Thursday, March 15, 2012

Forecast Predictability and the Utah Sirocco

In previous posts, I have discussed some of the predictability issues associated with this weekend's storm. I still think this is a remarkably difficult forecast for northern Utah.  There's much to talk about, but I'm going to concentrate primarily on Saturday, which may prove to be a very notable weather day.

The GFS forecast for 1800 UTC (1200 MDT) Saturday 17 March puts a digging upper-level trough off the California coast.  Strong southwesterly flow ahead of this feature contributes to the development of a strong Intermountain cyclone centered near Wendover.

Yesterday's model forecasts for Saturday placed the frontal precipitation band associated with the cyclone in various locations (see previous post), which reflects one of the predictability challenges for this event, determining where the cyclone and frontal band will be located.  The GFS forecast from last night keeps the frontal band to north and west of Salt Lake City and the central Wasatch through Saturday evening.  Similarly, so does this morning's NAM, which puts the cyclone center over the West Desert at 0000 UTC 18 March (1800 MDT Saturday 17 March).

While we can't rule out the front sagging a bit further south, especially on Friday night when the models suggest it comes close before being shunted back north (not shown), the model consistency I'm seeing suggests the Salt Lake Valley and central Wasatch will remain in the warm, windy airmass ahead of the Intermountain cyclone and front through Saturday evening.   We could see some valley rain/mountain snow showers pop up in the southerly flow, possibly accompanied by a little thunder and lightning.

In addition, there is significant potential for high winds across much of western Utah on Saturday, including the Salt Lake Valley and Wasatch Mountains.  The GFS forecast above calls for 55 knot (27.5 m/s) at 700 mb near Delta, with a very strong sea level pressure gradient (nearly 12 mb from Page, AZ to Wendover).   The NAM is also "breezy."  This is a recipe for strong southerly winds and dust, which I like to call the Utah Sirocco as it reminds me of the Mediterranean Sirocco, a warm, southerly wind that often transports dust from the Sahara into Europe.

Beyond Saturday, it remains a very difficult forecast with a wide range of possibilities.  This is a situation where the moisture accompanying the system is torn up into pieces and predicting position, timing, and intensity is very difficult at these long lead times.


  1. Dr Jim, Are you betting on a south west flow. It looks like the SW favored areas could get a real slug of snow. I am starting to plot the weekend skiing.

  2. Could this be a total dud for us, like less than 6 inches dud?

  3. With regards to both these questions, right now the models call for the heaviest precipitation *this weekend* to fall late Saturday night to early Sunday morning with the frontal passage. There is not a sharp wind shift in this case. The large-scale mountain-top flow shifts from southerly to southwesterly. The NAM puts out .31" of liquid precipitation at Alta from midnight-6 AM Sunday, the GFS less. Going for 4–8" based on that is probably the best forecast at this stage, but Mother Nature may ultimately have different plans for this event. As mentioned in the post, this is a situation where the crystal ball is cloudy as much is going to depend on the exactly how things set up.

  4. Dr Jim, We need to consult the magic eight-ball:)