WELCOME BACK! Be sure to check out our inaugural Anti-Seasonal Outlook, Seasonal Outlook: A Four Part Series. We will have a new post each day, Monday through Thursday (10/20-10/23). Also, we will have new and improved content through the season, so be sure to come back often!

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Friday, December 19, 2014

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A break in the action

After a brief hiatus Thursday, another round of light snow has re-coated the north-central Sierra slopes today, especially just west and north of the Lake (~ 3-6 inches) with lighter amounts east and south. Intermittent light snow showers will continue overnight and part of Saturday with a couple additional inches possible above 6000 ft and slightly more above 7000 ft. Later Saturday, snow showers should become isolated with drier and milder conditions Sunday-Tuesday
  • Intermittent light snow showers overnight into Saturday
  • 1-2" around 6000 ft with a few additional inches above 7000 ft
  • Drier and milder conditions developing later Saturday through Tuesday
  • Milder afternoons with partial sunshine developing, but persistent cloudiness at times will not be uncommon with lingering moisture
The next 24 hours will be a transition period with another weak disturbance this evening ahead of warm advection developing early Saturday. Snow shower activity should be widespread this evening and then gradually taper through Saturday. All of this is a prelude to a developing upper-level ridge that will assume control of our weather into early next week (Figure 1).

Figure 1. Upper-level ridge at 250 mb building into the West coast Saturday evening. From the GFS model; image courtesy of UHawaii.

Looking ahead, the models show amplification of the longwave pattern over the central Pacific on Monday-Tuesday; a reflection of the primary core of momentum associated with the polar jet encountering the upstream West coast ridge. This prompts the polar jet to buckle and break upstream over the central-east Pacific (Figure 2) and preempts a potentially strong storm system affecting the Sierra Wednesday-Thursday (Figure 3).

Figure 2. Rossby Wave break over the central-east Pacific at 250 mb from the GFS model. Courtesy of UHawaii.
Figure 3. Potential storm impacting Sierra Wednesday-Thursday as reflected at 250 mb in the GFS model. Courtesy of UHawaii.

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Wednesday, December 17, 2014

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A lighter version of the same

Since Monday afternoon healthy snow accumulations of 6-15 inches have blanketed the Sierra slopes, particularly near-above 7000 ft north and west of the Lake and south along the higher terrain toward Mammoth. Limited amounts have occurred east of the Lake. Figure 1 provides an overview of accumulated liquid across the area.

With another small shortwave tonight and weaker one Friday, light snow, approx. 2-6+ inches, will continue re-coating the north-central Sierra, especially north and west of the Lake (Figure 2). Temperatures will gradually inch up as we approach the weekend as an upper-level ridge builds in and quiets activity Saturday through early next week.

  • Light snow tonight 2-4" above 7000 ft, an inch or less near Lake level, limited amounts east
  • A brief lull Thursday, snow showers becoming scattered allowing a few cracks of limited sun
  • Short-lived light snow on Friday 1-2" above 7000 ft
  • Quieter conditions and milder temperatures approaching for the weekend
Will issue a new post and a look ahead Friday.

Figure 1. 48 hour (Monday-Wednesday afternoon) liquid accumulation across the north-central Sierra.
Figure 2 illustrating the 72 hour snow accumulation (Wednesday-Saturday night). Courtesy of NOAA-WPC.

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Monday, December 15, 2014

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Staying Active

I apologize for not posting this weekend. To make up for this shortcoming, this post contains some phenomenal (-ish?) content. 

First things first. Three waves move through the Sierra this week. One moves through today, another one Tuesday night/Wednesday morning, and the final one near the end of the work week. Intermittent snow continues through Friday and possibly into the weekend across the Sierra. Enough cold air accompanies the first two waves that snow levels will not be a problem, and the third wave is only slightly warmer. 

Figure 1: 72-hour snowfall accumulation probability forecast at the 50th percentile via WPC.

Figure 2: Snow level and 1-hour precipitation forecast valid Wednesday evening. Image via CEFA, CANSAC. 
  • 6-12" of new snow by Thursday morning (Figure 1)
  • If we get lucky we could see some totals above 7000' approaching 18"+
  • Snow levels will be 4000-5000' (Figure 2)
You read the what and when, but now the how and why. The upper level jet across the Pacific continues to rip zonally (west to east; Figure 3). When this happens, good things (i.e. snow/rain) come to the Sierra. I don't want to complain about the past 2+ weeks because we have snow to ski/board on. However, upon closer examination, we can figure out why these storms didn't quite live up to their potential for snowfall in the Sierra. 

Figure 3: 250mb (upper level) winds across the Pacific. Via University of Hawaii Department of Meteorology. 

I discussed why the storms did not affect the Sierra as the models predicted 3-7 days in advance with the other Powdiction meteorologists and our esteemed mentor Dr. Michael Kaplan. The lack of observations across the Pacific affect the initialization and forecasts models generate, which always presents a challenge. Secondly, the overall long wave pattern across North America and its adjacent oceans contributed greatly to the storms' effects on the Sierra. 

The increasing meridional (south-north) structure of the winds and temperature of the past few waves moving onshore contributed to the lack of snowfall in the Sierra (note the break in the jet stream in Figure 3). It represents a departure to the mostly zonal flow across the Pacific, so why is this happening? As we have mentioned many times before, ridges control weather. 

Figure 4: 500mb (mid-level) heights, temperature, and winds via San Jose State University valid today. 

The cross North Pacific jet slams into a negatively tilted, strong upper level ridge over the Pacific Coast (Figures 4-5). This causes the wave to break and the flow to split. Think of a stream where it encounters a rock or boulder. Eddies form on the sides and the main flow splits around the rock. The Sierra represents a rock in the stream as well, but the fluid depth of the atmosphere is 2-3 times higher than the Sierra. The Sierra or any large mountain range affects the atmosphere, but a strong upper level ridge greatly exceeds the impact of the Sierra or any mountain range on weather patterns. 

Figure 5: 250mb (upper level) winds and heights with isotachs shaded via NCEP valid today.

A strong, negatively ridge has generally been present over central North America. The long wave pattern shifted to a ridge shifted over central North America from a ridge over the Pacific Coast. Therefore, when the strong jet encounters the ridge, the polar jet emerges with a northern and southern branch with Rossby wave breaking as another result (Figures 3-5). The leads to a storm track across the southern United States (Figures 4-5). These signals represent some of the teleconnections of El Nino (save this for another post).

For the Sierra, the increase of southerly flow decreases the orographic lift, which decreases the orographic precipitation. 15-45 degrees difference in wind direction drastically changes orographic influence and the probability of precipitation falling east of the Sierra crest (i.e. spillover precipitation).  Additionally, the waves tend to push south due to the splitting and the negative tilt, which moves the moisture plume and best dynamics further south before it can really impact and move across the Sierra. For further reading, Dettinger et al. (2004) 'Winter Orographic Precipitation Ratios in the Sierra Nevada-Large Scale Scale Atmospheric Circulations and Hydrologic Consequences' details orographic precipitation in the Sierra in great depth. 

In conjunction with the long wave pattern, the Madden-Julian Oscillation (MJO) was active and in preferable phases for the western United States during the past 2-3 weeks. The MJO signal weakened, but there is some net lag effect still lingering. The Arctic Oscillation (AO) appears to be heading back towards negative in the next 1-2 weeks, which usually indicates more cold air intrusions in central and eastern North America. The MJO weakening, the AO negative forecast, and a forecast retrograding the long wave pattern, it appears we are heading towards Ridgeasaurus next week. 

For now, let's enjoy the next few waves to provide a powdery refresher to a solid base in the Sierra. 
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Wednesday, December 10, 2014

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Shastafari Rejoice

In the coming days:

  • ~1 ft @ 6000ft, 2.5 ft above 8000ft with wind loading pushing 4ft in choice locations by Saturday.
  • Strong southerly flow will limit spillover and totals in the Sierra and boost them in the Coast Ranges and southern Cascades but all is welcome at this point!

Get psyched for an eventual migration to one of the finest ski runs in all the land! Oh wait, just kidding, how about TWO of the finest ski runs in all the land? Two images both showing 12 hour accumulated precipitation sums it up:

Thank an upstream intensification of Pacific jet momentum just east of the dateline (happened this afternoon). Doh!

The southern Cascades and Northern California reservoirs look to enjoy the gold medal performance. The Sierra Nevada will still enjoy a podium finish, nothing to be ashamed of when you recently have been lurking amongst a series of disappointing boreal winter performances, last week notwithstanding. While the more meridional orientation of the jet will reduce total Sierra awesomeness, we will score a bit lower snow level in trade. This is much needed from both a skiing perspective as well as a hydrologic one.

Keep your eye on places like Lakes Shasta and Oroville at first then turn south to the Central Coast. These places will receive a nice soaking and long-awaited filling as well as some high elevation snow (in the Sierra) to support later season streamflow. Get those soils saturated and produce some runoff! Our favorite descents of Northern California will be stackin' snowflake chips and Saturday morning should show some wonderful satellite views of stratovolcano snow-cones. California is about to get hosed down!

For the central Sierra Nevada, a solid foot of snow at 6000ft will be excellent, with favorable regions of the crest and higher lands seeing highly variable totals in feet  (2-4; watch for some sweet cornices on slightly non-standard aspects) due to crystal advection by everybody's favorite, the turbulent component of the wind! The high elevation faceted layer might just go in a few more heavily loaded places...Although from a general snowpack stability trend standpoint, the past week couldn't have been much better in minimizing continuous basal facet development and contributing to overall cohesion of the now awesome snowpack provided by the "sweet eastern tropical Pacific moisture export" (SETPME).

Your favorite wind pockets will be skiing quite nice due to the stronger winds and lower density snow (by Sierra standards, which is the GOLD standard for general awesomeness. Eagle's Nest. Need I say more?). At lower elevations watch out for the geology underfoot. 
Enjoy a few feet of wind tomorrow and good skiing ahead!
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Monday, December 8, 2014

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Off to a "normal" start, more on the way

After three drought stricken winters in a row, it's easy to forget that even an average snow year in Tahoe is still A LOT of snow.  The storm last week brought much needed short-term drought relief to Tahoe and all of CA in the form of heavy rain and high elevation snow, which helped to replenish the parched soil moisture.  But when we take a look at the SNOTEL data from Tahoe City (elevation 6,797 ft.) and Squaw Valley (elevation 8,029 ft.) you can see that the precipitation and snow water equivalent for the year are just about average, or slightly below average.  The bottom line is this: it's going to take a massive winter to fully recover from the extended drought.

There is hope though.  The 2005-2006 winter is shown below the current year, and it actually started out very similar to this season. Several warm storms early in the season boosted the precipitation totals, but the snow pack remained below average at low elevations.  After about mid-January, 2006, the storms stayed cold with low snow levels, and the fluffy white goodness piled up deep.  So, don't worry about the warm early season storms with high snow levels.  We're building a killer base, and we'll keep our fingers crossed for an active winter.  Speaking of active...

Tahoe City SNOTEL current water year (top) and 2005-2006 water year (bottom) accumulated precipitation and snow water equivalent. (NRCS)

Squaw Valley SNOTEL current water year (top) and 2005-2006 water year (bottom) accumulated precipitation and snow water equivalent. (NRCS)

There is a strong storm currently in the Pacific that will move onshore Wednesday night into Thursday, and dump some more Sierra cement.    Below are the forecasted snow totals for Wednesday-Friday.  Yes, you are reading that figure correctly: 24-48 inches of snow over the Tahoe crest and Mt. Rose!!  We are still a few days away and things can change quickly, but models have been consistent that  past several days in heavy precipitation amounts.  The thing that is still uncertain are the snow levels.  Feet of snow is looking very likely above 8,000 ft., but between 6,000 and 8,000 ft. there is less confidence in the snowfall forecast.

Snowfall forecast for Wednesday through Friday. (NWS Sacramento)

The 700 mb (~10,000 ft.) temperatures indicate that snow levels could fall to lake level overnight Thursday into Friday morning. Notice the -5 C line (blue dotted) to the east and south of Tahoe by 4 AM on Friday morning.  This is looking much better for snow levels compared to the previous storm.


We'll have a detailed update on Wednesday with snowfall amounts, snow levels, and timing of the incoming storm.  Get those skis and boards waxed up, it could be fun weekend of riding!
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