Sunday, January 22, 2017

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The Ghost in the Machine

Always good to some fieldwork!

My, what a week it has been. Dr. Morales above Washoe Lake:

East Shore trees looking like an East Atlanta Santa:

Lovely cloud reflections above Sand Harbor on a deep bluebird day:

Easterly flow favoring orographic ascent along the Carson Range produced perfect all-day pow conditions above Washoe Valley:

Why don't you kids get some Jobs!

Whoa! Still brrrrrrr kinda snow in the Carson Range trees above the gated community.

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Thursday, January 19, 2017


Can't Stop, Won't Stop

Well the Pacific continues to be charitable by dumping snow once again across the Sierra and really all of the western US. And the snow will continue through the weekend and into early next week. Let's pause and give thanks to the Pacific and the atmosphere...

Snow totals forecast

  • Expect another 2-4' of snow from base to mid-mountain elevations (~6000-7500')
  • Numbers continue to look silly for the highest elevations (> 7500'): 4-8' of new snow

Above are the forecast snowfall maps from the GFS (top) and NAM 3km (bottom) operational models through Monday night. The GFS has a lower resolution so the snow totals are usually more smoothed, which explains some of the differences between the two maps.

Below is a loop of precipitable water and surface pressure illustrating the wave train impacting the Sierra. We had one wave move through and looks like two more are lined up. 

The record setting winter continues and it's definitely a welcomed sight after a brutal stretch from 2011-2015. Also it's been beautiful to see all the snow across the Western US after flying over the region during the last two days. So continue to enjoy the cold smoke in the white room! 
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Tuesday, January 17, 2017

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Record Setting

Going to go through some of the record set with our impressive January and overall start to the water year (October 1 - September 30).

The first two images show the massive amounts of precipitation that have fallen over central and northern California in January. They also highlight how dry it has been recently.

Thanks to our friends at the WRCC for creating these plots. Some areas received more rain in one week than all of 2013 (top image)!!!!!! Also there are some large swaths of areas that have already seen 40%+ of annual precipitation in the first 15 days of the calendar year!!!! These are insane stats. South Lake Tahoe also had their wettest 5, 10, 15 days all in the first 15 days of January.

When looking at the total water year, we are well above the wettest year on record (green line 1982-83). And look at the slope of the line since 1 January!!! We are nearly at the annual average and have already eclipsed 2014-15 precipitation totals.

Now, let's discuss the snow because let's be honest, that's what we care about the most. We are nearly 200% of normal for snow water equivalent and most of the higher elevations of the Sierra have 100-200" of snow on the ground right now.

As you can see there is plenty of snow to play with in the Sierra and really across most of the western US. It's been awhile since we've had a big season (2010-11), and it seems longer due to the length and severity of the drought in the Sierra since that big season.

And more good news is on the horizon. More snow is in the forecast for this week/weekend and the long range model guidance shows another wave train setting up in the Pacific. So take a minute to soak in all this good fortune and then go shred!
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Wednesday, January 4, 2017

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Throwback to 1997? Or just 2005?

The 2016-2017 season has been pretty wild to start, with an extremely wet October up high, a dry November, and a somewhat classic December (mix of wet/cold/warmer but not too dry thankfully!). Then January comes along, keeps the light and dry pow coming for a few days, but follows it up with a potentially devastating round of very strong atmospheric rivers and high snow levels. Depending on how the snow levels work out, ski season could be in deep doodoo for the lower elevations and off the hook up high (high snow level scenario) or world class all around (lower snow level scenario).

The last two proper flood events are similar to the forecast event in time of year and general setup. They were both right around New Years and preceded by colder storms, which setup an excellent scenario for rain-on-snow flooding. This year we could see even more productive flooding due to the certainly saturated state of soils (the end of 1996 had some good rain events as well, but I can't recall 2005). The ultimate flood events include a coupling of the polar jet, subtropical jet, and the narrow, elongated plume of concentrated low to midlevel water vapor flux (atmospheric river). Shown below is the moisture transport and 500 hPa height field from the gold standard flood event in 1997. The moisture source originates near Hawaii, leading to the Pineapple Express term, whereas when the moisture plume originates further westward it is sometimes called the Mango Connection.

Get ready for a repeat! Below is the 72 hour integrated IVT.


The GFS Ensemble IVT forecast shows the Friday break and the weekend demolition derby of moisture flux. These values have come down a bit in recent forecasts, yesterday they were off the charts. Atmospheric river (AR) thresholds start at 250 kg/m/s (but are rightfully arguable as a function of latitude, just talk to some Arctic/Antarctic meteorologists) and strong ARs are typically in the 400-700 kg/m/s range. The weekend event will be memorable.

The pan-Pacific upper level atmospheric patterns conducive to the most extreme storms are well-known and characterized by a blocking high near the dateline that is undercut by a strong zonal jet (see figure below from Underwood et al. 2009 J. Hydrometeor.). Contours are 250 hPa heights, filled contours are 250 hPa winds.

Figure from Underwood et al. (2009 J. Hydrometeor.)

Looks a lot like what is forecast for Saturday!

The same signal appears in a composite of 500 hPa heights and water vapor transport anomalies I made for days characterized by upside down storms and high avalanche hazard in the northern Sierra (Hatchett et al. in prep). The thick black lines show positive height anomalies (ridging) developing near the dateline, and as one gets closer to the day of the event, the anomalous trough develops offshore of California with strong vapor transport favoring copious precipitation (panel d, star shows the Tahoe Basin). Right now, we are looking like panel c, about three days out (see above GFS forecast). UDSE means 'upside down storm event', or layering of more dense snow upon less dense snow. As Donald Trump would end his tweet on this: "Excellent for avalanches! Nice!"
Figure from Hatchett et al. in prep.

The forecast for precipitation is outrageously high and not worth showing. It will precipitate jaguars and wolves like a good old hurricane of the west (see Ralph and Dettinger 2012 Bull. Amer. Meteor. Soc.). The biggest question for hydrology and snow enthusiasts is WHERE IN THE WORLD WILL SNOW LEVELS BE?

First order of business is 700 hPa temperatures, even though there are a multitude of physical processes that control the snow level. Before we look at 700 hPa temperatures, here's a diagram I made highlighting just a few of the snow level/precipitation processes that are occurring at a range of scales. 
You can see that there are a lot of things going on. This version is also lacking the potential Sierra Barrier jet (Parish 1982 J. Appl. Met. Clim.) which strongly controls orographic precipitation gradients in the Sierra Nevada (Lundquist et al. 2010 J. Hydrometeor.) and is a key reason for the bullseye in precipitation that often is seen in Plumas County (e.g., Ralph et al. 2016 J. Hydrometeor.). If you are into snow science and look at crystals and snowpack stratigraphy, try to relate what you see to processes that produce those layers and crystal habits. It will help you understand the snowpack and mountain weather that much better.

Things look OK Saturday (above). It is Sunday where things get bad, and both the GFS and NAM agree on this.

After a 12 hour period where several billion gallons of rain will fall, we will shift back into a cooler regime late Sunday evening/early Monday. Hopefully there will be some snow left!

Stay tuned!

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Monday, January 2, 2017

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Snowy start to January

I spent the afternoon at Alpine Meadows today. The snow was fantastic and the lift lines were fantastically long. Upper mountain was shut down all day due to high winds, and that trend will likely continue through at least Thursday. About 8-10" fell overnight and snow continued throughout the day making for a good start to this exciting storm cycle coming up. The first wave is winding down and several more inches will fall at the resorts overnight.

Not much break before the next wave arrives tomorrow afternoon. The water vapor satellite image below shows a classic setup for a good Sierra snowfall. We have a cold upper level low currently off the coast of Oregon that will merge with a good moisture tap that can be seen off the coast of central California.

Tahoe resorts should see several feet of snow through Thursday Morning, with snow levels not being an issue with this storm. The 72 hour forecast from the GFS show impressive snowfall over much of the length of the Sierra.

However, temperatures will get warmer with this next storm (as seen in the image below), and snow will contain more water (less fluffy powder). This is actually a good thing for our snowpack, but will make for some sketchy avalanche conditions.

We should see a break in the action Thursday into Friday, but then things get really interesting as we go into the weekend. It is looking more likely that another atmospheric river will impact the region with significant precipitation through the weekend and maybe into next week. The big question right now is snow levels. There is a good chance snow levels will rise to lake level and maybe much higher. Details with these storms change rapidly so we will have better snowfall estimate later this week.

The ensemble shown below, consisting of about 20 different forecast runs, shows a very consistent wet pattern ahead with 10-15"+ precipitin through the next 7 days with 80-100" of snow. This at Donner Summit. If it stays cold enough, these numbers could verify, but if it warms too much snowfall will be much less.

Enjoy the snow this week and be safe out there. Avalanche danger will get progressively worse over the next several days, so use caution and have fun!
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