Thursday, November 21, 2013

Nothing Good Comes from the East

Like Napoleon, meteorologists know that nothing good comes from the east.  Most of the time, our weather comes from the west, so when it comes from the east, the pattern is unusual and anomalous and strange things happen.

Wasatch Front residents are all too aware of what often happens when our flow comes from the east: downslope winds.  Indeed, we will be seeing a downslope wind event develop later today and amplify tonight as the large-scale flow between a developing ridge over northwest North America and a closed low near southern California strengthens and becomes more easterly.  This easterly flow is quite apparent in the NAM forecast valid 8 am MDT tomorrow (Friday) morning (although I've elected to show tomorrow morning, strong easterlies expected tonight too).

A closeup of the corresponding NAM 850 mb (~6500 ft) wind and temperature forecast for 1500 UTC (0800 MDT) tomorrow that time shows a low-level flow of cold air from interior Wyoming that is blocked by the Uinta Mountains and extends westward toward the Wasatch crest.

1200 UTC 21 November 2013 NAM 800 mb temperature and wind forecast valid 1500 UTC (0800 MST) 22 November 2013
Flow across the northern Wasatch crest is expected to generate what meteorologists call a high-amplitude mountain wave, with strong flow from crest level plunging down the western face of the Wasatch Mountains and generating strong winds along the northern Wasatch Front as indicated schematically in the image below.  

Source: Whiteman (2000)
One of the most challenging aspects of forecasting events of this type is determining the strength of the winds.  Most of our computer models lack the resolution to fully resolve the influence of the Wasatch Mountains.  We have some locally run models that do a better job, but these events are relatively rare, so we don't know how well calibrated that they are.  Nevertheless, we glean what we can from the models, experience, and past events.  Based on this knowledge, the National Weather Service High Wind Warning issued at 10 am this morning calls for easterly winds of 30–40 mph with gusts in excess of 70 mph.  

Source: NWS
Right now we don't think this event will be as strong as the December 1st, 2011 event which featured a maximum gust of just over 100 mph near Centerville.  Let's hope that's the case, but don't use it as a reason to be complacent as this is still a strong event (and one can't rule out something stronger even if that's not a likely scenario).   Secure those objects that can become airborne during strong winds. 


  1. I've always been curious as to why the winds are always strongest in the Centerville/Farmington area, versus elsewhere that the downslope winds happen. Any explanation for it? I'm assuming its topography related, but could be wrong.

    1. I am unaware of any study that has looked very carefully at this issue. There are a few possibilities:

      1. Exposure to easterly flow (blocking by the Uintas may help direct the strongest flow to this area - but this is going to vary some from case to case) yielding stronger cross barrier flow at that location.
      2. Shape of the terrain (effects the intensity of the downslope)
      3. Add your best idea here

      Centerville frequently has the strongest winds, but its now always the case in all events. The area in NE Salt Lake County, including the U of U, sometimes has stronger winds, and at other times it can be to the north. The direction of the flow plays some role in this variability.

  2. I have noticed that where I work (Sandy/Cottonwood heights area) the wind tends to be remarkably calm during these events, and this was the case in this event. The Wasatch mountains here are higher and tend to divert the air around them instead of having it pour over the top. Also, I agree that the Uintas are a big factor particularly when there is much colder air coming from the east side of the Continental Divide through Wyoming.
    Observations from the airport showed light westerly winds during much of this, apparently in the rotor area of the hydraulic jump. Meanwhile, where I live in Taylorsville (about 7 miles directly south of there) there was a moderate (20-30 mph) northeasterly wind. I have noticed these same patterns in the SL valley in other cases like this also.