Wednesday, February 28, 2018

The Devil Is in the Details

Nothing gives me heartburn more than the hyping of winter storms days in advance in Utah.  The devil is in the details and snowfall at any given point in our state is strongly dependent on frontal position, wind direction, and the like. 

With those words of caution, let's take a quick look at forecasts for the storm later this week and this weekend.  As I mentioned yesterday, there will be a period of warm-frontal precipitation in southwesterly large-scale flow Thursday afternoon and evening that should some snow to the mountains.  Then, things get interesting.

The 6Z GFS forecast shows a developing trough and strong cold front over the Great Basin and northwest of Salt Lake City at 1500 UTC (8 AM MST) Friday.  We are in the strong southerly flow ahead of the front, with precipitation primarily lagging it. 

At 700-mb (roughly crest level), strong southwesterly flow reaching over 30 m/s (60 knots) covers much of western Utah. 

If happening in the afternoon, this would be a recipe for very strong southerly flow and wide-spread blowing dust.  At night and in the morning, it's a bit more of a crapshoot what will happen in the valleys as sometimes we are able to maintain a nocturnal stable layer or inversion that helps reduce the strength of the surface winds.  I'm inclined to think in this case we will see that inversion mix out and strong southerly winds will develop Thursday night and give us a good blow Friday morning ahead of the cold front. 

The models differ somewhat on the details of the frontal passage.  The 6Z GFS pushes the front through Salt Lake City around noon Friday, with an active zone of frontal precipitation draped over the region Friday afternoon.  Post-frontal precipitation would fall as snow at all elevations.

The GFS is positively bullish on frontal precipitation blanketing our area for much of Friday afternoon and Friday night.  Note the lack of movement from the forecast above, valid 2 PM MST Friday, to the forecast below, valid 11 PM MST Friday. 

The 12Z NAM has a remarkably different forecast as it stalls the front to our northwest, keeping us in prefrontal southerly flow Friday and even Friday evening.  This is a dramatically different forecast that will surely give forecasters heartburn. 

Eventually the NAM pushes the front through late Saturday and Saturday night.  Thus, we can have confidence we're going to have winter weather, but the timing and amounts vary dramatically between these two model forecasts. 

In situations like this, I tend to lean toward the GFS, at least for the gross large-scale details for the following reason.  The GFS is a global model with a later cutoff for ingesting weather observations because it runs later.  The NAM is a regional model, with an earlier cutoff, and it is driven on it's boundaries by the previous GFS forecast. 

Perhaps another reason to lean toward the GFS is that the Euro also favors a frontal passage on Friday and a solution that is more GFS-like than NAM like, at least as far as large-scale features are concerned.

That being said, in terms of the amount of precipitation produced, the GFS is a bit of an outlier and I consider it's precip amount forecast to be a lower-probability outcome.  NCEP has finally corrected a bug that was providing incorrect hourly precipitation amounts in the station output that we use for our model-derived upper cottonwoods forecast guidance (info here).  Thus, I've started processing that data again and as can be seen below, the total amount of water produced for the period, including the warm-front tomorrow, is 3 inches, with nearly 50 inches of snow due to relatively large snow-to liquid ratios. 

This is substantially higher than the Euro, which is producing a bit over an inch.  It is also higher than every member of our downscaled SREF product (Note that the SREF only goes to 0000 UTC 4 March/5 PM MST Saturday, although this still captures most of the storm). 

For the GFS to verify, we'll probably need everything possible to align just right.  Although not out of the realm of possibility, the odds favor water and snowfall totals in the upper Cottonwoods that are lower than forecast by the GFS. 

The bottom line is that we're still a couple of days out from this one and that much is going to depend on small-scale details including the timing and movement of the cold front.  Monitor official forecasts from the National Weather Service and be prepared for winter weather Friday and over the weekend. 


  1. Yeah!! The GFS is back on the LCC guidance page w 3 feet of snow by Saturday and 4 feet of snow by Sunday.

    Guess we'll know if it continues to be a dream come Monday, like before you took it away. At that time it consistently over-forecasted by a factor of 2 or 3.

    You seem confident the bug has been corrected. Think you'll leave it up if it continues to provide erroneously high forecasts by a factor of 2 or 3? Of course, if we actually do get 4 feet at Alta by Sunday, then maybe the bug actually did get fixed. Probably you'll give it a few storms to see how bad the high error is. I must say I always enjoyed looking at the GFS forecasting feet, knowing not to be let down when inches or a trace showed up.

    1. The bug is fixed, which means that the data being extracted out of the GFS is right. This does not mean that the GFS isn't biased.

      We did some validation work last winter and found that on average the GFS produces about 50% more precipitation than observed at the Snowbird SNOTEL.


  2. I'm hearing most of the models are now saying the cold front could stall to our north and the snow will hold off until later.

    With that said, do you think Saturday will be a powder day in the central Wasatch?

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