How Bout Some Late April Pow

Interesting start to the weekend as a strong polar jet amplifies over the northeast Pacific this evening (see Figure 1) prompting a moderate shortwave pulse to impact the Sierra. Two bursts of precipitation are expected on Friday. The first in the morning associated with strong cold advection, and the second during the afternoon-evening as the upper-level energy works through. Snow accumulations of 12-18 inches will remain concentrated near and above 7,500 ft, particularly further south in around the Mammoth Lakes area, with a coating to a couple inches possible below the 7,000 ft level.

Figure 1 illustrating the location of the upper-level amplification of the polar jet west of British Columbia. 

The mid to upper level cold air associated with this late April storm is quite impressive (Figure 2) and shall enhance the volume of snow accumulation at high elevation. Figure 3 reveals a broad area of 1-2+ inches of liquid precipitation across the Sierra. As cold fropa approaches tonight, winds will continue to ramp up from the southwest. Cold air arrives early Friday morning along with the first burst of precipitation. Intermittent to moderate snow showers continue through the day Friday before a heavier second burst in the afternoon-evening associated with the upper-level vorticity advection. The vorticity advection and central core of the upper trough appear to take a more southern trajectory across the Sierra prompting a bit more orographic enhancement further south along the Crest (Figures 4).

Figure 2 illustrating the cold air advection at 700 mb. Note the perpendicular nature and intersection of the blue-dotted isotherms and black-solid height contours. 

Figure 3 illustrating the broad swath of liquid precipitation across the Sierra through Sunday AM. 

Figure 4 illustrating vorticity advection (red) further south coming into the Sierra. 

As this system exits to the east Saturday, residual snow showers will be frequent with minor additional accumulations likely. Then on Saturday evening, another weak shortwave attempts to press south from the Pacific NW, although the amplifying ridge off the southern California coast assumes control Sunday night. Ahead of this ridge building, a period of convective snow showers associated with a pulse of mid level moisture will be the feature on Sunday. 

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Hey It's Sum...NOT SO FAST, MY FRIEND!

Mother Nature pulls a Lee Corso on us and says NOT SO FAST! to summer. The next 7 to 10 days will be a reminder that it is still spring. MJO is in an active phase for the west coast, PNA is in a somewhat favorable phase, and AO is more favorable than unfavorable, which all together leads to stormy weather.

 Mother Nature pulling a Lee Corso and saying, 'NOT SO FAST!' for summer weather.

The first storm will impact the Sierra tonight and tomorrow, dropping 0.1-0.5" of precipitation and 1-3" of snow. It will get cold with snow levels dropping below lake level tomorrow. The next storm beginning Thursday evening and lasting through Saturday will be stronger and drop 0.5-1.5" of precipitation and 4-8" of snow with some high elevations areas possibly reaching 1". That is a little further out, so we will keep monitoring the storm for any changes.

HPC 5-day precipitation forecast. 0.5-2" across the Sierra.

 9-10,000' winds, temperatures, and heights. Will get cold tomorrow and again late this week. 

Now you may ask how rare is this? Well after digging around with the help of WRCC data, I found that it is not that rare. Looking at the last 15 days of April since 1960 the Central Sierra Snow Lab (~7000' near Soda Springs) has had 294 days with precipitation and 197 days with snow. The average precipitation per day was 0.15" and average snowfall per day was 1.18". The average snow depth for the 2nd half of April was 63" or just over 5'. The average maximum temperature was 47 degrees and minimum temperature was 25 degrees.

So, about 1 out of every 3 days during the 2nd half of April has precipitation and 1 in 4 days has snow. Compare this to Reno for the same period and only 1% of days saw snow and 4% of days recorded precipitation.

Bottom line is, this is pretty normal and we should be happy because it will slow the decline of the snowpack and extend spring skiing/snowboarding just a bit longer. We will have another post up mid-week getting more into the specifics of the late week storm.

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Big Line Spring Window: Get It While It's Open!

Many of our favorite spring ski areas now have road access, including Tioga Pass (120, to the bathrooms), Sonora Pass (108), South Lake (167) and Lassen (89 to the Devastated Area). The low snowpack allowed these roads to open soon, but the sharp edge of the sword means that good skiing will not last as long as we like. The North Peak/Conness zone and Brewer Creek sides of Shasta will be some of the only good May options, barring a final Pacific blast of winter. As awesome as snowboarding North Peak in June or July is (as JimW caught me doing in 2010), those months will be better spent on the rocks, trails, or other ranges.

To keep things rolling into June and onward, the Cascades (Mt. Hood north) and Tetons are going to be the best options.

When on the search for the ultimate corn descent, especially on volcanoes, the following two parameters are of key interest:

• 700mb winds and temperatures: These can make or break your spring ski run. Too cold and breezy and you better hope you sharpened your edges. Too warm and it's a slush fest complete with wet slides. A volcanic note of importance is that due to their mass ripping into the troposphere, cloud formation can take place very quickly while all other free atmosphere regions at the same level remain clear. This makes for changing conditions (lack of melting, rapid refreezing) and visibility challenges. Always watch the clouds.
• 
• Overnight lows: If it doesn't get close to 32 (or lower in ideal cases), you are hosed. Slarving slush is fun at the resort but laying globatacular trenches on beautiful stratovolcanoes is poor form. Recently, we have had rather warm overnight temps that have wrecked havoc on the snowpack, as melting was able to occur 24 hours a day instead of during a 10 hour window. 34 or 35F is around the limit for acceptable freezing of the snow surface, but it is far better when a solid freeze takes place with regards to safety and fun.

Yikes! Let's hope for some cooler weather.

Conditions on Tioga have been reported as very good, we skied Lassen's north face with excellent results last week and the Trinity Chutes on Shasta (6400 vertical foot run to the car!) were stellar. Get it while you can!!!!

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Sunday Update

As the upper-level ridge arcs in over the Great Basin this morning, residual mid-level moisture remains persistent undermining the nice blue forecasted sky. Moisture should erode south to north by afternoon, and hopefully this afternoon will re-impress your memory as you enjoy Pow. Nice warm stretch ahead through Tuesday.

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Sunday is the Day

Cold air aloft associated with weak disturbances has been the scenario across the Sierra for the past couple days. This along with morning sunshine continues reveal afternoon convective snow showers across the area. Accumulations will remain light this evening with a couple inches possible above 8,000 ft. Over the next 24 hours, this pattern will continue to erode as an upper-level ridge returns to the Sierra Saturday-Sunday. After a clear start, Saturday will feature snow showers again, however showers will be much more isolated with accumulations less than inch above 8,000 ft. Through Saturday afternoon, winds should begin to turn more north to northeasterly and showers should begin to dissipate. And if you don't mind a brisk northeasterly wind at times, Sunday is the day to get out and really investigate the small Pow left this past week. This pattern will continue and modify into early next week. Bring the chapstick/sunscreen and enjoy!

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BC Boost!

The past week has provided a monumental boost to Sierra ski psyche. Conditions have been stellar both in and out of bounds, particularly if one chooses their terrain and timing wisely. Steep lines are filling in, but sharks still lurk. Overall the mountains have finally transformed to the magical canvas where turns can be drawn at will. This transformation does allow localized instabilities to be realized as terrain complexity is still high. Zones of tension and windloading (read: convexities) must be carefully evaluated with regards to larger scale slope stability to enhance safe travel conditions. The ever-increasing solar radiation inputs add a complicating factor that must be continually assessed. Interpret the solar and topographic signals correctly, and ye shall receive glorious turns!

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