Huge Storm Coming to Sierra

 Continuing with our theme of lyrics from 70s pop songs in this week's posts, Albert Hammond once sang, "it never rains in southern California, it pours." 

I'll take artistic license and extend that more broadly to the Sierra Nevada and snowfall.

Model forecasts for the Sierra in the coming days are pretty incredible.  The setup is a deep, cold cyclone and upper-level trough dropping down out of the Gulf of Alaska, with a slow-moving cold front and atmospheric river stalling over central California and the Sierra Nevada.  Below is the GFS forecast for 1500 UTC 27 January (8 MST Wednesday) showing the situation during the early stages of the storm, with the front just south of the Bay Area and heavy precipitation in the Sierra Nevada.  

Below are GFS integrated vapor transport forecasts showing the southwesterly flow and transport of moisture associated with the atmospheric river near and ahead of the front at 1200 UTC 27 Jan (0500 MST Wednesday) and 1200 UTC 28 Jan (0500 MST Thursday).   

Source: CW3E

Source: CW3E

Persistent and large moisture transport oriented perpendicular to the Sierra Nevada is a recipe for heavy orographic precipitation.  However, this is also a relatively cold storm.  The National Weather Service has issued a winter storm warning covering the northern Sacramento Valley and the Sierra Nevada foothills for example.  

Source: NWS Sacramento

Our downscaled snowfall products are spitting out some huge numbers for the Sierra.  Below is the downscaled GFS forecast for the 24-hour period ending at 0600 UTC 28 January (2300 MST Wednesday).  Local accumulations > 36" at upper-elevation locations in the Sierra Nevada near and north of Lake Tahoe and widespread accumulations > 48" in the high Sierra south of Lake Tahoe. 

It's worth a look at how we're getting those amounts.  To do this, we'll look at a time series produced from the GFS and our downscaled products by University of Utah graduate student Mike Wessler for Kirkwood near the Sierra crest south of Lake Tahoe.  At this location and in general in the high Sierra, the downscaling doesn't change the precipitation much.  Through 1800 UTC 29 January (1100 MST Friday) the GFS with and without downscaling produces a bit more than 6 inches of water-equivalent precipitation (upper-left panel), including a peak 3-h accumulation of almost 0.7 inches.    

The average snow-to-liquid ratio in the Sierra Nevada is about 8 to 1, so if we got 8 inches of snow out of that, the total would be 48 inches.  That's impressive, but not insanely so.

However, this is a cold storm.  The temperatures for the storm at 700 mb, for example, are near or below -8˚C (center-left panel).  In Utah, that would mean snow down to the valley floor and probably a snow-to-liquid ratio in the mountains of perhaps 13:1.  

Indeed, our snow-to-liquid ratio algorithm is spitting out snow-to-liquid ratios of 12.5:1 or greater (center-right panel).  This leads to snowfall totals > 80".  

The plot below illustrates the relationship between snow-to-liquid ratio and 700-mb temperature in past storms at Soda Springs along I-80 north of Kirkwood.  For temperatures between -8 and -10˚C, as forecast by the GFS for most of the period, the linear fit snow-to liquid ratio is about 10-11 to 1.  Numbers like we're getting from our snow-to-liquid ratio algorithm are in the upper part of the distribution.  

Graph source: Van Cleave (2014)

We can also look at our downscaled SREF products.  The mean for water equivalent is right on 6" and the GFS forecast, with a range of about 3-9".  Mean snowfall is also right on the GFS, around 80", with similar snow-to-liquid ratios.  

Both water equivalent and snow-to-liquid ratio must be considered when making snowfall amount forecasts.  This will be a significant snow event in terms of water equivalent, but snow-to-liquid ratio will strongly affect how huge the snowfall amount goes.  The algorithm we use for snow-to-liquid ratio right now is temperature dependent and doesn't consider wind or precipitation amount.  While temperature favors larger ratios, the wind and precipitation amount do not. 

I'm excited to see where this all ends up.  Another 2 meter (80") storm?  Let's hope so.