Arctic Surge or Not?

This is my first post since Saturday's tragic avalanche in Wilson Glade.  My heart goes out to the victims and their friends and family.  Let me also express my gratitude to all of the first responders, rescue and recovery personnel, and Utah Avalanche Center staff for all of their efforts.  

I've decided to focus on temperature for this post rather than snow.  Often temperature is an "easier" forecast variable than precipitation or snowfall, but that's not the case for this weekend.  

A look at the GFS forecast for 1800 UTC (1100 MST) Saturday shows northern Utah in an arctic airmass that has surged across the Continental divided, dropping 700-mb temperatures in the Salt Lake Valley and Wasatch Mountains to -20˚C or colder.  

In fact, the GFS forecast 700-mb temperature for the Salt Lake City International Airport is -21.4˚C at that time.  

Readers of this blog know that I get excited about 700-mb temperatures above 20˚C or below -20˚C as they are relatively unusual.  The chart below shows the distribution of 700-mb temperatures observed by upper-air soundings collected at the airport.  The thin blue line shows the lowest recorded each day.  You will noticed these brief "tooth like" minima from time to time during the winter months.  These were produced by extreme cold surges.  They don't happen every year and are most common in December and January.  

Source: NOAA/NWS/SPC

From 7 February to 25 February, a 700-mb temperature below -20˚C has only been observed at two sounding times, 1200 UTC 15 February and 0000 UTC 16 February.  This isn't because there's anything special about mid February, it's just that these are unusual and episodic events and we just haven't seen a cold surge that extreme during this 2.5 week period.  

Now that I've attracted your interest, let's take a look at the large-scale setup. Below is the GFS forecast for 1800 UTC (1100 MST) today.  There is a deep upper-level (500-mb) trough centered over southern Canada with an upper-level ridge to the north in the high latitudes.  This produces strong easterly flow between them with and embedded short-wave trough indicated by the red line.  

This short-wave trough migrates westward in the easterly flow and reaches the coast of British Columbia at 1800 UTC (1100 MST) 11 February.  

It then rotates southward and eventually southeastward around the large-scale trough reaching Utah at 1200 UTC (0500 MST) 13 February (Saturday).  

A closeup with 700-mb temperatures shows the accompanying cold front (blue line) is farther downstream in southeast Utah and northeast Colorado.  The coldest air at 700 mb is east of the Continental Divide, but 700-mb temperature in northern Utah are still unusually low and near -20˚C at the airport (they eventually bottom out at -21.4˚C six hours later).  

This is all a wonderful story, but the GFS forecast may be one of the colder ones out there in the model suite.  We don't regularly produce ensemble 700-mb temperature products at weather.utah.edu (maybe we should, but who has time...), so below is the 700-mb GEFS mean temperature and spread from eldoradoweather.com (there is a NOAA logo on this graphic, so I suspect NOAA is the original source). The average 700-mb temperature of the GEFS members is only -12˚C at the airport and the spread is quite large and maximizes near the Continental Divide region. 

https://eldoradoweather.com/current/models/North-Pacific/00-GEFS/gefs-00-700mb-temp.php

All of this indicates that there is a great deal of uncertainty in the temperatures for Saturday as much depends on the characteristics and location of the upper-level trough and whether or not cold air can penetrate across the Divide.

In fact, the ECMWF IFS keeps the cold air near and east of the Divide and northern Utah in the westerly flow at 700 mb, with a 700-mb temperature of -7˚C at 1800 UTC (1100 MST) Saturday, which would be close to average for this time of year.  

Source: Pivotal Weather

I'm not going to pick a winner here.  Instead, I'm going to say that we are playing blackjack with a full deck for Saturday and a wide range of possibilities are possible.  The GFS solution appears to be one of the colder possible outcomes.  It's a possibility, but not the most likely outcome.  More likely is the coldest air doesn't get here and we don't feel the full brunt force of the arctic surge.  Be aware of this possibility and we'll see what time ultimately tells.