Monday, October 28, 2019

How Hell Freezes Over

October is the new January, at least in Salt Lake City where if you think it is cold this morning, just wait.

Really, the past month has been quite cool relative to climatological norms.  The mean temperature at the Salt Lake City International Airport for September 27 through October 27 was 49.8˚F, the lowest mean for that period since 1982 and the 4th lowest on record at the Salt Lake City International Airport (records back to 1928).

The reason for this cool weather has been a persistent, long-wave, upper-level pattern featuring ridging over the Gulf of Alaska and troughing over the northwest United States.  This has led to northwesterly flow over the western United States, with a series of systems dropping into our area from the northwest or north.

Yesterday, steady light snow fell along the east bench in the wake of the first of two major upper-level troughs that will plunge us into a deep freeze this week.  Snowfall rates were perfect for creating lovely scenes without too much impact on roads or trails.

This morning, we had a minimum temperature of 22˚F at the Salt Lake City International Airport, a new record for the day.  Minimum temperature records have become relatively rare, so this is cause for celebration.  Hooray!

However, this is just the tip of the spear of the cold air as another system pushes in tonight and tomorrow, which will really make it feel like hell has frozen over.

Believe it or not, the circumstances leading to this deep freeze involve typhoon Neoguri, the remnants of which were off the coast of Japan at 0000 UTC 23 October (circled in red).

Neoguri then underwent extratropical transition and greatly perturbed the midlatitude jet stream, building a high amplitude ridge over the eastern Pacific.

The amplification of this ridge (identified at 1200 UTC 27 October by a red line in the image below) led to an amplification and southward "digging" of the downstream trough over the western United States (long brown dashed line below), giving us yesterday's snow.  This process is known as downstream development.

But Neoguri isn't finished yet.  If you look at the image above, there is a second short-wave trough over the Northwest Territories (brown dashed line), and it is coming to Utah tonight and tomorrow.

The analysis below shows the situation at 700 mb (about 10,000 ft above sea level) a day earlier at 1200 UTC 26 October or 0600 MDT Saturday morning.  The cold front associated with the first upper-level trough was just north of Utah.  Cold air associated with the second upper-level trough (circled in red) was north of Alaska.

Now let's put that into motion. The cold front pushes through Utah, and then, the really cold air associated with the 2nd trough moves southward and into the state.

The GFS forecast valid 0000 UTC 30 October (1800 MDT Tuesday) calls for 700-mb temperatures of -19˚C in Salt Lake City.  As noted in an earlier post (A Scary Forecast with Halloween Approaching), temperatures that cold have never been observed in October at the Salt Lake City Airport (upper-air records dating back to the mid 20th century). 

The NWS forecast for the Salt Lake City International Airport thus is calling for a low Tuesday night of 13˚F. 

The record low for the month of October is 16˚F. 

Indeed, this is as close as you can get to hell freezing over. 

And, for God's sake I leave you with this PSA.  TURN OFF YOUR SPRINKLERS. 


  1. We usually don't get in on these arctic blasts in Utah because the mountains tend to block the coldest air to the east and north. Why aren't the mountains doing their job this time?

    1. The tendency for cold air to "stay east of the divide" reflects terrain effects, but also storm track.

      In this case, the storm track is amplified enough for a cold surge into Utah. The terrain is still doing its job in the sense that the air in Utah, while cold by our standards, will still not be as cold as what will be observed to the east.