Monday, January 5, 2015

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The Ridge of Steel

Since this past weekend, temperatures have been on the increase and will continue to do so through Thursday as the ridge of steel continues to build and park itself over the west-central Great Basin. The scenario for this week in a nutshell features:

  • Morning inversions in the valleys with moderate air stagnation; 
  • Unseasonably mild with temps pushing 60+ during the day in the arid valleys; 
  • High elevations will be most pleasant with snow re-softening by mid-late morning; 
  • Lots of sun and blue sky with only occasional high clouds at times; 
  • Friday the ridge breaks temporarily and allows for some westerly flow and air cleansing

The current longwave pattern for us and the rest of the Country reveals a classic (positive) wintertime Pacific/North American (PNA) signature with a ridge over the Pacific NW and a broad trough over the upper Midwest and Great Lakes region. Over the next 24 to 72 hours, this pattern amplifies significantly bringing unseasonably mild temperatures to the West and a healthy bout of Arctic air across the eastern two-thirds of the US (Figure 1).

Figure 1. 250 mb (Heights/Winds) GFS model prog illustrating the ridge of steel over the West and a broad trough over the eastern two thirds of the Country. Courtesy of NCAR RAL web page.

A positive phase PNA anomaly is quite common for this time of year, particularly the first week or two in January. In general, the prominent weather that ensues with such a pattern is above (below) average temperatures (precipitation) over extreme western Canada/US and the opposite scenario for the central-eastern US. A negative phase PNA anomaly tends to feature the reverse ridge/trough signature over the US.

The dynamics of the PNA anomaly are a function of the upstream environment. In large, it is controlled by the sea surface temperature anomalies over the south Pacific (e.g, El Nino/La Nina), which modulate the behavior of the Asian-Pacific jet. The phase duration of the PNA can be persistent or highly variable at times where changes every 7-10 days are not uncommon. Persistence or (high variability) has been linked to stronger (weaker) signals in the El Nino Southern Oscillation (ENSO). The short term forecast for the PNA anomaly is depicted in Figure 2. As noted in an earlier post, a few weeks back, the current ENSO signal teeters toward weak El Nino. Therefore, we can watch and see if the positive PNA (and the associated ridge of steel) is short lived...let's hope so, or the beast reforms and drives us crazy.

Figure 2. The daily PNA pattern from September 2014 to present, including a 7-day model projection set. Courtesy of the NOAA Climate Prediction Center. 


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