Pacific Storm Track Makes Holiday Visit

The next week looks very active as the Pacific storm track has finally got its act together and is going to bring a parade of storms to northern Utah and much of the western U.S. 

Below is the GFS 500-mb analysis for 0600 UTC 27 December (11 PM MST Monday) showing strong westerly flow extending across nearly the entire Pacific basin.

Coincident with the strong jet is potent atmospheric river that also extends across nearly the entire Pacific.  

The downscaled NAEFS plume for Alta-Collins is known for overforecasting, but is laying down some big numbers with 6–13" of water equivalent and 100+ inches of snow over the next seven days.  

Those numbers are eye popping, but I often use the plume for Brighton as a better approximation and even it is in the 5–10" range for SWE and 60–120" for snow.  

Let's take a closer look at the event for today and tonight, beginning with the situation as I prepare this post on Tuesday morning.  At 7:40 AM MST, it was quite mild across the region with temperatures in the high 40s and low 50s in the Salt Lake Valley, including some readings in the high 50s along the east bench.  Even in the mountains, temperatures were in the 40s at many stations.  Even on Mt. Baldy and Hidden Peak it was 34˚F!

The GFS forecast for 1800 UTC 27 December (11 AM MST Tuesday ) shows strong southwesterly flow at nearly all elevations with a broad atmospheric river extending across California and the Great Basin to Utah.  

Given all of that tropical moisture, it is going to be very warm today, with 700-mb temperatures as high as +2˚C in western Utah.  Beneath a band of precipitation that will be moving in this morning, however, it is a bit cooler (-2 to 0˚C) due to cooling from evaporation and melting.  

Nevertheless, the GFS wet-bulb zero level reaches as high as 9600 ft at 10 AM at Alta.  

I would not be surprised for there to be rain this morning or in the early afternoon to altitudes as high as 8500 feet and wet flakes falling at even higher elevations.  Later in the afternoon, the snow level will decline to closer to 7000 feet before dropping further overnight.  

For Alta-Collins, at 9600 ft elevation, the GFS is putting out 1.55" of water and 15" of snow through 9 AM Wednesday.  Water contents are forecast to be 20% to start today, but declining to 5% tomorrow, so this will be a right-side up storm.

The reason for the lowering snow level and right-side up snowfall is the passage of a cold front overnight with the flow shifting to northwesterly, as can be seen in the time-height section below.  

In terms of water totals, the 12Z HRRR is even wetter, putting out 2.42" for Alta and even bigger numbers in areas that are typically "favored" in the moist southwest flow we will see ahead of the front including 4.32" at Sundance and 3.03" at Ben Lomond Peak by 1600 UTC (9 AM MST) tomorrow.  Unfortunately some of that precipitation will fall as rain today at those locations.  

The bottom line is expect mild, windy weather with occasional rain, heavy at times, at mid elevations and wet, high-density snow at upper elevations today.  Conditions improve later today and tonight as temperatures decline and snow levels drop.  

Buckle up.  This is just the first of many.