Wednesday, September 25, 2013

Donald Rumsfield Forecasts Powder

Cold air, northwesterly flow, lake stink.  Ah, fall is here, and so is the post-frontal crapshoot.

Let's talk about the easy stuff, or what Donald Rumsfield would call the known knowns.  A deep upper-level trough with the coldest air we've seen thusfar this fall will be rumbling through the Intermountain West over the next couple of days.  The coldest and juiciest air is expected to be over northern Utah on Friday morning.

At that time, the 700-mb temperature is forecast to be about -5 to -6ºC, which is why the National Weather Service is calling for snow levels down to about 6000 feet.  If we got a strong convective or lake-effect storm, I wouldn't be surprised if we say snow down to the benches.

The hard parts of this forecast are what Donald Rumsfield would call the the known unknowns and the unknown unknowns.  The known unknowns are things that we know we don't know.  In this case, we know we don't know how to predict with precision the distribution and intensity of precipitation that falls a post-frontal environment like the one that is picture above.  The problem is this.  Small changes in the position of the upper-level trough, the wind direction, the temperature of the airmass, and the humidity of the airmass can greatly alter precipitation structures and patterns.  Further, this is a lake-effect situation, and meteorologists don't call it the Dreaded Lake Effect for nothing (we actually call it something saltier than that, but I try to keep this blog clean).

The unknown unknowns are those things that we don't know we don't know.  It's hard to speculate about something you don't know you don't know, but it is pretty clear that we haven't yet unlocked some of the key ingredients of the Great Salt Lake Effect.  Some situations with very similar atmospheric structures and lake temperatures produce lake effect, while others don't.  We haven't figured out why.  We also haven't figured out why lake-effect is sometimes banded while other times it is wide spread.  We have some ideas, but they don't really help us discriminate events in advance.

Different types of lake-effect: (a) Widespread, (b) hybrid, (c) banded. Source: Alcott et al. (2012).
These known unknowns and unknown unknowns are why I call the postfrontal environment a crapshoot.  There are few sure things in the post-frontal environment.  Sometimes we get it, sometimes we don't.  This is an instance where we're going to get something over the next couple of days, but we'll have to see if it proves to be a lot.  Personally, I'd be happy if it held off until early November, as discussed in the now classic Wasatch Weather Weenies post from 28 September 2011: Patience Young Jedi Knight

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