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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Tuesday, December 6, 2016

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Multiple Atmospheric Rivers to Impact Sierra Over the Next Week

While November brought decent snowfall and a nice start to the ski season, storms were all small to moderate with limited moisture for Sierra standards. December will start off much different. A deep plume of moisture originating in the tropics can be seen on satellite that will begin to reach the West coast starting tomorrow afternoon. Wave one will occur Wednesday night into Thursday night, and a second wave will impact the region Friday night into Saturday.

Currently we are transitioning from a cold and mostly dry continental airmass to a much warmer and moist Pacific air mass. The map below shows temperatures at 700 hPa (~10,000 feet) with cold air (purples, -10 to -12 C) over Tahoe and really cold air (whites, -15 to -20 C) over the Pacific Northwest and Northern Rockies.

By tomorrow afternoon temperatures begin to rise a bit, but will still be cold enough to snow down to lake level and probably Reno, too. You can see the yellow colors (warm) out in the Pacific being advected (transported) towards CA. 

By Thursday morning temperatures continue to rise, and then things get interesting. The cold air in place from the day before may be difficult to mix out and snow levels could stay low longer (we've seen this in the past). It is also possible that snow will transition to rain, or even freezing rain, Thursday morning at lake level. Snow level could rise above pass level by Thursday evening. 

Snow level will remain relatively high with second wave Friday night, but the snowpack should build up pretty nicely above ~7,000-7,500 ft. The second AR is likely to impact the region early next week, but we won't get into those details yet. The bottom line is several storms will bring abundant precipitation to the region through the next week. The graph below shows the accumulated precipitation (bottom) and snowfall (top) over Donner Summit based on many different forecast ensemble members. The thin lines are individual forecast members and the thick line is the average of all forecasts. Snowfall will heavily depend on temperatures and snow levels. 

Wave one will bring 0.5-1 inches of liquid to the Sierra Crest Wednesday afternoon Thursday afternoon. Wave two could bring about the same of maybe more Friday night into Saturday.

Lake level could see a slushy accumulation that will likely melt by Thursday afternoon, mid elevation (7,000-8,000 feet) could see 6-12 inches by Saturday morning, and upper elevations may see 1-2 feet of base building pineapple pow by Saturday morning! More later on next weeks storms. 

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Saturday, November 19, 2016

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Nasty Shadowing Today, Weak Spillover Begins This Evening

Sure is a blustery one on the leeside of the Sierra Nevada today! Look at those gusts in the valley! 49mph at the airport!

This is a classic rain shadowing case. The radar and satellite confirms it.
Beautiful shadowing along the crest!

Confirmed. Awesome.

Note the inversion between 690 hPa and 660 hPa. This is the ideal mountain wave/downslope windstorm setup. Not shown, but some serious vertical windshear as well from 850 to 600 hPa. Flights are likely cancelled at KRNO/KTRK. Trashcans are migrated about the neighborhood.

As the upper level jet dips equatorward, it weakens, but this will bring the precipitable water plume into the central Sierra and lead to enhanced precipitation due to the more favorable orientation. Check out the difference in PDub from tha RightThRRR model output from this morning to early Sunday morning:
Snow levels should come down somewhat in the next 18 hours, but still likely will not fall below pass level (at least that's what Chingy's output sez!):
Plot on right shows time series of freezing elevation (in feet) for the gold dot (Tahoe Basin) at right. Currently, snow levels are pretty high!

The waves keep rolling in over the next week, a very nice active early winter pattern to at least get some sort of base going. Moisture tends to be rather anemic with the shorter overwater trajectories and lack of deep coupling with the subtropics/tropical moisture reservoir, but we'll take it!
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Sunday, November 13, 2016

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The Ridge Breaks Down...Too Much

The good news: The ridge breaks down and we get snow.

The bad news: The ridge really breaks down and the main storm track goes south and east of the Sierra...

Figure 1. Snowfall totals through Thursday afternoon.

But hey at least we get some colder temperatures and a little bit of snow. The snow should start falling early Wednesday morning and finish up by Thursday night. But how much snow? A dusting to about 6 inches of snow is expected with the 'higher totals' in the southern Sierra (Figure 1).

 Figure 2. Precipitable water valid Monday late morning.

There is good Pacific moisture associated with the storm (Figure 2), but the trajectory of the storm and a strong push of very cold air on the backside of the trough doesn't allow the deepest moisture to affect the Sierra (Figures 3-4). You always need a good moisture source and strong lifting (orographic, frontal, convection) to dump snow in the Sierra, and this setup just doesn't provide that.

Figure 3-4. Top: Precipitable water valid mid Wednesday morning. Bottom: 500mb heights and winds valid mid Wednesday morning. Notice the strong N/NW winds on backside of trough over the eastern Pacific.

There is a slight chance the storm track/trajectory changes and we get more snow, but I'm not holding out much hope. The Pacific remains active with a few more storms traversing the east Pacific and across the western United States. We'll keep an eye on those and hope for the best.

As many of you know, once the ridge breaks down, we need as much snow as possible before it builds back in. Let's hope the Pacific provides us with a good base during this window of opportunity.

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Thursday, October 13, 2016

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Get Ready For A Mid-October Soaking

The general circulation is about to explode over the western United States. Snow levels will initially be quite high during the first wave lasting from Friday to Saturday morning. The second wave is colder and will drop snow levels by 1500+ feet between Saturday evening and Sunday morning. A third wave will be colder still and allow to snow to accumulate at Boreal.

Beautiful filamentary structures of concentrated midlatitude moisture.

The juicy plume of remnant typhoon moisture (a common ingredient for October extreme precipitation events in the Sierra, recall back in 2009) will supply ample water vapor leading to 3-6 inches of liquid over the weekend. A few days ago, the Pacific looked like this:

A tropospheric river in all its glory.
It would not be surprising to see a foot or more of the stickiest, ickiest snow on the Summit by Monday morning with several feet coating the higher peaks. Shorter people could be buried further north. The orientation of the plume will favor the northern half of the Sierra and southern Cascades from Sonora to Shasta. The Pacific-wide zonal flow regime is excellent for a high impact, high snow level precipitation event.

Classic setup for a high elevation drenching!

Winds will easily allow north shore surfing to occur.

This weekend's events will be absolutely superb for several reasons. 1) as the ground becomes saturated, it will be set to freeze and cause minimal damage to vegetation after a very dry and rather hot summer, 2) the trails will be in tip top shape once they dry out after a few days (don't ride too soon! nobody likes rutted berms) and 3) streams and lakes will receive a nice boost in flow/level.
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Thursday, April 30, 2015


Climate Data Mining in the Sierra Nevada

California and the Sierra Nevada are well know for the Gold Rush years of the mid 1800s, but for weather weenies like myself (and you likely fall into this classification if you are reading this blog) there is a wealth of beautiful long term climate records for the region just waiting to mined. As a weather and climate researcher data mining is a regular part of my job, and the long term observations really fascinate me.

Dr. James E. Church, who originally came to Reno in the late 1800s to teach foreign languages at UNR, developed the Mount Rose snow sampler in 1906, and the Mount Rose site now has one of the longest snow water equivalent records in the country.

Figure: Dr. James E. Church,  http://www.wcc.nrcs.usda.gov/factpub/wc_ss.html

Donner Summit and Mount Rose both have snow course records archived back to 1910, and can be found at the same pages as the Natural Resources Conservation Service SNOTEL data (http://www.wcc.nrcs.usda.gov/snow/). Below I have plotted up both sites showing the measured snow water equivalent on April 1st, which is considered about the time of maximum snow pack in the Sierra.

Some interesting things stand out when comparing the two sites. First, when considering the current drought 2014 and 2015 are the two lowest values in the Donner record, but at Mount Rose 2015 is tied for third lowest rank with 1948. The lowest value at Mount Rose was in 1924 followed by 1931. There are likely two main factors contributing to these difference: (1) orographic effect (Donner situated just west of the Crest, and Mount Rose in the Carson Range) and (2) elevation. From a skiers perspective, we know that during many storms, Donner and Rose can get similar snowfall or often times Rose is shadowed and Donner gets more, and rarely (during upslope events) Rose can be the big winner. The high freezing levels (ridiculously warm temperatures) during the past two winters is likely the main reason for Donner snow water equivalent being so much lower than Rose (~2000 feet elevation difference), and has played big role in the "snow drought" (here and in the Pacific Northwest).

Streamflow records can tell you quite a bit about drought conditions as well, particularly when you accumulate the streamflow throughout the course of the water year. The Merced River at Happy Isles in Yosemite has one of the longest records in the Sierra, dating back to the early 1900s. 

Shrad Shukla and Andy Wood developed the Standardized Runoff Index (SRI; http://www.hydro.washington.edu/forecast/monitor/info/Shukla_Wood_SRI_GRL08.pdf) to quantify streamflow drought based on the historical distribution. Below is the water year SRI from the Merced River, which is showing the accumulated streamflow throughout each water year. For the SRI, negative values mean drought, and positive values mean wet. This one only goes through 2014, but tells a similar story as the Mount Rose snow course: 1924 and 1931 were worse single year drought than 2014. Based on this SRI 1977 was actually the worst single year drought. Looking at the abysmal snow pack for this year, the 2015 water year SRI may actually be the new worst. 

There are many things to consider when looking at the current drought, and depending elevation, orographic positioning, and variable being considered you will find different results. I've only considered single year droughts here, and this multi-year event (four years now) is certainly one of the most severe in the past 100 years.

Well, I'm droughted out! Hopefully in the near future when can discuss extremes on the opposite end of the spectrum!
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