Monday, April 25, 2011

Prospects for Deep Powder Skiing

Feeling the itch for one last deep powder day before the season comes to and end?  Tomorrow could be the day, but it could pay to wake up and check the Alta-Collins snow stake or Snowbird Snowcam before calling in sick.

Although there are times when one can bank on a powder day, there are other times when there are synoptic possibilities, but whether or not we get the goods for a great powder day depends on mesoscale processes that cannot be confidently forecast 24 hours in advance with current technology.  A good analogy is storm chasing in the midwest where the potential for tornadic thunderstorms can be forecast a day or two in advance (sometimes longer), but success on a storm chase depends on factors that are not predictable at such long lead times.

This cartoon cracks me up, but have no idea who created it!
Thanks to the anonymous comedic meteorologist. 
So, tomorrow we have the possibility of a deep powder day, but whether or not Mother Nature brings the goods depends on mesoscale processes that are not well handled by our computer forecast models and are difficult to predict a full day in advance.

What we do know is that a cold front will move across northern Utah this evening.  The NAM model puts this front on our doorstep at 9PM this evening.

The NAM time-height section for Salt Lake City shows the frontal passage quite clearly at about 0300 UTC (9 PM), with moist, potentially unstable flow with and following the frontal passage.

The post-frontal flow is northwesterly, and the NAM produces heavy orographic and lake-effect precipitation overnight.

If this were to verify, it would be epic late April skiing in the Cottonwoods tomorrow, provided the convection doesn't produce too much graupel (and even if it does, sometimes that skis good too).

On the other hand, we have to remember that this is a model simulation.  Of concern:
  • The NAM model has a grid spacing of 12 km.  The processes that generate orographic and lake-effect convection are NOT resolved well (if at all) at that resolution.  
  • The duration, intensity, and location of orographic and lake-effect convection are very sensitive to small changes in upstream temperature, moisture, and wind, which limits predictability. 
  • The NAM uses (we think) climatological Great Salt Lake temperatures, that are probably higher than the actual lake temperature.  
  • What is the actual lake temperature?  We don't know because there are no publicly accessible real-time lake-temperature sensors over the interior of the lake.  Ridiculous!!!
So, we have synoptic possibilities, but ultimately the quantity and quality of the we get depends on mesoscale processes that are poorly resolved by the NAM and are sensitive to upstream flow characteristics.  I think things look pretty good for a powder day tomorrow, but what ultimately happens depends on fickle mesoscale processes.

Update: 9:45 AM

Here's the latest AVHRR 7-day mean Great Salt Lake temperature courtesy of Erik Crosman

which yields a 7-day mean lake temperature of about 11.3C.  Climatology if 15.1C.  Thus, the NAM is likely "Jacked," although that doesn't mean lake-effect won't occur as even with the lower observed lake temperatures, we have the possibility of lake effect tomorrow morning.  Unfortunately, for the reasons noted above, that's about all we can say.


  1. Regardless, a deltaT of 23C or so is more than sufficient to kick the lake off.

  2. Yes, it should be. However, if you look for situations when the lake-700 mb temperature difference is that large and the flow is out of the northwest, you can find cases where there is no lake effect. It appears instability is not enough by itself as moisture and lift are also important ingredients (perhaps vertical wind shear too). Unfortunately, the forecast models can't handle the convective scale dynamics, and they often have large errors in low-level moisture. Thus, this storm looks good, but unresolved mesoscale processes deserve some respect.

  3. Exactly. The deltaT is not going to be the limiting factor here. Most of my lake effect knowledge comes from lake superior and I can say the GSL is very, very unique.

  4. You can say that again. The predictability of these events goes down with lake size. A small error in wind direction makes a big difference. In addition, it seems we need some sort of focusing mechanism, like a land breeze, to get things going. We're working on trying to understand that piece of the puzzle.

  5. Interesting. So the smaller the lake, the more precise the wind direction has to be to catch the long fetch of the lake? Makes sense. It happens on superior too; when they can get an extreme deltaT and no lake effect or the flakes are so small accum is negligible. I agree that the land breezes causing some convergence helps a dominate band form. The Keewenaw has what is called "the bayfield bomber" and is the result of an ultra long west fetch and land breezes causing convergence and enhancing lift. I think there is more we don't know than we do about this fickle process.

  6. That is the hypothesis. Further, a short fetch implies less boundary layer modification, so the sensitivity to the upstream flow may be higher as well.

    I hadn't heard of the bayfield bomber before. Thanks!