UTC (1200 MST) very clearly shows the surface cold front, as defined by the wind shift, pushing into the Salt Lake Valley, well in advance of the trailing precipitation band.  

So, this is an instance where we will expect a mainly dry frontal passage, with precipitation moving in later (let's hope things fill in better than in the HRRR forecast above!).  

The visualization below depicts the structure of a similar front on the 27th of November when the surface front was moving across the Great Salt Lake Desert and into the Skull, Tooele, and Salt Lake Valleys.  The cold air behind the surface front was quite shallow and during this period the surface front was dry.  Deeper cold air was further upstream and accompanied by a wide cold-frontal rainband (rainband not shown).  I suspect todays frontal passage will be similar in structure.  

What controls the precipitation characteristics of cold fronts in our part of the world is not fully understood.  The topography appears in some instances to form a new surface trough ahead of the approaching Pacific cold front, and this trough sometimes becomes a new cold front ahead of the precipitation system.  On the other hand, we also see events that feature bonafide narrow cold-frontal rainbands.  These differences can, however, often be anticipated by the HRRR.

Which brings us to the forecast.  I'm sticking with a 3-6" storm total at Alta through noon tomorrow (Thursday), most with the wide cold-frontal rainband (or more correctly, snowband) trailing the surface front, with a few snow showers from the wrap around late tonight and tomorrow.  The snow will be of the low-density variety, which is a shame, as we really need a pasting with high-density snow right now.  

If Alta gets more than 8 inches, consider it a Christmas miracle.  If they get less than 2 inches, you had better take some time to reflect on your behavior this past year, because you are clearly on Santa's naughty list.

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