Monday, February 6, 2017

Why There Is Ginormous Spread in the Model Forecasts

We are transitioning into a warm, windy, and potentially wet (mountain snow, valley rain) pattern, but one in which the models are generating a wide range of possible outcomes for total snowfall (in terms of water equivalent) in the mountains.

Let's begin by taking a look at our downscaled NAEFS product for Alta Collins, focusing on the period through 0000 UTC 10 February (5 PM MST Thursday).  Water equivalent totals vary from about 3 inches to nearly 9 inches, the latter a truly astounding amount.  Note that most of the higher amounts (and nearly all of the members that are above the mean, are produced by the Canadian model.  You have to love our friends in the Great White North!

Shifting now to the NCEP Short Range Ensemble Forecast System (SREF), we see a similarly large spread through 0000 UTC 10 February, with a range from about 1 inch to 9 inches.
So what gives?  Part of the huge spread is the differences in the models.  The Canadian ensemble is often wetter than the GEFS and the ARW members of the SREF are typically wetter than the NMB members.  One can see the strong clustering by model system in both of the graphs above.

Another contributor to the spread is the sensitivity of the forecast to cyclogenesis (cyclone development) over the eastern Pacific.  This cyclogenesis in turn affects the strength of the developing ridge over the western US (note how the ridge over the western US amplifies in the loop below) and the track of moisture and storminess into Utah.

GFS forecast valid 1200 UTC 6 Feb – 0000 UTC 9 Feb 2017
The cyclogenesis also affects the strength, trajectory, and inland penetration of the atmospheric river ahead of the accompanying cold front.  The thumbnails below present analyses of integrated water vapor transport from all 20 members of the GEFS.  Shaded areas represent areas with atmospheric river conditions.  Note that there are members in which there is little spillover of atmospheric river conditions (color shading) into the interior west (e.g., ensemble #2, 5, 19, 20), and others where Utah is in the crosshairs (e.g., ensemble #4, #8, #9, #11, #12, etc.).
Source: Atmospheric River Portal,
All of this adds up to a huge range of possibilities for snowfall water equivalent through Thursday.

Had enough yet?  Let's talk about wind.  Yup, it's nuking the entire period.  Lots of wind barbs in the NAM time-height section.  Hold on to your hats!

So, this looks to be a stormy period.  The wind alone is going to wreak havoc on the mountains.  A wind range of possibilities exists, however, in the total water equivalent forecast through Thursday.  I'll try and find time later to discuss the period from tonight through Tuesday night in more detail as that looks to be quite interesting.

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