Monday, October 23, 2023

A Cold October Trough

The last week of October is going to be an interesting one for mountains of the western United States.  I've been watching forecasts for a cold trough to drop into the Pacific Northwest for about a week now.  It's going to bring significant October snows to portions of the western United States.  

It's worth a look at the origins and forecast evolution of this trough.  It all began with the formation of a midlatitude cyclone off the coast of Asia late last week and over the weekend.  This cyclone built a ridge that was over the Behring sea at 1800 UTC 22 October 2023 (1200 MDT Sunday, orange line).  Just east of this ridge was a weak trough and jet streak that I've circled in red. These represent the incipient cold trough.  

Both the ridge and the incipient cold trough are forecast to amplify through 0000 UTC 24 October (1800 MDT Monday) with the trough digging southward over the Gulf of Alaska. 

By 0000 UCT 25 October (1800 MDT Tuesday), the now well developed high-amplitude ridge is parked over the Gulf of Alaska with the cold trough over Vancouver Island.  This is a dream mid-winter pattern for Northwest Skiers and potentially a horror show for lowland snow in the Puget Sound area, although in October, the latter won't happen.  Still, it is a recipe for the first big snow of the year in the Cascade Mountain Passes.  

Finally, that trough swings eastward across the interior Pacific Northwest.  By 1800 UTC 26 October (1200 MDT Thursday), the trough axis is moving over Utah and a second trough is dropping into the Pacific Northwest.

Let's start with a look at what this will do in the Cascades through 0600 UTC 25 October (Late Tuesday Night).  Our HRRR derived snowfall product shows 24-h accumulations in some mountain areas of over 16 inches. Although the snow starts a bit before this accumulation period, this is the bulk of the snowfall through that time.

Below is a time series from a HRRR grid point very near Stevens Pass Ski Area.  The HRRR is going for 1.25 inches of water and about 13 inches of snow, with more to come from the second trough after the end of the HRRR forecast period.   

Here in Utah, the forecast is trickier as much will depend on the track and structure of the trough and the models have been all over the place on those two storm characteristics.  Over the weekend, I saw a GFS run that was putting out about 18" for Alta with the trough passage, now it's down to about 9". 

Let's have a look at the latest GFS-derived forecast guidance for Little Cottonwood. In this forecast, the bottom drops out with a frontal passage around noon on Thursday.  Temperatures at Alta-Collins and Mt. Baldy drop, the wet-bulb zero crashes from 9000 ft to 6000 feet (and then lower Thursday night), and there is a pulse of snow adding up to about 3 inches.  This is followed by a second round on Friday that brings the storm total to an inch of water and about 9 inches of snow.  

The latest Euro too is right around an inch of water.  

The normally jacked downscaled NAEFS is a bit more optimistic with a mean of about 1.3" of water equivalent with the frontal passage and then a few members that are excited about the second trough pushing storm totals over 2 inches of water and nearly 30 inches of snow.  

These are fairly extended forecasts covering a period of 72 to about 144 hours out, so uncertainties are to be expected.  Note that the NWS is also advertising a range of outcomes depending on storm characteristics.  

Bottom line.  Keep an eye on the forecasts. 

1 comment:

  1. I appreciate the Stevens Pass callout! Planning on heading up to sample the storm Tuesday night. I hope the warm ocean and cold airmass turbocharges the convergence zone and provides some intense snowfall rates