When Almost Anything Can Happen

A few days ago, some of the "deterministic" global forecast models, namely the GFS and the ECMWF HRES, forecast a truly spectacular cold surge for the continental United States this coming week.  There were a few forecast runs that brought the cold surge directly into the mountain west.  Below is an example from the ECMWF HRES initialized on 0000 UTC 13 December, which produced a 240 hour forecast [valid 0000 UTC 23 December (5 PM MST Thursday) with a spectacularly low 700-mb (10,000 ft) temperature of -34˚C (-29˚F) over southern Wyoming and -30˚C over Salt Lake City.  

How insane is cold air like that for Salt Lake City?  A 700-mb temperature of -30˚C has only been observed once in the upper-air sounding record for northern Utah, which began in 1950.  

However, today we run ensembles of forecasts to get a handle on the uncertainty in the forecast, and other members of those ensembles had different ideas.  Some didn't bring the cold surge into northern Utah.  

It is relatively easy to view forecasts from the GFS or ECMWF HRES out to 10 or more days online.  The GFS and ECMWF HRES are higher resolution than the members of their respective ensembles (known as the GEFS and EPS, respectively) and there may be some validity to weighting them a little more heavily in a short range forecast, but there is little validity to doing so in a medium range forecast.  After a few days, the higher resolution GFS or ECMWF HRES aren't significantly more accurate than the lower resolution members of their respective ensembles.  

This is because large scale forecast uncertainty trumps resolution at long lead times.  The development of high and low pressure systems and waves in the jet stream is too uncertain to place great faith in a single high resolution forecast model.  Going all in on the GFS or the ECMWF HRES at day 10 makes little sense.

Now let's forecast to today.  The ECMWF is still forecasting a potent cold surge, but at 0000 UTC 23 December (5 PM Friday), which corresponds to the same time presented above, the coldest air is located farther east and centered over Nebraska and the high plains.  

Northern Utah, however, is in the strong temperature gradient on the southern and western edge of the cold surge, and this is still a very very difficult forecast situation.  If cold air shifts a bit eastward or westward, it will make a HUGE difference in our temperatures.  HUGE.  

The National Weather Service "National Blend of Models" or NBM is a forecast system that pulls in data from the various global ensembles and produces not one but many forecasts based on those members.  This allows one to look at the range of possibilities, or what one might call a distribution.  Below are box-and-whisker plots of the maximum and minimum temperature forecasts for Salt Lake City based on the NBM forecasts from 6 AM this morning.  Prior to the 21st, the boxes are relatively squat.  Then on the 21st they get very tall. This continues through the 23rd, when the get squat again.  

In the box and whisker plots above, the middle half of the forecasts lie within the box.  The fancy word for this is interquartile range.  Whiskers then extend above and below the box and those cover the middle 80% of the forecasts.  Essentially, this is a visualization of how large the range of forecasts is, and that range is very large from the 21st to the 23rd because we are on the edge of the cold surge.

If the cold surge remains to the east or just barely trickles in, the highs on Thursday could be around 40 or even higher since there are a few forecasts that are above even the top whisker.  If the cold surge pushes west and we get deep in the cold air, highs could be a very nasty 15˚F or colder.  Minimum temperatures also show a huge range during this period.  Basically, almost anything could happen (within reasonable limits).

For maximum and minimum temperature, the NBM suggests that the period covering Thursday and Friday is the most difficult to predict of the next 10 days.  Although we think of uncertainty as increasing with increasing lead time, for Salt Lake City, the uncertainty actually goes down again in this instance because of the extreme sensitivity of what will happen with the cold surge later this week.  

We will see where all of this ends up.  Right now there are a lot of possibilities that Mother Nature could roll with her weather dice.  

Cold Surge Coming

 There's a pretty good cold surge expected to push into northern Utah later this week.  By Friday afternoon, yesterday's high of 88˚F will be long forgotten.  

Below is the GFS forecast for 2100 UTC (3 PM MDT) Friday showing a short-wave trough moving through northern Utah with 700-mb (10,000 ft) temperatures near -8˚C.  

The GFS-derived Little Cottonwood forecast shows temperatures on Mt. Baldy dropping from 43˚F tomorrow afternoon to 13˚F Friday morning.  Your Friday morning dawn patrol will feature slide-for-life conditions in the Baldy Chutes.  

Right now, the system looks fairly dry.  The GFS produces only 0.13" of water equivalent with it on Friday at Alta.  There are a few members of NAEFS (11 out of 52) that produce more than 5" of snow at Alta Collins, but most are under 2".  

Unless a low-probability snowfall occurs, the best recreation options later this week and weekend are probably at lower elevations.

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.