Friday, December 15, 2017

Storms that Stretch Like Taffy

Over the past several weeks, we've had a parade of storms that, to steal a line from Howie at Alta, "stretch like taffy."

This is a symptom of the large-scale patter that has dominated our weather during the period.  Upper-level troughs either move through or over the ridge over the eastern Pacific and western North America into an area characterized by strong deformation.  Deformation is a word that describes flow patterns that tend to "stretch" and change the shape of weather features.  A good analogy is what you do with silly putty.  Push it down on something like comics in the newspaper, pick it up, and stretch it.  The figures on the silly putty get stretched out and deformed.  In other words, deformation.

An example is provided by the forecast for the next couple of days.  The loop below depicts the forecast evolution of the dynamic tropopause, basically a map of the upper-level pattern.  Troughs are blue, ridges are red, and the wind vectors depict the jet-stream level flow.  Such a map shows deformation very well.

One can see two pieces of taffy in this loop.  The first is early in the loop over Mexico and the central US.  That is the trough that brushed by us on Wednesday and Wednesday night.  Note how it gets stretched out int eh first part of the loop before a piece breaks off over Mexico and gets entrained back into the large-scale flow.

The second is the trough that is associated with our storm tomorrow.  Note how it too gets stretched out as it moves across the eastern Pacific and the western U.S. before it eventually forms a "treble clef" like structure over the southwest U.S. (nod to musical friends).

As I mentioned, just about anything slipping through the net of the persistent ridging the past several weeks has experienced this.  Ultimately, the amount of precipitation the mountains gets depends on the gory details.  Troughs have been infrequent, but when the come through, some have been productive, others non-productive.

Which brings us to the Saturday storm.  If you want an optimistic solution, look no farther than the 6Z NAM (Note: I"m writing this at 6 AM, so you are stuck with the early model runs for this post).  It holds the trough together in a way that the central Wasatch get some frontal precipitation tomorrow morning with some additional post-frontal snow showers through tomorrow evening.

Total precipitation through Saturday evening is .32" of water and about 4.5" of snow at Alta Collins.  Not much, but this season, we'd be happy to get it.

The GFS solution is only slightly different, with a less productive frontal period.  Maybe in this case we're looking at 2-4" at Alta.

True depression is provided by the 0Z ECMWF forecast which is the driest of all (not shown) with less than 0.1" of water at Alta.  The total precipitation forecast through 12Z Sunday from the ECMWF shows that the front basically dies before it gets to the central Wasatch and says NO SOUP FOR YOU!

Our downscaled SREF product has most members producing between 0.2" and 0.7" of water at Alta Collins, but this might need to be tempered by the fact that the downscaling is based on average mountain enhancement, and this is largely a frontal event with weak flow, so it may be overdoing the mountain effects.

So much will depend on exactly how and where the taffy stretches.  Keeping expectations low and hoping for the best seems to be the key to happiness this season.  A total for Saturday Morning through Saturday evening of 2-4" represents the most likely scenario for Alta Collins, but I would not be surprised if we got a little less or a little more.  More than 8 inches is quite unlikely.  Ho Hum.


  1. Jim I'm crushed, you took away the GFS from the LCC guidance.

    I loved dreaming about the GFS coming true.

    While it never did come true, it was always heartwarming to see 20 inch dumps forecast, despite the fact they never verified.

    Is this why you dropped it, it was always far too high and therefore basically useless, except as a fantasy.

    More seriously, the NAM3 now looks like it lines up w the NAM12. Did you work on the downscaling, or did the NAM folks make improvements, or is the NAM3 still bad, it's just coincidentally tracking NAM12 this run.

    1. Hey! It came to be once or twice a year :)

    2. We discovered a mild bug with the GFS processing code this week and, after digging more, we found another. However, we have not been able to ferret through all the issues. It will appear again, eventually.