Wednesday, January 17, 2018

What Happens to Fronts over the Western US

It's a question that fascinated me 30 years ago and continues to fascinate me today.

What happens to fronts over the western US?

The short answer is a lot, and it varies from case to case.

Let's begin with a loop of the front making landfall later today and tonight.  This loop presents sea level pressure in black contours, 850 mb (1500-m) temperature in red contours, and either radar or 3-hour forecast accumulated precipitation in color fill.  In this loop, note how the front appears to decelerate as it pushes inland across Oregon and northern California and ultimately takes on an "S" shape with the inflection near the trough that develops downstream of the Sierra Nevada. 

These and other effects are more apparent if we look at individual times.  At 1800 UTC 17 January, the system has the appearance of a classic occluded cyclone with a relatively smooth and continuous cold front extending from just off the Washington coast into the subtropics. 

Eighteen hours later, the front is pushing into Oregon and northern California (I've given up analyzing the fronts north of this location as they have become difficult to track).  The front at this time is beginning its inland transformation and is decelerating over southern Oregon and northern California.  At the same time, there is a temperature contrast just ahead of the front over California and the eastern Pacific (circled with a thin light blue line).  This is an important feature as a new front is beginning to form at the leading edge of this zone of temperature contrast.   

By 1800 UTC 19 January, the front and the temperature contours have taken on an "S" shape with the inflection over northern Nevada.  This S-shape is the result of several processes, including the deceleration of the front over northern California and Nevada, and the development of a new front ahead of the old Pacific cold front over the eastern Pacific.  Note in particular that the precipitation band accompanying the old Pacific front is well behind the leading edge of cold air at this time, consistent with a new front forming ahead of it. 

By 1800 UTC 19 January, the front has a strongly distorted into an S-shape.  One can quibble with the precise position of the cold front in my analysis over southern California and Baja California, but the S-shape is very clear in the temperature contours.  Again, note the separation between the cold front and the model precipitation. 

The processes responsible for this evolution are complex and not well captured by simple models of frontal evolution.  Where to position a front in an analysis is always a subject for debate, but the development of the S-shaped appearance is common as cold fronts make landfall in this part of the world. 

If you are wondering what all this means for snow, tough luck.  I've already spent too much time on this post and need to get some work done!  Maybe tomorrow.  Maybe. 

1 comment:

  1. I would probably credit most of the S-shape to the effects of the Sierra Range, at least in this case. Another mountain range that may have a particularly substantial influence on cold fronts (more locally) is the Uintas, including effects that extend far westward across the SLC area. Clearly the Wasatch do as well, here, but I think the Uintas may be especially significant for larger mesoscale wind patterns. I would be curious what thoughts others have.