Saturday, October 23, 2021

The Coming Storm and Cautionary Words about Atmospheric Rivers in Utah

Everything remains on track for the big storm in California and eventually more October snow for the central Wasatch.  

Current forecasts show a very well-defined precipitation system moving across California and the Great Basin Sunday and Monday.  The GFS forecast valid 0600 UTC 25 October (0000 MDT Monday) shows heavy precipitation (>1" water equivalent in 3 h) in the Tahoe area, to the southeast along the Sierra Nevada, and then along the cold front in the northern Great Basin.   

By 1800 UTC 25 October (1200 MDT Monday), the action has shifted southeastward to the southern Sierra and Mammoth Mountain and to central Nevada.  It is impressive to see a continuous precipitation band extending across the Sierra and into the Great Basin.  We will see if that verifies, but often there's a break due to shadowing downstream of the Sierra.  

Much has been made of the "atmospheric river" conditions that will penetrate into the Great Basin and indeed at 1800 UTC (1200 MDT) Monday northern Utah is solidly in an atmospheric river ahead of the cold front with large values of integrated vapor transport (see below).  However, as can be seen in the GFS forecast above, there's virtually no precipitation over northern Utah. More on this in a minute.  

Source: Center for Western Weather and Water Extremes

Ultimately, the GFS brings the frontal band into northern Utah late on Monday.  Below is the forecast for 0300 UTC 26 October (2100 MDT Monday) with the frontal band over northern Utah.  

The time height section for Alta helps summarize the next few days and sheds some insights into the the atmospheric river forecast for Utah.  Recall that time increases to the left in this diagram.  First up is today's storm, which will bring 2-5" of wet-snow scraps to upper Little Cottonwood.  On Monday, warm, moist southwesterly flow develops aloft. Freezing levels (thick blue line) rise to over 11,000 feet, which sounds scary, but the low levels are drier with lower relative humidity levels.  This, and the lack of any well define warm-frontal feature to drive large scale lift, is why the GFS isn't generating much in the way of precipitation until the arrival of the cold front later in the day.  

The GFS terrain is not as "impressive" as reality, so perhaps we will get some showers at times before the approach of the front, but I suspect accumulations will be limited.  Thus, I'm not panicked about a big rain-on-snow event.  I think it is more likely we will see a few mountain showers on Monday ahead of the front, perhaps a bit of rain to high elevations as the front approaches, but then snow levels will drop abruptly as the cold front moves in.  

Supporting this narrative is our GFS-derived forecast for upper Little Cottonwood (time increases to the right in this graphic).  I've identified the front with a light blue line.  Ahead of the front, temperatures on Mt. Baldy exceed 35˚F and the wet bulb zero level (typically the snow level is a bit below this level) over 11,000 feet.  

However, during this period, the GFS is generating just a skiff or two of precipitation.  Temperatures and wet-bulb zero levels drop abruptly with the front.  At Alta Collins, the GFS-derived precipitation total is a bit over 1.5 inches and snowfall about 20 inches with and following the late Monday front.  If you are wondering, the ECMWF is just a bit behind that.  Total water equivalent from the GFS, including today's storm, is a bit over 2 inches.  In the Euro 1.83".  

Wednesday is still a long ways off to be talking about details, but right now I see the Monday to Tuesday storm as a 10-20 inch event at Alta Collins, with the possibility for more if the post-frontal period is especially productive.  It's too far off to have confidence about what that will bring.  

To wrap up, a few cautionary words about atmospheric rivers in Utah.  There is a strong correlation between integrated vapor transport (IVT) and precipitation in California, especially in the mountains.  This is shown really well by the left hand figure below from Rutz et al. (2014), which shows correlation coefficients between IVT and precipitation in the coastal range, southern Cascades, and most of the Sierra above 0.65.  IVT and atmospheric river intensity have a good deal of value there.  In Utah, however, the correlations are lower and the predictive value of IVT is lower and perhaps about 0.35 to 0.40 in our mountains.  This is an issue we've discussed in previous posts.   

Source: Rutz, Steenburgh, and Ralph (2014)

This doesn't mean that an atmospheric river can't bring heavy precipitation to our mountains, but it does mean that you shouldn't have AR-myopia.  Also needed is large-scale forcing for precipitation and/or lower-level vapor transport with higher relative humidities to drive orographic precipitation processes.  Right now, it doesn't look like we will have those on Monday until the front approaches.   

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