We Will Likely Be Kissed by Arctic Air

As mentioned in prior posts, a major surge of cold air will affect a significant portion of the continental United States over the next several days.  

To be sure, an impressive pool of cold air lies over Alberta this morning with many stations at or below -30˚F and a few in the -40s.  Cold air is pushing into the northern Plains already with temperatures in North Dakota already as low as -29˚F.  At least it's windy too!

The large-scale setup for this cold surge involves interactions between high-latitude and midlatitude circulation features.  Last night, at 0600 UTC 29 Dec (11 PM MST Monday), an upper-level ridge was parked just north of eastern Siberia in the high latitudes with a weaker ridge in the midlatitudes over the northeast Pacific. 

Over the next two days, the upper-level ridge north of eastern Siberia strengthens further into an upper-level anticyclone while the midlatitude ridge migrates eastward.  Concurrently, an upper-level trough drifts across central Asia with the closed low to the north impinges on the ridge north of eastern Siberia.  This results in a remarkable upper-level flow pattern in which there is quasi-continuous ribbon of upper-level flow from central Asia across the high latitudes, and down into the central United States (red line).  

Little wonder that this will be a potent cold surge.  

Current forecasts suggest, however, that the Salt Lake Valley will only be kissed by the cold surge.  Below is the forecast for 1200 UTC 22 December (5 AM MST Thursday) when the coldest air should be over northern Utah.  While temperatures across much of central and eastern Wyoming are generally near or below -20˚F, and even the Snake River Plain has temps in the single digits (even colder in the upper Snake River Plain) they are much higher in our part of the world.  This is due to the blocking of the coldest air by the terrain to our west and north, but also the fact that what air does make it into the Great Salt Lake Basin experiences some downslope warming.  

Source: tropicaltidbits.com

Still, it is likely to feel quite cold in the Salt Lake Valley on Thursday with highs in the 20s and a stiff north wind.  If you want to be colder, try Logan or Evanston.  

There is still some uncertainty in those numbers.  The coldest forecast models have highs in Salt Lake City in the mid teens, whereas the warmest are in the low 30s.  The bulk lie between the high teens and the high 20s.  

For the mountains, there is also some notable weather.  What caught my eye are the strong winds Wednesday night as the arctic air approaches.  Our GFS-derived forecast for Little Cottonwood shows peak wind gusts on Mt. Baldy of 80 mph.  Earlier runs had gusts as high as 95.  

The reason for this is the strong northwesterly jet aloft and above the edge of the arctic air.  GFS forecast 700-mb winds are at or above 45 knots in northern Utah with the upper-level jet parked right over northern Utah.  Buckle up if you are flying into our out of KSLC during this period!

The models are also calling for a hit of snowfall in the mountains Wednesday and Wednesday night, which would be greatly appreciated, assuming it doesn't end up in Evanston.  

A curious aspect about the current forecasts is that the arctic air bleeds into Utah in a way that temperatures at upper-elevations are not that much colder than they are on the valley floor.  The GFS forecast sounding for Salt Lake City, for example, shows surface temperatures around -5˚C (23˚F), but at 700 mb (10,000 ft), the are -10˚C (14˚F).  

However, with very cold air lurking though just to the north and east, I would be cautious about counting on that forecast at this time.  It could verify, but depending on how strong the cold surge is and the depth of the cold air, there are a wide range of possibilities.  It might be warmer, for example, at upper-elevations than in the Salt Lake Valley if the cold air bleeds in from the north in a shallow layer.  On the other hand, it could be much colder than indicated above if the cold surge comes in deep.  This is a time I would pay very close attention to the observations and the forecasts before packing my pack for touring.