Thursday Will Bring a Shock to the System

It has been a remarkable fall across the entire Northern Hemisphere.  The large-scale upper-level pattern has been totally locked in.  As shown by the mean (left hand side) and departure from average (a.k.a. anomaly, right hand side) 500-mb analyses below, the pattern has featured anomalous troughing over the northeast Pacific, ridging over North America, and ridging over the North Atlantic and Arctic.

Source: NOAA/ESRL

That large-scale pattern remains in force today, leading to well above average temperatures in the Arctic, where sea-ice coverage is at a record low and above average temperatures across much of North America.  In contrast, ,below average temperatures exist across much of Eurasia, especially Siberia. 

Source: ClimateReanalyzer.org

This weekend, I watched some of the World Cup skiing races from Levi, Finland and it was like viewing another planet as it sure looked like winter there, whereas here in Utah I threw on shorts and a t-shirt to mountain bike.  

In fact, the past month (13 October - 13 November) rates as the 2nd warmest on record at the Salt Lake City International Airport, with an average temperature of 55.6ºF.  Just 0.1ºF ahead of it is the comparable period from 1927.  

Source: NOAA Regional Climate Centers

However, dramatic and abrupt change is coming.  The GFS 500-mb and 6-h accumulated precipitation forecast below shows a deep upper-level trough centered over Utah at 1800 UTC (1100 MST) Thursday.  WHOOPIE!!!

The transition from Tuesday through Thursday is going to be enormous.  The GFS calls for a 35ºF temperature swing at 11,000 feet.  

Basically, we are looking at a transition from temperatures that may reach 70ºF at the airport tomorrow (Tuesday) to snow on the valley floor possibly as early as Wednesday night and Thursday.  

There is a great deal of spread amongst the models in terms of the how everything will go down Wednesday and Thursday.  The 0600 UTC GFS favors an amplified trough, which leads to a later frontal passage and later precipitation, as well as a longer period of precipitation following frontal passage.  In contrast, the 0000 UTC ECMWF and 1200 UTC NAM (which just came in) favor a weaker trough, which leads to an earlier frontal passage on Wednesday and precipitation moving in as early as late Wednesday afternoon or Wednesday evening.  

In either event, Thursday is going to bring a shock to the system.  Given the spread in forecasts, it's too soon to discuss with any confidence the details of how this storm will evolve, but prudence dictates that you take care of your car and commute winterization needs by Wednesday afternoon. 

Should you get the skis waxed?  Well, I still see a wide range of outcomes given the diversity of model forecasts in the various ensembles, with perhaps a storm total of 6 inches being on the low end of possibilities and something like 30" at the high end if everything were to come together wonderfully.  

Below are our downscaled snow estimates from the North American Ensemble Forecast System and these show the mean of all ensemble members being around 18" for upper Little Cottonwood with about a 90% chance of 6 inches or more, 70% chance of 12 inches or more, and 20% chance of 24 inches or more.  

We've made some changes to this algorithm over the summer and it's not as "jacked up" as it has been in the past.  My take, however, is that if one were to add the European ensemble to the mix, these numbers would shift downward a bit.  Keep expectations low and hope for the bigger solutions to verify.