Friday, September 17, 2021

Keep an Eye to the Sky this Weekend

It's a somewhat surprisingly complicated weather forecast for the weekend, but not that atypical for September as we transition from the so-called monsoonal flow pattern to the cool-season westerly storm track. 

We'll begin with a look at the GFS forecast valid 0600 UTC 18 September (0000 MDT Saturday).  At this time, a deep upper-level trough (black contours) and frontal system are pushing onto the northwest coast.  Ahead of this system is a much weaker trough over California.  It doesn't look like much, but it contributes to a surge of monsoon moisture (color contours are precipitable water – a measure of the total atmospheric water vapor) into Utah (red arrow).  

As the deep upper level trough and cold front push into the Pacific Northwest, the weak trough slowly moves northeastward and the monsoon moisture streams into northern Utah.  By 1800 UTC 18 September (1200 MDT Saturday) the GFS is generating some scattered precipitation in northern Utah, mainly in a narrow region coincident with the monsoon surge. 

Thus, I would be aware of the threat of showers and thunderstorms tomorrow in northern Utah, especially from about noon through midnight.  These storms could be strong, as indicated by the NWS infographic below.  

Source: NWS, downloaded 8:35 AM MDT 17 Sep 2021

As indicated by that graphic, keep an eye to the sky and on forecasts and radar imagery if you have outdoor plans.  It's not possible to precisely predict the timing and distribution of these storms a day in advance.  

Your outdoor plans on Sunday should also consider the weather forecast as that is when the upper-level trough and the cold front arrive.  The timing of the surface front varies by a few hours amongst the various models.  The NAM brings it through the Salt Lake City International Airport after 6 PM MDT Sunday, whereas the GFS is much faster and brings it through just before 3 PM.  

The Euro also calls for frontal passage closer to 3 PM.  Based on this, I suspect the morning will be prefrontal, with the front coming through at some point in the afternoon.  Expect the temperatures to drop precipitously following the frontal passage and on Monday morning they will be around 20˚F at 11,000 feet.  

Although cold enough for snow in the higher elevations Sunday night, most of the model runs are favoring little to no snowfall at this time.  Below is the downscaled SREF for Alta-Collins as an example and of the 26 members, only 1 generates more than 2" of snow and the rest are 0.6" or less.  

  Still, I suspect the airmass, even in the valley, will definitely make it feel like fall on Monday.  

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