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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