Utah: Winter Returns on a NW Flow

By:  Lionel Hutz
February 4, 2011 11:37 am | Category: Weather

I know it has been a tough month for LCC.  A sloppy rainy once in 30 year storm over MLK day turned parts of LCC into the Adirondacks.  And since there isn’t an edge grinder within 900 miles of Alta, the locals weren’t happy.  However, after 36 inches of mixed density snow in the last two weeks, if you tucked the wha-mbulance away in the garage you could find some mighty decent skiing.

And it should only get better.  Beginning this weekend, a high pressure off the coast, and digging troughs in the central US will conspire to funnel a moisture stream right down into Utah from the NW.  Yay.  Lets go in depth.

First, a high pressure parked off the Cali coast will drift to the west.  This will move the mean flow of air around the high westward as well.  At the same time a long wave trough will deepen in the central states.  This will create a conveyor belt of air between these two systems that will ride down from the NW right over the Wasatch.  (Good). shortwave impulses of energy of moderate intensity will round the high and dive s/e on this conveyor belt of air.  A steady stream of moisture off the ocean should also be established with this pattern.   (Good).  Essentially then, these shortwaves will work to spike atm dynamics in a moist environment and spark steady snowfall for the next 5 days.

With this pattern established we can take a look at the time/height maps showing air saturation and upward vertical motion.  Why is this important? Well because studies have shown that for little cottonwood canyon, storm duration and upward vertical motion are possibly the most vital elements in getting big dumps. (and here you thought it was taco bell).

Looking at the NAM (which runs through the first 75% of the event). P.S.  the last sentence of the annotation should say “NOTE the long duration…”:

Now turning to the GFS we see a markedly similar solution not just for the first event but also for the second and third wave:

I’ve annotated both to focus on the upward motion.  Ideally you want to see the max upward motion occurring within air that is at least 90% saturated.  Here both models clearly depict this. That’s good.

Looking at some models to ground our sense of how much precip is possible, we see the high res models really picking up significant moisture in the NW flow favored terrain.  This is for the period ending early sunday am.

For snow growth, recent studies have show that temps in the minus teens Celsius  with winds in the high teens to low 20s are key for development of dendrites.  Looking at the temp profiles I see some degree of variation early on and then with the later waves, good cold snow growth temps. 

NWS/SLC noted the possibility for rime, and I’ll mention graupel since they are both possible.  Rime forms when super cooled droplets of air freeze on surfaces.  Supercooling is often a result of very fast cooling via upward motion.  Graupel is achieved through as similar process. Snow falls through this layer of super cooled water and rime accumulates on the flakes until they take on that ball shape. (And here Dwyer, you thought it was the result of magic snow elves pooing).  With the upward motion and saturation predicted it’s very like at some period rime and/or graupel will exist. It happens. Nothing is perfect.

Now lets talk totals.

For the first wave ending sunday the high cottonwoods should see decent totals….something like 6-12 is a good 25-75% call right now.  Adding the next few waves I think the high cottonwoods could see 2-2.5 feet by the wed. of next week.  Not bad. Not bad at all.

Here’s the caveat however. Yea…there’s always a catch.  There is going to be wavering about whether the shortwaves dive s/e through the great basin or more west into the Grand Tetons.  For this system to play out they need to dive S/E.  I’m pretty confident they will but if for some reason this doesn’t pan out, that will be the reason why.  Stay tuned.

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  1. Dwyer
    wrote on February 4th, 2011 at 12:53 pm  

    Damnit Lionel! Why must you dash out my illusions-of-snow-elf-grandeur?

  2. Porter Haney
    wrote on February 4th, 2011 at 4:28 pm  

    Dwyer and his magical, pooping snow elves.

  3. Skimohr
    wrote on February 5th, 2011 at 12:45 am  

    Cool synopsis Lionel. Thanks!

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