Monday, September 23, 2013

Downstream Development and Utah Snow

Chances are you've heard rumors of flakes in the Uintas yesterday and snow coming to the Wasatch this week.  The setup for the potential snow later this week is something that frequently happens over the north Pacific, especially in the fall.  On Saturday morning [1200 UTC (0600 MDT) 21 Sep] and upper-level short wave trough (red line) was moving eastward over the Sea of Japan, while a plume of moisture (green line) extended northward from the subtropics near the dateline.  

IR satellite image and GFS 500-mb geopotential height (black lines) and integrated water vapor (color fill)
analysis valid 1200 UTC 21 Sep 2013
 Over the past two days, the upper-level trough amplified (western most red line below) and phased with that plume of moisture (green arrows).  This led to an amplification of the pattern downstream with a ridge building over the Aleutian islands (blue line) and a trough digging over the eastern Pacific (easternmost red line).

IR satellite image and GFS 500-mb geopotential height (black lines) and integrated water vapor (color fill)
analysis valid 1200 UTC 23 Sep 2013
This process is known as downstream development.  Here's a loop of the process.  Note in particular how the large scale pattern (the flow roughly parallels the black 500-mb height contours) over the entire  north Pacific transitions from something that is relatively "straight" (what meteorologists call zonal) to one that is "wavy" (what meteorologists call amplified).

The amplification of this pattern is expected to continue the next couple of days with the trough over the eastern Pacific strengthening and moving into the western U.S.

Forecasting pattern transitions like this couldn't be done reliably before the development of computer forecast models.  Nevertheless, although those forecast models are helpful, the forecast challenge for later this week is figuring out the details, such as how deep with the trough get, how quickly will it move downstream, and where will it generate precipitation.  That's why we can be fairly confident that we're going to see cooler weather and some mountain snow later in the week, but when, where, and how much remains a bit of a mystery.  The most likely scenario is a few inches at high elevations, but there's a wide range of possibilities depending on how everything comes (or doesn't come) together.

1 comment:

  1. On the afternoon of September 24th, my wife and I drove down to American Fork to pick up some furniture and noted clear snow patches on the mountains east and above the town.